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5.2. What Is Expected to Happen?evade detection. Hence, new filtering techniques are required to counterthis new type of spam. Third-party applications can also exploit vulnerabilitiesin users’ browsers, conduct cross-site scripting attacks, compromise theirmachines, form a botnet to launch malicious activities such as DoS attacks,propagate malware, or send spam email. One example is the Koobface botnet[93], which abuses social network sites. It starts by sending a spam messagecontaining a malicious link to a video which, once clicked, redirects the victimsto a website where they are asked to install an executable file. This file theninfects their machine with the Koobface malware.5.2.3 Sybil AttackIn addition to the propagation of spam and malware, multiple fake identities insocial networks can be used to out-vote honest users, influence online ratings,and manipulate search results [225]. Attackers can either compromise existingaccounts or generate fake (Sybil) accounts. The compromised accounts havealready established friendships with other users of the social network and aretrusted by them. Sybil accounts, however, need to establish friendships andgain trust before launching attacks, such as sending spam. Selling fraudulentor compromised social network accounts is even starting to overtake stealingcredit card numbers [275, 334].5.2.4 AuthenticationIn order to mitigate attacks from compromised accounts, mechanisms requiringmore than a password have been introduced, such as Social Authentication(SA) in Facebook [173]. These types of mechanism require a user to providetwo distinct pieces of evidence in order to be authenticated. For example, in SA,users must provide a password and recognize pictures randomly chosen fromtheir friends’ pictures. Unfortunately, this type of authentication is vulnerableto advances in face recognition techniques [104, 318]. Different approaches toauthentication and authorization, as well as general problems that exist arecovered in Chapter 7.5.2.5 Third PartiesThird-party applications, which are widely deployed in online social networks,can also perform malicious activities, for example exploit vulnerabilities inusers’ browsers, conduct cross-site scripting attacks, compromise their machines,form a botnet to launch attacks such as denial of service [87], propagatemalware, and send spam.Moreover, malicious third-party applications that access private user datatend to store the information, or send it to advertising and Internet trackingcompanies, thus violating user privacy. Unfortunately, any data harvested37