Software Engineering Institute

The Security Assertion Markup Language (SAML) is a standard for transmitting authentication data between two or more security domains. In SAML language, XML security packets are called assertions. Identity providers pass assertions to service providers who allow the requests. In the Google Single Sign on (SSO) implementation, the authentication response did not include the identifier of the authentication request or the identity of the recipient. This may allow a malicious service provider to impersonate a user at other service providers.

More technical information about this issue is available in the Formal Analysis of SAML 2.0 Web Browser Single Sign-On: Breaking the SAML-based Single Sign-On for Google Apps whitepaper which is available here: http://www.ai-lab.it/armando/GoogleSSOVulnerability.html

Note that to exploit this vulnerability, the attacker would have to convince the user to login to their site.

A malicious service provider might have been able to access a user's Google Account or other services offered by different identity providers.

Google has addressed this issue by changing the behavior of their SSO implemenation. Administrators and developers were required to update their identity provider to provide a valid recipient field in their assertions.

Do not log into untrusted sites

To mitigate future vulnerabilities, users should use caution when providing their credentials to log into Google services via third party service providers.