A Web developer has documented a proof-of-concept attack that could have been used to hijack accounts and steal private information on Facebook and MySpace. The attack leveraged a serious security oversight in XML configuration files used to define the cross-domain access policy for Flash applications.

The discovery was made while 24-years-old Dutch Web developer Yvo Schaap tried to find a workaround to a problem he was having with one of his Facebook applications. “I found a solution to one of my function limitations. Surprisingly, when looked into more carefully my solution allowed full access and control to the Facebook user account that accessed my application,” he notes on his blog.

As the programmer goes on to explain, this is not actually a bug, but an insecure configuration issue in crossdomain.xml. This file can be used to tell a Flash application, loaded from a URL and executed locally on the client, if it is able to access a file served from its domain. For example, if the owner of domain A wants to let an application from domain B access its files, it can specify domain B in the crossdomain.xml on its server.

While the crossdomain.xml on facebook.com whitelisted only other domains and sub-domains associated with Facebooks, the similar file on connect.facebook.com contained an <allow-access-from domain=”*” /> directive. This meant that any Flash application, hosted on any domain, was allowed to access data on the connect.facebook.com sub-domain.

“This wouldn't be a big deal if the subdomain only hosts images, but unfortunately this domain hosts the whole Facebook property, including a facebook user session,” Mr. Schaap notes. The web developer goes on to point out that if a user has auto-login enabled and is served with the URL to a specially-crafted flash file, an attacker could obtain full control of their account.

MySpace also suffered from a similar crossdomain.xml misconfiguration, though not as obvious as in Facebook's case. In particular, the crossdomain.xml on myspace.com granted access to a farm.sproutbuilder.com sub-domain. Sprout Builder is a web application building platform, which amongst other types of media, allows its clients to upload and incorporate .swf (Flash) files into their projects. As it turns out, these files are hosted on farm.sproutbuilder.com, thus opening the opportunity to create MySpace self-propagating social networking worm.

Yvo Schaap contacted both companies in advance of making this public and they moved swiftly to address the security problems. However, if two of the biggest social networking platforms left such gaping security holes opened, it is reasonable to assume that many smaller websites also have them.