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A new type of vulnerability is described in which publicly available information from social network sites obtained out of context, can be used to identify a user in cases where anonymity is taken for granted.
This attack (dubbed Cross Site Identification, or CSID) assumes the following scenario: A user that is currently logged on to her social network account visits a 3rd party site, supposedly anonymously, in another browser tab. The 3rd party site causes her browser to contact the social network site and exploit the vulnerability resulting in her identity being disclosed to the attacker. The 3rd party target site is not necessarily controlled by the attacker. It could also be, for example, any site allowing user provided content that includes an image link (basically any forum or blog site). Other possibilities exist.
While the information that is received by the attacker is technically publicly available, obtaining it in this manner effectively lifts the veil of anonymity from the user when interacting with the 3rd party site.
Three social networks were tested and all were found to contain the vulnerability. These are Facebook, Orkut and Bebo. Some of the vulnerabilities were design flaws. The vulnerabilities are described and demonstrated. The sites were contacted in advance yet some of the vulnerabilities are still open.
CSID is not bound only to social network sites but might be found on any site that authenticates its users. Various flavors of the attack are discussed.
The post below contains a detailed description of the attack and its implications. It also includes details about the live vulnerabilities found.