=Paper=
{{Paper
|id=Vol-447/paper-9
|storemode=property
|title=Privacy Concerns of FOAF-Based Linked Data
|pdfUrl=https://ceur-ws.org/Vol-447/paper7.pdf
|volume=Vol-447
|dblpUrl=https://dblp.org/rec/conf/esws/NasirifardHD09
}}
==Privacy Concerns of FOAF-Based Linked Data==
https://ceur-ws.org/Vol-447/paper7.pdf
Privacy Concerns of FOAF-Based Linked Data
Peyman Nasirifard, Michael Hausenblas and Stefan Decker
Digital Enterprise Research Institute
National University of Ireland, Galway
IDA Business Park, Lower Dangan, Galway, Ireland
firstname.lastname@deri.org
Abstract. In this position paper, we introduce a potential problem that arises
with the emergence of publicly-available, FOAF-based linked data. The
problem allows a spammer to send context-aware spam, which has a high click-
through rate. Unlike online profiles within social networks, FOAF-based
structured data provides a more reliable and accessible “food” for spammers
and attackers. Current solutions (e.g. Digital Signatures) and proposed methods
to restrict unauthorized accesses to FOAF files can prevent a subset of such
activities; however we show that they are not widely used. Moreover, some of
these solutions may be contrary to the mission of the Semantic Web and open
data initiative.
1 Introduction
“Congratulations! You have won the National Lottery!” is a common subject line in
unwanted emails (i.e. spam), which we receive in our inboxes or junk folders.
Although we delete them or mark them as spam, further spam emails sometimes
arrive. Key industry players (e.g. Microsoft) invest a huge amount of money in
fighting spam and spammers1, but it seems that the latter is the winner.
Various techniques have been initiated and developed for spam fighting. Labels
that are identifiable by humans (i.e. CAPTCHA2) are currently used by major email
providers to restrict the sending of automated spam. Services like tinymail3, which
aims to hide email addresses, or Email Icon Generator4, which creates an image out of
an email address, are samples of such efforts for fighting spam. Some spammers hire
people to circumvent these techniques5. The unwritten rule of the game between
spammer and “spammee6” is the less information we provide on the Web regarding
our contact information and personalities, the lower the probability that we will
receive spam.
1 http://www.cbsnews.com/stories/2004/01/24/tech/main595595.shtml
2
http://www.captcha.net/
3
http://tinymail.me/
4 http://services.nexodyne.com/email/
5 http://www.ibm.com/developerworks/web/library/wa-realweb10/
6
We define “spammee” as a person, who receives spam
Context-aware spam, unlike common spam, has a high click-through rate, as it
contains more relevant information. This issue can be easily (ab)used by a spammer
or an attacker for sharing phishing links and/or other malware. Brown et al. [1]
studied the vulnerabilities of major social networks (e.g. Facebook) against context-
aware spam. They estimated that around 85% of Facebook users could be accurately
targeted with context-aware spam.
FOAF7 profiles are used by many people, including Semantic Web researchers and
professionals, as a means to structure contact information and social networks. FOAF
profiles are considered to be “machine-interpretable”, as they are based on a formal
model. FOAF-based linked data helps towards enabling the mission of the Semantic
Web and/or Global Giant Graph8.
In this position paper, we describe a potential (privacy) problem of publicly-
available, FOAF-based linked data. We argue that a spammer can potentially benefit
from FOAF profiles by sending context-aware spam and we support this argument by
presenting an example of such an attack. We believe that FOAF profiles provide a
ready input for spammers, who may utilize them to personalize the spam message and
increase the click-through rate of the emails. We also discuss the possible partial
solutions that currently exist, but are not widely used.
2 FOAF: Structured Data for Spammers
FOAF profiles are structured, decentralized and extensible. They are probably the
most explicit and true representation of our social networks, as people we know are
clearly listed, and our contact information and interests are disclosed. FOAF profiles
are an honest representation of a person’s attributes, as the user takes into account the
fact that they will probably only be used by machines (psychological effect). Unlike
automatically-generated FOAF files9, manually-generated FOAF files are usually
hosted on personal homepages. Manually updating FOAF profiles is a time-
consuming and error-prone task. Therefore, some tools have been developed to help
users to create / update their profiles (e.g. FOAF-a-Matic10).
This structured and honest knowledge about a person’s life is what spammers
and/or attackers are looking for. In the following section, we demonstrate how a
context-aware spam message can be constructed and sent, using a FOAF profile plus
publicly available search engines (e.g. Google). Although we performed the process
manually, it can be automated using available APIs and packages and/or simple text
parsing.
As FOAF profiles are linked data, we need to choose some seeds which are well-
connected. In our example, we used the FOAF file of “Axel Polleres11”, as it is
7 http://www.foaf-project.org/
8
http://dig.csail.mit.edu/breadcrumbs/node/215
9
As an example, FOAF profiles on http://www.deri.ie/about/team/ were generated
automatically using a script
10 http://www.ldodds.com/foaf/foaf-a-matic
11
We had his permissions to use his FOAF profile
complete and error-free. Our goal is to send a context-aware spam message to Axel
using his FOAF profile.
Figure 1 demonstrates the overall view of the process. First step is to use Google to
find his FOAF file to use as a seed. Using Google API and/or parsing the HTML
results can automate this process. Moreover, using some available techniques [2],
such as utilizing the filetype option, can help us to reach the desired FOAF profile at
first or higher ranks (i.e. filetype:rdf). Our experiment with the query “Axel Polleres
FOAF” returned the requested FOAF file as the first retrieved link. We followed the
link and accessed Axel’s FOAF profile. As it is well-structured linked data, it can be
parsed using common RDF processing libraries, like Sesame or Jena. Parsing Axel’s
FOAF profile gave us valuable information about his friends and contact information.
The next step is to find the seed’s email address. FOAF profiles include SHA1
hash code of email addresses. As SHA1 collisions are far from current power of
computers12, they are good sources for “verification” purposes. Therefore, for finding
email addresses, we follow a similar approach to that for finding FOAF profiles. Our
experiment with Google using query “Axel Polleres Email” returned the requested
email as the first retrieved link. However, the result is a HTML page and needs
further processing, but having the SHA1 hash code of the email, is used as a help.
Many people, including our seed, try to mask their email addresses using various text-
based techniques such as replacing “@” with “[at]” etc. After gaining access to a
HTML page that contains the seed’s email address, we first search for the keyword
“email” or “e-mail” on that page. We then apply the following common patterns to
retrieve the email:
• Remove spaces
• Replace ’at’ or ‘[at]’ with ’@’, and ‘dot’ or ‘[dot]’ with a period
• Replace ‘firstname’ with the user’s actual first name, and ‘lastname’ with the
user’s actual last name (gleaned from FOAF profile)
• Remove ‘removeme’, ’[removeme]’
• other possible rules
As generating a SHA1 hash code is fast, we may continuously check if we have a
valid email address. If we did not succeed at the first retrieved link, we may refer to
the second or third results of search engine. Note that some people generate SHA1
hash code of their emails without the “mailto:” prefix.
Using these techniques, we can successfully identify Axel’s email address. In order
to send a context-aware spam message, we need to identify Axel’s friends and their
contact information as well. Finding Axel’s friends from his FOAF profile is
straightforward, as they are clearly listed in the profile. In order to find their emails,
we repeat the previous method.
One may claim that a possible problem that arises from this approach is resolving
name ambiguity. As emails are unique and we have access to the SHA1 of the email,
we always ensure the hash code of a person’s email matches the SHA1. This will
automatically resolve name ambiguity issues. Obviously, this may require parsing
more results from the search results.
Returning to the scenario, we now have the email address of the seed plus the
email addresses of his friends. In order to send a context-aware spam, we may use
12
http://www.schneier.com/blog/archives/2005/02/cryptanalysis_o.html
some pre-defined templates. Based on the granularity of information that people
provide in their FOAF profiles, we select the appropriate template.
Fig. 1. Overall view of sending spam using FOAF
For example, Axel put his birth date in his FOAF profile. A context-aware spam
may be sent on his birth date from one (or more) of his friends, which encourages him
to visit an online greetings card, that contains some harmful/harmless links. Another
template may use Axel’s friends for advertisement purposes. Figure 1 demonstrates
two sample spam messages that have been created using some templates: one for a
birthday and the other for sending a product recommendation. Using techniques and
tools like URL shortening13 can potentially be used to further hide the content and
target of the links.
For our experiment, we did not use a template. We created a simple email with an
embedded link and sent it via a customizable SMTP server to our victim (seed). He
clicked that link, as it seemed to be from one of his friends.
This approach is recursive. The seed’s friends may be used as new seeds and the
process continued. Taking into account the small world phenomenon (i.e. Human
Web), we can expect to reach all people in the FOAF-o-sphere. If not, common
(Semantic) search engines can be used for identifying new seeds. We can also
consider mutual links between people in FOAF profiles may represent a stronger /
more reliable relationship, which can in turn be used for increasing spam’s click-
through rate.
3 Discussions and Potential Solutions
Privacy of FOAF profiles has been discussed by some researchers. Reagle [3]
proposed to encrypt parts of the FOAF file to restrict unauthorized accesses. Frivolt
and Bieliková [4] proposed to partition FOAF files, where each partition has a
specific visibility. These solutions are not widely used and FOAF files are accessible
to all and are indexed by major (Semantic) search engines. Moreover, it seems that
hiding FOAF profiles can be contradictory to the open data initiative.
In comparison to common social networking sites, using FOAF for sending
context-aware spam can be considered as a more reliable and accessible approach for
spammers, due to several reasons:
• Finding users’ email from online social networks could be very difficult, as most
social networking sites hide the email addresses of the users. SHA1 hash code of
the emails within FOAF profiles can be used as a means for verifying the valid
email address.
• Crawling heterogeneous and highly customizable social networks (e.g. MySpace)
offers a huge overhead for spammers, whereas FOAF is unique structured data.
• Someone may generate fake user profiles with incomplete names within online
social networks, whereas FOAF is considered to be “reliable”, as they are hosted
on personal homepages and/or automatically generated from reliable data.
Although, we are not aware of any study on the degree of “honesty” of FOAF
profiles.
Generally, generated context-aware spam using FOAF can be categorized into two
main groups: the first is where the sender of the email is supposedly a friend of the
seed, then second is where the sender of the email is unknown to the seed, however
the email may refer to friend(s) of the seed. Figure 1 illustrates examples of both
categories.
13
http://tinyurl.com/
Digital Signatures can potentially obstruct spam of the first type, but they are not
widely used. The result of a survey that we did within our institute showed that just
one person out of one hundred and twenty researchers and staff is using digital
signatures permanently. Three researchers claimed that they use it, whenever they
want to look serious or they want to contact somebody officially. One participant
claimed that he used to have digital signature, but as his friends do not use it, he found
it useless and he stopped using it. One participant claimed that he always wanted to
have one, but due to time limitations, he did not investigate how he can install it on
his email client. One participant claimed that managing private key / public key is
very time-consuming. He said, however, there exist some tools that can help towards
key management, but they require user verification (i.e. human-in-the-loop) at the end
which brings lots of overhead for users. One participant with sufficient technical
background tried to install a certificate on his email client, but after half an hour
struggling, he stopped and complained about its complexity and argued that the
process can be very time-consuming for a common user with no or little technical
background. The result of the survey showed that 114 out of 120 participants never
used digital signature within their email clients. The fact that lots of people are not
really using digital signatures undermines the usefulness of digital signatures for
those, who are using them permanently.
The other possible solution for the first group of spam is looking at detailed
headers of the emails. This will show the “traversed” path of an email through its
journey to reach the target person. This is probably a technical point which many
users are not aware of and even for technical users, it is a time-consuming issue.
Moreover, there exist some RFCs (e.g. RFC 4408 – Sender Policy Framework (SPF))
that help towards this direction, but they are not widely used.
For the second group of spam, spammers may always use major and free email
providers for sending spam. This is still an open problem. Thanks to publicly
available FOAF profiles, spammers can increase click-through rate of the spam by
making them context-aware.
Generally, some partial solutions can be also considered to increase privacy of
FOAF profiles:
• Remove SHA1 hash code from FOAF
• Use various hashing functions within FOAF, and not only SHA1
• Mask person’s name and/or friends’ name within FOAF
4 Conclusion
We all receive spam. Although FOAF-based linked data can bring lots of advantages
for the community, it is necessary to be aware of malicious usage of such data. In this
paper, we presented a potential privacy leak of publicly available FOAF profiles,
demonstrated a context-aware spam that was sent just by using FOAF files, search
engines and a customizable SMTP server and our victim clicked on the embedded
link. We also argued that current partial solutions (e.g. digital signatures) are not
widely used and if we put our FOAF profiles online, we may expect context-aware
spam.
Acknowledgement. We thank anonymous reviewers for their feedbacks. We also
thank all people, who participated in our survey. The work presented in this paper has
been funded in part by SFI under Grant No. SFI/08/CE/I1380 (Lion-2) and the EU
under Grant No. FP6-IST-5-35208 (Ecospace).
References
1. Brown, G., Howe, T., Ihbe, M., Prakash, A., Borders, K.: Social networks and context-
aware spam, in Proceedings of the ACM conference on Computer supported cooperative
work. ACM, San Diego, CA, USA (2008)
2. Calishain, T., Dornfest, R.: Google Hacks: 100 Industrial-Strength Tips and Tools. O'Reilly
and Associates, Inc. 352 (2003)
3. Reagle, J.: FOAF Spheres of Privacy. Available online at
http://reagle.org/joseph/2003/09/foaf-spheres.html
4. Frivolt, G., Bieliková, M.: Ensuring privacy in FOAF profiles. in Znalosti (2007)