According to OpenID, "If the site is only using AX to receive low-security information like a [user's] self-asserted gender, then this will probably not be a problem. However if it is being used to receive information that it only trusts the identity provider to assert, then it creates the potential for an attack."
No attacks have been spotted that exploit this flaw, according to the OpenID Foundation. But the threat is that an attacker could use it to "modify information passed between parties and impersonate a user," said John Fontana, an evangelist with Ping Identity, in a blog post.
The OpenID suggested fix for vulnerable applications is "modifying application code to accept only signed attribute values, as an initial step."
Already, "the researchers contacted the main websites impacted, and those sites have deployed a fix," said OpenID, although it declined to name the affected sites. It also said that "OpenID Foundation board members have worked to identify other websites that were impacted and similarly have them deploy a fix."
The most notable OpenID-based vulnerable application is OpenID4Java. To mitigate the vulnerability, OpenID recommends updating that application or any dependent libraries--such as Step2--to the latest version of OpenID4Java (0.9.6 final). Meanwhile, the Kay Framework, which was vulnerable, has been patched as of version 1.0.2. "Other libraries may have the same issue though the default usage of services/libraries from Janrain, Ping Identity, and DotNetOpenAuth are not susceptible to this attack," according to the security bulletin.
Essentially a form of single sign-on for the cloud, OpenID has been adopted by a number of websites and businesses, including Google, Facebook, Flickr, Microsoft, WordPress, Yahoo, and Zappos. As of December 2009, the OpenID Foundation reported that there were over 9 million websites that used OpenID to allow users to register and log in to at least some portion of the site. A number of government agencies have also backed OpenID.
Last year, Google proposed securing account data on websites that relay user credentials--for example, using OpenID--through a process known as PseudoID. The system, which is backwards-compatible with OpenID, would prevent attackers from linking a set of stolen sign-on credentials back to a user's account on referring websites.