Facebook announced last week that movie information site Rotten Tomatoes would join Docs.com, Pandora, and Yelp as a partner in the social networking service’s “instant personalization” program. Rotten Tomatoes will now be able to automatically identify and access public information for visitors logged in to Facebook, unless those users have opted out of the program. This marks the first new partner since Facebook launched the feature earlier this year.
Soon after that initial roll-out, security researchers noted vulnerabilities on Yelp’s website that allowed an attacker to craft pages which would hijack Yelp’s credentials and gain the same level of access to user data. TechCrunch writer Jason Kincaid reported on the cross-site scripting (XSS) holes, and made this prediction: “I suspect we’ll see similar exploits on Facebook partner sites in the future.”
Kincaid’s suspicions have now been confirmed, as the latest site with instant personalization also had an exploitable XSS vulnerability, which has now been patched. I’ll quickly add that Flixster, the company behind Rotten Tomatoes, has always been very responsive when I’ve contacted them about security issues. They have assured me that they have done XSS testing and prevention, which is more than could be said for many web developers. In posting about this issue, I primarily want to illustrate a larger point about web security.
When I heard about the expansion of instant personalization, I took a look at Rotten Tomatoes to see if any XSS problems might arise. I found one report of an old hole, but it appeared to be patched. After browsing around for a bit, though, I discovered a way I could insert some text into certain pages. At first it appeared that the site properly escaped any characters which could lead to an exploit. But ironically enough, certain unfiltered characters affected a third-party script used by the site in such a way that one could then execute arbitrary scripts. Since I had not seen this hole documented anywhere, I reported it to Rotten Tomatoes, and they promptly worked to fix it.
I’ve long argued that as more sites integrate with Facebook in more ways, we’ll see this type of problem become more common. Vulnerable applications built on the Facebook Platform provided new avenues for accessing and hijacking user accounts; now external websites that connect to Facebook open more possible security issues. As Kincaid noted in May, “Given how common XSS vulnerabilities are, if Facebook expands the program we can likely expect similar exploits. It’s also worth pointing out that some large sites with many Facebook Connect users – like Farmville.com or CNN – could also be susceptible to similar security problems. In short, the system just isn’t very secure.”
Overcoming such weaknesses is not a trivial matter, though, especially given the current architecture of how scripts are handled in a web page. Currently, any included script has essentially the same level of access and control as any other script on the page, including malicious code injected via an XSS vulnerability. If a site uses instant personalization, injected scripts can access the data used by Facebook’s code to enable social features. That’s not Facebook’s fault, and it would be difficult to avoid in any single sign-on infrastructure.
Of course, all of this applies to scripts intentionally included in the page as well, such as ad networks. With the Rotten Tomatoes roll-out, Facebook made clear that “User data is never transferred to ad networks.” Also, “Partner sites follow clear product/security/privacy guidelines,” and I assume Facebook is monitoring their usage. I’m not disputing any of these claims – Facebook is quite correct that advertisers are not getting user data.
But that’s due to policy limitations, not technical restrictions. Rotten Tomatoes includes a number of scripts from external sources for displaying ads or providing various functions. Any of these scripts could theoretically access a Facebook user’s information, though it would almost certainly be removed in short order. I did find it interesting that an external link-sharing widget on the site builds an array of links on the page, including the link to a user’s Facebook profile. This happens client-side, though, and the data is never actually transferred to another server.
I bring up these aspects simply to note the technical challenges involved in this sort of federated system. I think it’s very possible that we will eventually see ad network code on a Facebook-integrated site that tries to load available user data. After all, I’ve observed that behavior in many Facebook applications over the last few years – even after Facebook issued explicit policies against such hijacking.
Postscript: This morning, an XSS vulnerability on Twitter led to script-based worms (somewhat reminiscent of “samy is my hero”) and general havoc across the site. This particular incident was not related to any mashups, but once again emphasizes the real-world security ramifications of cross-site scripting in a world of mainstream web applications.
Update (Sep. 27): Today news broke that Scribd had also become part of Facebook’s Instant Personalization program. I took a look at the site and discovered within minutes that it has a quite trivial XSS vulnerability. This particular issue should have been obvious given even a basic understanding of application security. It also indicates that Facebook is not doing much to evaluate the security of new instant personalization partners. Update 2: Scribd patched the most obvious XSS issue right about the time I updated this post: entering HTML into the search box brought up a page that loaded it unfiltered. Another search issue remained, however: starting with a closing script tag would still affect code later in the results page. After about half an hour, that problem was also patched. I’m glad Scribd moved so quickly to fix these problems, but I still find it disconcerting they were there to start with. I’ve not done any further checking for other XSS issues.
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