Proof of concept code examined by El Reg shows how the platform can be used to steal Facebook user's session identification cookies, deliver pop-up messages or change the layout of Facebook pages. With a little extra work, an attacker could probably do much more, including send and read messages from a user's account, change privacy settings and add or delete Facebook friends.
"This is quite a big security hole," said Artur Wachelka, a Munich-based developer of online games who stumbled upon the bug while writing a chess game for Facebook. He said he decided to take the vulnerability public after reporting it to Facebook privately and receiving a single sentence reply that the security issue didn't exist.
A Facebook spokeswoman said members of the company's security team were investigating the report. As of Monday afternoon, the bug had yet to be squashed.
All your Facebook session cookies are belong to us
The failure to sanitize the content of third-party applications is one of several privacy and security gaffes that have threatened Facebook users over the past few years. In May they were poked by a cross-site scripting (XSS) flaw, and a separate security hole exposed the private pictures of Paris Hilton and who knows how many other users. Recently, security researchers have reported a worm that attacks users of Facebook and other social networking sites.
Says Parker: "It's certainly a flaw that needs to be fixed." ®