First Twitter and now Facebook: A researcher today began a round of daily disclosures of serious vulnerabilities in popular Facebook applications.

The researcher, who goes by "theharmonyguy," plans to disclose multiple cross-site scripting (XSS) flaws he discovered in various third-party Facebook applications this month, though he may not do so every day. He says he found major security holes in several of Facebook's top 10 most popular applications.

Today's bugs include XSSes in FunSpace, which has more than 8 million users; SuperPoke, which has 2 million users; SocialToo, which has nearly 2,000 users; and YellowPages.ca, which has nearly 1,200 users. FunSpace, SuperPoke, and SocialToo have been patched, but YellowPages.ca has not.

Theharmonyguy says Facebook's own applications tend to be secure, as does its site. He says he has studied several Facebook-written apps for the site, and it was tough to find any holes. "They seem to know what they're doing with security," he says. "I found a cross-site scripting hole on the Facebook page and contacted them, and they patched if really quickly," he says.

Even so, the researcher says Facebook doesn't vet third-party apps. "In fact, four of the five apps I'm going to post in the next week or so are Facebook-verified ones," he says.

Theharmonyguy plans to release source code for the attacks once he has completed the disclosures. He says he gives each application developer 24 hours notice before posting the flaws.

The problem, he says, lays in Facebook's API -- problem that has been well-documented by other researchers, as well. The API gives the application developer full access to a Facebook member's profile when a user runs that application.

"A lot of the security problems I and others are finding [on social networks] seem to come in through third-party apps," theharmonyguy says. "Once you start allowing this third-party content, you start opening yourself up to problems."

Theharmonyguy says XSS is simple to exploit in a Facebook application by inserting iFrames or scripts. And the XSS hole can then open the door to cross-site request forgery (CSRF) attacks, as well, he says.

There's no surefire way for Facebook users to protect themselves from these application-borne attacks, he says. "My No. 1 tip to users would be not to put anything on their profile that they don't want to be public. Treat your Facebook profile as if it's already public," he says. "And be careful what applications you install," even though Facebook-verified ones can also be abused, he says.

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