On Profile Linkability despite Anonymity in Social Media Systems.

A number of works have recently shown that the privacy offered by pseudonymous identities on social media systems like Twitter or Reddit is threatened by cross-site identity linking attacks. Such attacks link the identities of the same user across websites. Therefore, assessing linkability, i.e., the risk that identities are linked across different websites, remains an important open problem. In this work, we analyze whether anonymity within a single social media site can protect a user from being linked across sites. To this end, we first introduce a relative linkability measure ranking identities within a social media site by their anonymity. We show that anonymity alone is not sufficient to assess linkability risks, by evaluating this measure on a data set comprising 15 million comments gathered from the Reddit social media system. Second, we mitigate this insufficiency and present our absolute linkability measure, which in addition utilizes information about matching identities. Then, we confirm the validity of this measure on our data set. The measure is able to accurately assess the linkability risk in almost 75% of the cases and, more importantly, is shown to never underestimate the linkability risk.