=Paper= {{Paper |id=Vol-3329/paper-04 |storemode=property |title=Federated Learning as Enabler for Collaborative Security between not Fully-Trusting Distributed Parties |pdfUrl=https://ceur-ws.org/Vol-3329/paper-04.pdf |volume=Vol-3329 |authors=Leo Lavaur,Benjamin Coste,Marc-Oliver Pahl,Yann Busnel,Fabien Autrel }} ==Federated Learning as Enabler for Collaborative Security between not Fully-Trusting Distributed Parties== https://ceur-ws.org/Vol-3329/paper-04.pdf
Federated Learning as enabler for Collaborative
Security between not Fully-Trusting Distributed
Parties
Léo Lavaur1,2 , Benjamin Costé3 , Marc-Oliver Pahl1,2 , Yann Busnel1 and
Fabien Autrel1
1
  IMT-Atlantique, IRISA
2
  Chaire Cybersécurité des Infrastructures Critiques (Cyber CNI)
3
  Airbus CyberSecurity


                                         Abstract
                                         Literature shows that trust typically relies on knowledge about the communication partner. Federated
                                         learning is an approach for collaboratively improving machine learning models. It allows collaborators
                                         to share Machine Learning models without revealing secrets, as only the abstract models and not the
                                         data used for their creation is shared. Federated learning thereby provides a mechanism to create trust
                                         without revealing secrets, such as specificities of local industrial systems.
                                             A fundamental challenge, however, is determining how much trust is justified for each contributor
                                         to collaboratively optimize the joint models. By assigning equal trust to each contribution, divergence
                                         of a model from its optimum can easily happen—caused by errors, bad observations, or cyberattacks.
                                         Trust also depends on how much an aggregated model contributes to the objectives of a party. For
                                         example, a model trained for an OT system is typically useless for monitoring IT systems.
                                             This paper shows first directions how heterogeneous distributed data sources could be integrated
                                         using federated learning methods. With an extended abstract, it shows current research directions and
                                         open issues from a cyber-analyst’s perspective.

                                         Keywords
                                         Federated learning, cybersecurity, intrusion detection, distributed trust




1. Introduction
A common cybersecurity goal is thwarting attackers through detection of their actions, un-
derstanding of their methodologies, and increasing the resilience before their next attempts.
However, the considerable variety, thus complexity, of the tools and techniques used by at-
tackers drown their traces in the high volume of legitimate network traffic and endpoints logs.
One way used by defenders (often called “Blue teams”) to act against threat actors is sharing
knowledge about their abuses. However, knowledge gathered during security monitoring or

 C&ESAR’22: Computer & Electronics Security Application Rendezvous, Nov. 15-16, 2022, Rennes, France
£ leo.lavaur@imt-atlantique.fr (L. Lavaur); benjamin.b.coste@airbus.com (B. Costé);
marc-oliver.pahl@imt-atlantique.fr (M. Pahl); yann.busnel@imt-atlantique.fr (Y. Busnel);
fabien.autrel@imt-atlantique.fr (F. Autrel)
DZ 0000-0002-0379-7946 (L. Lavaur); 0000-0002-5631-9120 (B. Costé); 0000-0001-5241-3809 (M. Pahl);
0000-0001-6908-719X (Y. Busnel); 0000-0002-8403-515X (F. Autrel)
                                       © 2022 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
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    Workshop
    Proceedings
                  http://ceur-ws.org
                  ISSN 1613-0073
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Federated Learning as Enabler for Collaborative Security between not Full...


incident response is often coupled with private data which cannot be shared for confidentiality
reasons (GDPR, NDA, etc.).
   Machine Learning (ML) approaches can help here, as they result in abstract models that are
typically not reversible to their input data [1]. A fundamental problem for observation-based
security is the amount of data needed for having a trustable and reliable impression. Typically,
more data allows for better behavior characterization, thus improving either anomaly detection,
or event classification. Collecting data requires either a long observation time or many data
sources, as comprehensiveness is difficult to reach.
   Federated Learning (FL) has been introduced to enable the sharing of local models and to
federate them towards better joint models. Each participant computes on its own an ML-model
using its own data. The resulting model is aggregated with the ones of other participants,
typically by a trusted party, then the new model is shared between each of them.
   Consequently, all parties benefit from each other while no one have access to their private
data and algorithms. Sharing models however still faces issues as models can be modified with
backdoors [2], poisoned with adversarial approaches [3] or simply suffers from bad quality
training dataset.
   This paper discusses FL sharing approaches through aggregation issues. Section 2 introduces
the concept of Machine Learning (ML) based intrusion detection, and Federated Learning (FL)
as a collaboration enabler. In Section 3, we summarize the literature around federated intrusion
detection, with an extended abstract of a survey. Section 4 outlines an experimental use-case
for Federated Intrusion Detection System (FIDS) application. Section 5 discusses open issues
and the envisioned solutions. Section 6 concludes our proposition.


2. Background: ML for collaboratively defend cyberattacks
One prominent application of FL in the cybersecurity field is for security monitoring and col-
laborative intrusion detection. This section defines the basis for the remainder of the paper.
   Security monitoring systems frequently use signature-based Intrusion Detection Systems
(IDSs) to detect known attacks to safeguard companies [4]. However, this strategy suffers
from serious limitations against zero-day and one-day attacks, as well as Advanced Persistent
Threats (APTs), such as Stuxnet [5]. Furthermore, the IoT’s heterogeneity and irregular traffic
cause IDSs to be less effective or inadequate [6]. As a result, researchers began to look into
anomaly detection as a way to improve detection systems.
   Here, multiple approaches coexist, depending on objectives and available data. On the one
hand, anomaly-based detection systems compare monitored events to a baseline profile trained
on nominal traffic to assess whether they are malicious or not [7]. On the other hand, pattern-
based classification, aims at extracting patterns from known attacks that have been previ-
ously labelled as such, and then characterize input data according to the extracted patterns.
Therefore, anomaly-based approaches are particularly relevant to detect unknown behaviors,
whereas supervised ones are more equipped for threat characterization.
   The source of data will also influence the available features for detection. For example,
endpoint-oriented sensors such as Sysmon monitor processes, use of windows’ registry key
or system calls while network-oriented sensors provides a lot of information about protocols,




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packet length, or IP addresses. Preprocessing can be used to extract additional features from the
raw data, to provide more information. The literature distinguishes three key non-exclusive
approaches: feature extraction, feature embedding, and feature selection [8].
   In the context of knowledge sharing, the choice of the input feature is critical to transfer
knowledge between participants. For instance, IP addresses are specific to the local layout of
the network, and might have another meaning in another environment, if they even exist. On
the other hand, given one class of devices, inter-arrival time (IAT) should not vary much be-
tween networks, thus making potentially making it a good choice for transferring knowledge.
   With a designated set of features and an input dataset, one can establish a model of the data.
A model is an abstraction that can be used afterward for other tasks, such as characterization.
Algorithmically speaking, a model is a set of mathematical parameters that are inferred from
input data by an algorithm. It can be the statistical parameters of a distribution function, the
condition nodes in a decision tree, or the weights and biases of a neural network. The ability
of a model to be shared partly depends on how easily these parameters can be aggregated.
   Over the years, ML have been applied to intrusion detection with substantial results. Three
approaches coexist:
  (1) Anomaly detection becomes a binary classification problem with supervised learning.
      For effective training, a balanced labeled dataset is required. However, because local
      training data is infrequently labeled and models can be affected by unbalanced class
      distribution, supervised learning is more difficult to apply in real-world scenarios [9].
  (2) For unlabeled data, unsupervised learning is more appropriate. In the case of IDS, we
      assume that (i) benign traffic is substantially more common in the testing set than anoma-
      lies [10]; and (ii) abnormal packets are statistically different from normal packets.
  (3) Semi-supervised learning is a hybrid method that labels only a portion of the training
      data. It can be used to bootstrap a detection model with a publicly labeled dataset before
      training it on locally collected data afterward.
   However, ML algorithms also require a lot of training data to avoid learning biased model,
from the lack of exhaustively or from an overrepresentation of a class in the training set.
   To cope with the limitations of ML, especially when training data is locally collected, col-
laborative IDSs have emerged in the literature. However, they are almost always built in a
centralized manner, which induces its own set of issues: (i) centralizing a system typically in-
troduce a single point-of-failure (SPoF) [11]; (ii) centralized IDSs imply sending local data for
training or detection, increasing the risk of information disclosure [12]; (iii) communicating
data over networks also increase bandwidth consumption and latency, which are critical for
intrusion detection [13].
   Introduced in 2016 by Google [14], FL promises to cope with these issues. In FL, model
learning is distributed among the participants of the federation. Therefore, local data stay in
the participant’s system, and collaboration is achieved by sharing and aggregating the gener-
ated models. Aggregation can be done by a server [6], [15]–[17], but it might lead to concerns
with trust and privacy. Due to challenges in terms of traceability, integrity, privacy, and trust,
recent research has favored the usage of trusted distributed ledgers [18], [19], multi-party com-
putation (MPC) [20], and privacy-preserving mechanisms like Differential Privacy (DP) [21].




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Federated Learning as Enabler for Collaborative Security between not Full...


3. The example of collaborative intrusion detection
Since its introduction, FL has been applied to multiple domains, such as intrusion detection,
whose presence is increasing in the literature. In this context, FL allows local detection of
attacks—thus offering low latency and bandwidth—while jointly enriching participant’s models
and preserving data privacy.
   This section is an extended abstract of the study published in IEEE Transactions on Network
and Service Management (TNSM) [8]. Contributions of the study are as follows: (1) it examines
the application of FL to the detection and mitigation of attacks; (2) it proposes a reference
architecture that generalizes the selected work and can serve as a starting point for the design of
future FIDSs; (3) it establishes a taxonomy of FIDSs, which provides a framework for comparing
the selected works in this study; and (4) it highlights open questions regarding FIDSs and
identifies associated research directions. The findings presented in this study can easily be
extrapolated to other cybersecurity applications of FL, such as automated forensic analysis, or
malware detection and classification.
   Section 3.1 presents the results of the study and the established taxonomy, whereas Sec-
tion 3.2 reviews identified research directions.

3.1. Literature review
The study performs two analyses of the literature. The quantitative analysis is based on ob-
jective metrics that can be extracted from the literature, such as publication date and venue.
Figures 1a and 1b show the evolution of the literature on FL and IDS over time, both being
building blocks of FIDSs.

                3000                                                      800


                                                                          600
 Publications




                                                           Publications




                2000
                                                                          400

                1000                                                      200


                                                                            0
                       2000   2005   2010    2015   2020                         2010 2012 2014 2016 2018 2020 2022
                                      Year                                                     Year
 (a) Keywords: intrusion detection system                                  (b) Keywords: federated learning
Figure 1: Evolution of related domains until 08/10/2021, data from Microsoft Academic [22]
—Figures from Lavaur et al. [8] © 2022 IEEE


  The qualitative analysis is based on the comparison of existing approaches, using the pro-
posed taxonomy. They can be grouped into four categories: data, local operation, federation,
and aggregation. A fifth meta-category is dedicated to the implementation and evaluation of
the approach.




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                                                              L. Lavaur, B. Coste, M.-O. Pahl et al


  The taxonomy (Figure 2) provides twelve characteristics on which all approaches can be
compared:

   1. Data source and type: what data is collected and how; heavily depends on the use case.

   2. Preprocessing: strategies for data curation, such as normalization and feature selection.

   3. ML location: where the ML model is trained and executed.

   4. Local algorithms: how the ML model is trained, and its impact on performance.

   5. Defense capabilities: the ability of the approach to mitigate attacks.

   6. Federation strategy: how the federation is organized; e.g. client selection, architecture.

   7. Communication: how data (i.e. models) is exchanged between participants, including
      protection mechanisms.

   8. FL type: type of FL strategy, depending on the objectives and available data.

   9. Aggregation strategy: how models are aggregated, especially with heterogeneous clients.

  10. Model target: i.e. the balance between specialization and generalization.

  11. Analyzed dataset: the dataset used to evaluate the approach; often Information Technol-
      ogy (IT)-focused, and not always available.

  12. Costs and metrics: how the approach is evaluated, depending on the use case and objec-
      tives.

   The structure provided by the taxonomy also allows comparing the selected works. Table 1
summaries the results of the comparison. It shows that most approaches focus on IT network
traffic, using a gateway to collect data and host both learning and detection processes. Most
also use Deep Learnings (DLs) to perform supervised learning and classify traffic. A majority
use unmodified FedAvg for the aggregation, which is the initial FL algorithm that was proposed
by Google [14].

3.2. Open issues and research directions
As the FL topic gets more mature, research tends to focus on secondary aspects, such as security
and privacy, or on the application of FL to other use cases. This subsection summaries the open
issues and research directions; more details are available in [8].

  (i) Performance — Like any detection system, FIDSs are looking for an absolute perfor-
      mance: a system with a perfect classification score, producing no false positives or nega-
      tives. To this end, several avenues have been identified in the literature, such as the use
      of Generative Adversarial Networkss (GANs) or the improvement of feature selection as
      input to the model. Moreover, the link between the performance of the system and its
      hyper- and meta-parameters is not yet established.




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Federated Learning as Enabler for Collaborative Security between not Full...

                                                                                                                Non-IID
                                                                                Distribution
                                                                                                                IID
                                                 1. Data source and type
                                                                                                                Usage
                                                                                Source
                              Data                                                                              Communication
                                                                                Dimensionality reduction

                                                 2. Preprocessing               Feature extraction

                                                                                Feature selection

                                                                                On server

                                                                                Dedicated device
                                                 3. ML location
                                                                                On gateway

                                                                                On device                       Outlier detection

                                                                                Unsupervised ML                 Dimensionality reduction

                              Local operation                                                                   Clustering
                                                 4. Local algorithms
                                                                                                                Regression
                                                                                Supervised ML
                                                                                                                Classification
                                                                                On-device counter-measures
                                                 5. Defense capabilities
                                                                                Network reconfiguration (e.g. SDN)

                                                                                Architecture (e.g. P2P)

                                                 6. Federation strategy         Reputation

         Taxonomy of FIDS     Federation                                        Client selection

                                                                                Overhead reduction
                                                 7. Communication
                                                                                Encryption

                                                                                Federated Transfer Learning

                                                 8. FL type                     Vertical Federated Learning

                                                                                Horizontal Federated Learning

                                                                                MPC-based aggregation

                              Aggregation        9. Aggregation strategy        Weighted algorithms

                                                                                Parameter aggregation (e.g. FedAvg)

                                                                                Generic model
                                                                                                                By performance
                                                 10. Model target               Personalized model
                                                                                                                By-class
                                                                                By-device

                                                                                Unpublished (as of now)

                                                 11. Analyzed dataset           Published but custom

                                                                                Public / Known
                                                                                                                Time (e.g. convergence)
                              Experimentation                                   Training
                                                                                                                Resources (e.g. hardware)

                                                                                                                Aggregation
                                                 12. Costs and metrics          Federation
                                                                                                                Communication

                                                                                                                Latency
                                                                                Execution
                                                                                                                Performance (e.g. accuracy)




Figure 2: Proposed taxonomy for FIDS—Figure from Lavaur et al. [8] © IEEE


  (ii) Adaptability and scalability — Distributed systems such as FL are often used to cope
       with resource limitations, especially in terms of computation and bandwidth. However,
       as pointed out by several selected works, FL faces limitations when dealing with numer-
       ous clients. Therefore, further research is needed on FIDSs client selection: dynamic
       fusion based on score or time, reputation, number of detected attacks, etc.

 (iii) Knowledge transfer — Current solutions focus on federating training and detection for
       devices and resources that belong in the same domain. Therefore, open issues include the




70                                                                         Proceedings of the 29th C&ESAR (2022)
                                                                                                                                                                         L. Lavaur, B. Coste, M.-O. Pahl et al


Table 1
Comparative table of selected works—Data from Lavaur et al. [8] © 2022 IEEE


                                      Sa




                               Fe


                                 Fe
                                 te Aut hys chn ing




                                  de
                              In



                                   llit on ic ol s




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                                    Cy ati rnet




                                     ra
                                     fo Int




                                       e- om al ogi




                                       ra F r L l
                                        te
                                        rm e
                                         be on of




                                         Pe U up ise
                                          te




                                          te ed ea FL
                                           d
                                            r P Te T




                                             rso ns erv d
                                             rre ou yst es




                                             d er r
                                              Tr V onta




                                               Se Su arn
                                                M at ni
                                                an er l F




                                                On




                                                Ne
                                                str s V em




                                                na up ise
                                                 m
                                                 Ho




                                                  im ed ng
                                                   sfe tic L




                                                   liz erv d

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                                                   Us -bas
                                                    ial eh s




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                                                      ic M
                                                       riz




                                                        e d is

                                                        or
                                                        ag
                                                        el
                                                         ne icle




                                                          Le T



                                                           p
                                                           S




                                                           k

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                                                             tw s




                                                              ar L




                                                               ba
                                                               h




                                                               od
                                                                a
                                                                 or




                                                                 ni




                                                                  se
                                                                  i
                    Ref                                                                                 Training location           Data type                Local Algorithm           Federation Algorithm            Dataset                     Strengths




                                                                  els

                                                                   e
                                                                    ng
                                                                    ks




                                                                     d
                                                                                                                            Abstracted network         BIRCH                                                                           relatively lightweight,
2018   Pahl et al. [23]                    # # # #         # # # #            # #                #      Device                                                                   Parameter addition               Generated
                                                                                                                            traffic (middleware)       K-means                                                                         online, no labels
                                                                                                        Edge-controller                                                                                                                offers mitigation,
2019   Rathore et al. [11]             #      # # #        # # # #        #       # #            #                          Network traffic (SDN)      ANN                       Vector concatenation             NSL-KDD
                                                                                                        (SDN)                                                                                                                          decentralized
                                                                                                                            IoT network                                                                                                online, offers per-class
2019   Nguyen, Marchal, et al. [6]         # # # #         # # # #            # #                #      Gateway                                        MLP                       Weight and biases average        MIMIC
                                                                                                                            traffic (TCPdump)                                                                                          models, no labels
                                                                                                                            Encrypted network
2019   Zhao et al. [24]                #      # # #       # # #       #   #       # #            #      Gateway                                        GRU                       FedAvg                           Generated            versatile (multi-task)
                                                                                                                            traffic (CICFlowMeter)
                                                                                                                                                                                                                  CICIDS2017
2019   Schneble et al. [25]            # #       # #       # # # #            # #        #   #          Gateway             Healthcare sensor values   FC (shared layers) → FC   Weight and biases average        ISCXVPN2016          high adaptability, no labels
                                                                                                                                                                                                                  ISCXTor2016

2019   Cetin et al. [26]               #      # # #        # # # #        #       # # #          #      Gateway             Network traffic (WIFI)     SAE                       FedAvg                           AWID                 –

                                                                                                                            Air conditioner
2020   Zhang et al. [18]                   # # # #         # # # #        #       # # #      #          Gateway                                        CNN-GRU → MLP             Homomorphic parameter addition   CPS dataset          offers traceability (blockchain)
                                                                                                                            sensor values

2020   Li, Wu, et al. [15]             # #       # #       # # # #        #       # # #          #      Gateway             MODBUS traffic             DAGMM                     Parameter addition               KDD 99               confidentiality (encryption)

                                                                                                                            IoT network
2020   Rahman et al. [27]              #      # # #        # # # #        #       # # #          #      Device                                         ANN                       CDW_FedAvg                       Generated            –
                                                                                                                            traffic (TCPdump)
                                                                                                                                                                                                                  CICIDS2017
                                                                                                                            IoT network
2020   Chen, Zhang, et al. [28]        #      # # #        # # # #        # # #          #       #      Gateway                                        CNN                       Parameter aggregation            NSL-KDD              no labels
                                                                                                                            traffic (TCPdump)
                                                                                                                                                                                                                  Generated
                                                                                                                            Network traffic                                                                                            segmented (performance-
2020   Sun, Ochiai, et al. [29]        #      # # #        # # # #        #       # #            #      Gateway                                        ANN                       FedAvg                           NSL-KDD
                                                                                                                            (PCAP)                                                                                                     based models)
                                                                                                        Gateway             IoT network traffic                                                                   LAN-Security         knowledge transfer between
2020   Fan et al. [30]                     # # # #        # #     # #     #       # #            #                                                     CNN                       Parameter aggregation
                                                                                                        (MEC)               (TCPdump, CICFlowMeter)                                                               Monitoring Project   public and private datasets
                                                                                                                            Network traffic                                                                                            enhanced privacy
2020   Al-Marri et al. [31]            #      # # #       # # # #         #       # # #          #      Gateway                                        ANN                       FedAvg                           NSL-KDD
                                                                                                                            (TCPdump)                                                                                                  (mimic learning)
                                                                                                                            Network traffic
2020   Kim, Cai, et al. [32]           #      # # #        # # # #        #       # # #          #      Gateway                                        MLP                       FedAvg                           NSL-KDD              –
                                                                                                                            (TCPdump)
                                                                                                                                                                                                                                       very lightweight,
                                                                                                        Gateway                                                                                                   CICIDS2017
2020   Qin, Poularakis, et al. [33]    #      # # #        # # # #        #       # #            #                          Network traffic (SDN)      BNN                       SignSGD                                               line-speed classification,
                                                                                                        (SDN)                                                                                                     ISCX Botnet 2014
                                                                                                                                                                                                                                       P4 language compatible
                                                                                                                                                                                                                  CICIDS2017
                                                                                                                            Network traffic                                                                                            robust to poisoning,
2020   Chen, Lv, et al. [34]           #      # # #        # # # #        #       # # #          #      Gateway                                        GRU-SVM                   FedAGRU                          KDD 99
                                                                                                                            (CICFlowMeter)                                                                                             scalable
                                                                                                                                                                                                                  WSN-DS
                                                                                                                            Network traffic                                                                                            online, offers traceability
2020   Hei et al. [35]                 #      # # #        # # # #            #     # #          #      Device                                         MLP                       FedAvg                           DARPA 1999
                                                                                                                            (TCPdump)                                                                                                  (blockchain)
                                                                                                                            Network traffic (PCAP,                                                                                     relatively lightweight,
2020   Li, Zhou, et al. [36]           #      # #          # # # #        #       # # #          #      Gateway                                        CNN                       Homomorphic parameter addition   Generated
                                                                                                                            CICFlowMeter, Argus)                                                                                       confidentiality (encryption)
                                                                                                                            IoT network
2021   Popoola et al. [37]                 # # # #         # # # #        #       # # #          #      Gateway                                        MLP                       Parameter aggregation            KDD 99               zero-days detection
                                                                                                                            traffic (TCPdump, Argus)
                                                                                                                            Network traffic                                                                       Bot-IoT
2021   Qin and Kondo [38]              #      # # #        # # # #        #       # #            #      Device                                         ANN                       FedAvg                                                relatively lightweight
                                                                                                                            (TCPdump)                                                                             N-BaIoT
                                                                                                                            Network traffic
2021   Liu et al. [39]                 # # #          #    # # # #        #       # # #          #      Device                                         ELM + AE                  FedAvg                           NSL-KDD              decentralized
                                                                                                                            (TCPdump)
                                                                                                                            Network traffic                                                                       LAN-Security         segmented (performance-
2021   Sun, Esaki, et al. [40]         #      # # #        # # # #        #       # #            #      Gateway                                        CNN                       Parameter aggregation
                                                                                                                            (PCAP)                                                                                Monitoring Project   based models)


                                           Use case         FL type           Training       Approach




                   ability to federate clients across different domains. In addition, current methods often
                   consider that all local models share the same architecture and hyper-parameters. This
                   limitation makes current FIDSs less versatile and transferable.

       (iv) Security and trust — Using ML or FL to detect intrusions can introduce new threats
            to the system, such as poisoning. Several works have examined the vulnerabilities of
            FL systems and proposed countermeasures. With FL, poisoning becomes easier, as any
            participant can theoretically impact everyone else’s model. Structure of models depends
            on the architecture of the underlying algorithm, models trained cannot be aggregated
            easily [41]. Furthermore, as ML, and especially DL, lacks explainability, the content of a
            model is difficult to infer. Its aggregation with others is therefore made more risky. Fu-
            ture works are required in this direction to properly assess the content of a model before
            aggregation with others. Current solutions require an increase in the trust attributable
            to clients for model aggregation, inspired by the state of the art in collaboration systems
            and information sharing platforms.

       (v) Self-defense and self-healing — Current research on FIDSs focuses on intrusion detec-
           tion and attack classification. Defense is barely represented in the literature. However,




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Federated Learning as Enabler for Collaborative Security between not Full...


      technologies such as Software-defined networking (SDN) offer rapid resiliency capabili-
      ties; and recent work studies the effectiveness of such defense mechanisms. New emerg-
      ing applications such as self-defense and self-healing systems could benefit from FIDSs
      and other FL-based technologies.
 (vi) Model convergence — Models can differ from one client to another, especially in het-
      erogeneous contexts like intrusion detection. Consequently, model convergence is made
      more difficult. Current research focus on optimizing parameters by considering aggre-
      gation as an optimization problem. For instance, Charles et al. [42] use meta-learning to
      infer the right parameters, thus optimizing the aggregation afterward. Weighting mech-
      anisms are also present in the literature to improve the convergence [43].
 (vii) Dataset representativity — Existing public datasets are not representative of FIDSs
       potential deployment environments. Indeed, they are often datasets produced for tradi-
       tional machine learning algorithms but split for federated purposes, like NSL-KDD [44],
       UNSW-NB15 [45], or CIC-IDS2017 [46]. However, this approach introduces biases, as
       features or times series are all related to the same original event. Similar approaches us-
       ing adversarial examples for malware analysis which modify features instead of original
       binaries faces the inverse feature-mapping problem [47], [48].

   Some of these issues depend on works from other related fields, such as ML for performance
or FL for scalability. However, specificities of the FIDSs use case require more concrete research
questions. Especially, the topics of security, trust, and resilience, are critical for a collaborative
security use case.


4. Ensuring trust and personalization in FL-based intrusion
   detection
As introduced in Section 2, FL can be used in multiple cybersecurity settings and use cases,
from distributed statistic inference [49] to authentication [50]. Even in the more specific con-
text of intrusion detection (i.e. FIDS), numerous use cases are studied, e.g. IT networks [29],
Industrial Internet of Things (IIoT) [15], or smart healthcare [51]. This variety in use cases
comes with various data types, architectures, and constraints. Making knowledge transferable
among heterogeneous use cases and data is a long-term goal for FIDSs (see Section 3.2).
   However, heterogeneity is currently a major challenge for FL [52]. Therefore, we focus on a
specific use case, namely FIDS in IT networks. We study the impact of heterogeneity on FL in
this setting, and research solutions to mitigate it.

4.1. Use case definition
We consider a typical use case inspired from the industry, where actors invested in collabora-
tion are organizations aiming at improving their local detection. We assume that each organi-
zation has interests in sharing information, but has highly sensitive data that cannot be shared.
For example, Security Operation Centers (SOCs) perform security monitoring through the pro-
cessing of customer data (which can contain personal identifiable information) that cannot be




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                                                                L. Lavaur, B. Coste, M.-O. Pahl et al


shared. On the opposite, with the rise of cyber-criminal services [53], attackers tend to lead
similar attacks against different information systems. Two SOCs in such situation would share
their Indicators of Compromise (IoCs) or corresponding ML-models to detect all attacks after
the first that succeed. In this context, FL can be used to train a FIDS model on a distributed
dataset, while preserving the privacy of each organization. For instance, existing structures
such as Information Sharing and Analysis Centers (ISACs) or inter-SOCs could benefit from
such a system, which enables collaboration while protecting company secrets.
   This setting is called cross-silo [52], as opposed to cross-device. It is worth noting that FIDSs
do not exclude cross-device settings, for instance in endpoint detection [5]. However, cross-
silo is more relevant for our use case, as it is more relatable for the industry, and it is easier to
implement in a testing infrastructure. In cross-silo, fewer clients (10–1000) operate with more
data each, as well as more powerful computation capabilities. Participants are also deemed
more reliable in terms of availability, as the local learning process is performed on a dedicated
device. Hence, it is not dependent on whether the hosting device (e.g. an employee computer) is
turned on or off. Finally, organizations often operate with higher-stake privacy requirements,
as they process data from their customers, their employees, and themselves.
   We firstly also focus on horizontal federated learning (HFL), where participants have the
same features, but different samples. In HFL, participants have similar objectives (i.e. ML tasks)
and want to improve their models, but cannot build a centralized dataset due to privacy or legal
concerns. This is the most common setting in FIDSs, as it serves the goal of improving behavior
characterization, and having access to knowledge that cannot be inferred with only local data.
We also start with unsupervised learning algorithms, as the presence of labels not guaranteed
in real-world settings.
   Figure 3 depicts an example topology, inspired by IT networks from the industry. We assume
that each organization has a dedicated IT network, that might vary in terms of architecture,
probe location, or services (see Section 4.2).

4.2. Experiment presentation
The chosen use case inherently highlights two of the seven open issues identified in the litera-
ture: heterogeneity and trust. Furthermore, as mentioned in Section 3.2, the lack of a dataset
that is representative of this use case undermines existing works on the topic. Therefore, we
first focus on two complementary tasks: (1) the creation of a representative, distributed dataset;
and (2) the development of solutions to mitigate heterogeneity and provide trust between dis-
tributed participants.
   To cope with the lack of appropriated dataset in the literature, we propose to build a new
dataset generation platform with federation and distributed systems in mind. The platform re-
lies on virtualization topologies to generate normal traffic [54], and defined attack scenarios to
evaluate the detection performance. These topologies will be notably used to generate benign
traffic and train local unsupervised learning algorithms—literature typically use autoencoders
for this task [28], [38], [55]. The traffic generated at this step must be representative of a real
IT-focused network with users, internal resources (such as file sharing and web applications),
administration and supervision services, and access to the Internet.
   We then aim to evaluate existing approaches on this more realistic and demanding use case.




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Federated Learning as Enabler for Collaborative Security between not Full...


We expect most of them to yield less promising results, even when they claim to be able to
deal with heterogeneity. On the other hand, FL literature abounds of works on heterogeneous
data, and aggregation algorithms, such as FedProx [56] or Fed+ [57], might provide good re-
sults. In this case, proving empirically that such approaches are able to cope with the realistic
heterogeneity of our use case remains a major contribution.
   At the same time, we develop a new FL approach that deal with heterogeneity and trust. Sec-
tion 5 presents strategies envisioned to achieve this goal. When considered as a collaboration
system, FL is highly connected to research on trust and reputation systems. Hence, we plan
to leverage existing works on the topic, such as [58], to provide a reputation-aware FL system.
This approach will be evaluated on the new dataset, and compared to existing strategies.

4.2.1. Parameters
To measure the ways in which heterogeneity can manifest itself, we define varying parameters
that are used to generate the topologies. This can be used to generate a worst-case scenario,
where all the parameters are set differently in each topology.
  In regard to the use case presented in Section 4.1, we consider the following parameters:

     (i) architecture — the network architecture of the topology defines how services are inter-
         connected, how the traffic is captured, and where data collection is performed. For exam-
         ple, a topology with a single main gateway which captures traffic, and several services
         on the same network, will produce a different dataset when compared with a star-shaped
         topology with multiple subnets. Appropriated metrics are required to characterize the
         impact of these differences, e.g. size (number of hosts, of subnets), mean number of hops
         between a service and the last gateway, and so on.

  (ii) services — different services can rely on different protocols, and therefore generate dif-
       ferent kind of data, with different behaviors. For example, a service using TCP will
       induce connection establishment, and therefore a lot of traffic back-and-forth, whereas
       something based on UDP will produce a more continuous stream of data. Therefore,
       different services (and protocols) might have different normal behaviors, causing hetero-
       geneity among participants. The list of considered services must be adapted depending
       on the considered attack scenarios.

 (iii) maturity — security practices vary between organizations, depending on their threat
       model, previous expertise, and budget. For example, a company might have a dedicated
       security team, and therefore be able to implement a more mature security policy, whereas
       a small company might not have the resources to do so. This parameter is important to
       consider, as it can impact the quality of the dataset, e.g. by having unseen attacks in the
       training data, supposed to be benign traffic.

 (iv) probe location — while we assume that all participants extract the same features (due
      to HFL settings, see Section 4.1), the location of the probes can vary. In the same archi-
      tecture, one collection point at the gateway, or distributed probes in each subnet, will
      produce different datasets.




74                                                            Proceedings of the 29th C&ESAR (2022)
                                                                                                                                                                     L. Lavaur, B. Coste, M.-O. Pahl et al

                                                                                                                            LAN Users                                                           LAN Admin




                 WAN / Internet                                                                                   employee-pc1 employee-pc3
                                                                                                                   .101         .102
                                                                                                                                                                                 centreon-server   admin-pc           it-pc
                                                                                                                                                 insider-pc                                         .101              .102
                                                                                                                                                                                      .10
                                                                                                                                                    .105
                                                                                                                  employee-pc2 employee-pc4
                                                                                                                   .103         .104
       user-pc      remote-pc     outsider-pc
        .2.3        .5.101          .13.37                                                                                192.168.1.0/24                                                      192.168.100.0/24
                                                .1                                                                                                   .1                                  .1



                   10.0.0.0/8                                                                                                                                                                                    .2
                                                                                                                   192.168.201.0/24                                                  192.168.202.0/24
                                                                                                             .1                             .2                                  .1

                                                                           wan-fw                                                                                    lan-fw                                            monitor-fw
                                                                                                                                                      .1                                                         .1
                                                      .1

                                                                                                                                             LAN Local services                                                        LAN Monitor
                                                                          DMZ




                                                                                                                                                                                                           ids         syslog-server splunk-server
                                                                                                                                ad-server                     odoo-server
                                                     shopping-server         ftp-server
                                                                                                KEY   DATA




                                                                                                                                                                                                           .10                .11
                                                                                            0




                                                                                                                                   .10                            .12                                                                   .12
                                                                                            1

                                                                                            2

                                                                                            3

                                                                                            4




                                                           .10                    .12
                                                                                                                                            exchange-server             dns-server
                                                                tomcat-server             db-server                                                .11                      .13
                                                                     .11                    .13

                                                                                                                                                  192.168.10.0/24                                                192.168.99.0/24
                                                                       192.168.11.0/24




Figure 3: Example topology of a IT network


   Furthermore, some services are required to build working topologies, such as NTP, DHCP, or
DNS. We use a typical SME topology as a starting point for the experiments, which is presented
in Figure 3. This topology is composed of a dozen of machines, divided between employee
workstations, servers, etc. Variations of this topology will be also generated, and will be used
as other participants.

4.2.2. Evaluation
To evaluate approaches on any dateset, we measure their ability to detect relevant attack scenar-
ios. Typical attacks targeting such infrastructures are automated, like ransomware and botnets.
A lone attacker doing recognition and enumeration in the network is also to be considered, but
more advanced attacks such as APTs are out of the scope of this use case.
   To make a rigorous evaluation, we rely on the state of the art to select attacks, such as the
MITRE ATT&CK framework [59], or the most used attacks in dataset literature. The list of
attacks that are considered on a given topology is deeply correlated to the services available
in this topology. For instance, cache poisoning requires cache-based service in the topology,
such as a DNS relay-server. On the other hand, organizations establish threat models depend-
ing on the services they host or use. Considered attacks may include: Distributed Denial of
Service (DDoS) on the internet-facing web services, host and port scan,or web vulnerability
exploitation (injections, ...).
   Experiments are conducted on a private infrastructure. The test bed consist of three servers,
two for the virtualization, and one for computation that will be dedicated to executing the
required ML algorithms. While in real-world settings, the computation is performed on the
participants’ devices, we offload the computation to a dedicated server. This allows us to fo-
cus on the aggregation and trust aspects of the problem, and to avoid the complexity of the
distributed computation. To enable sound experiments [60], we plan to provide access to all
produced artifacts, including the dataset, the code, and the topology specifications, as well as
to the testbed itself.




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Federated Learning as Enabler for Collaborative Security between not Full...


5. Discussion
As the literature shows, FIDSs falter at aggregating heterogeneous models. Therefore, the
experiments detailed in section 4 provide means to measure and quantify the issue. While
literature on FL contains works on dealing with heterogeneous clients, this is still an issue in
the context of intrusion detection. Hence, we expect state-of-the-art approaches to perform
rather poorly, when compared to the results obtained on a homogeneous dataset. We consider
several approaches to cope with these issues.
   First, metric-based model weighting could be used to give less importance to models that
deviate too much from the others. Zhang et al. [18] use a centroid-distance weighting algorithm
that cope with the heterogeneity in IIoT. More generally, weighting algorithms can help to
merge only relevant models, depending on a set of defined metrics. Other metrics could be used
to further tune the model aggregation, like a numerical estimation of how much information
the model can bring.
   Another related topic is the measurement of the training data’s quality. In fact, as the learned
model is an abstraction of the data upon which it was trained, low-quality data would lead to a
low-quality model. Bringing a low-quality model in a federation could undermine every one’s
security. However, we must define what makes training data of quality, and how this quality
can be measured. Furthermore, weighting models according to their quality means the system
needs to be able to compare them, thus having access to the other’s data.
   Moreover, existing works on federated learning rely on participant clustering [58], [61] to
improve model specialization. Often, these approaches aim at either reducing heterogeneity
between clients, or detecting and excluding malicious participants. Part of the challenge here
reside in the metrics to chose to build clusters. Aforementioned metrics such as data quality, or
information estimation, could be also used in clustering. In the context of intrusion detection,
the formation of clusters can have an indirect impact on participants’ security.
   Finally, the lack of information inherent to the abstract nature of the model is a major hurdle
to estimate its value for aggregation. Therefore, we also consider adding metadata around
models, describing what the model contains without giving out too much information about
local data and configurations Such metadata would allow choosing which models one client
is interested in, depending on its use case. We believe an à-la-carte model aggregation would
help to cope with heterogeneity issues.


6. Conclusion
In this paper, we focused on the application of federated learning to the cybersecurity field.
Federated learning increases trust among partners as they do not need to share data, contrarily
to traditional ML-approaches. However, federated learning faces some limitations when feder-
ating models built on heterogeneous data. We therefore offer to address this issue within exper-
iments conducted on a platform with appropriate generated datasets. This platform displays
several use-cases with different partners in order to study aggregation-models parameters. We
then outline envisioned solutions which will be subjects of future work.
   These contributions can have a significant impact on increasing the security of organizations.




76                                                            Proceedings of the 29th C&ESAR (2022)
                                                                     L. Lavaur, B. Coste, M.-O. Pahl et al


In fact, while existing research has addressed the privacy aspects of collaboration through fed-
erated learning and other privacy-preserving mechanisms, maintaining trust in heterogeneous
collaboration is still a challenge. We showed that federated learning can help with the creation
of a common trusted security model for systems that are inherently distributed. Our approach
enables non-trusting parties to collaborate for the joint goal of increasing their cybersecurity
without revealing critical internals.


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