A critical vulnerability that could allow remote attackers to access sensitive enterprise log-in credentials and other data was fixed last week in Crowd, a single sign-on (SSO) and identity management tool used by large organizations to simplify access to their internal Web applications and services. According to Atlassian, the Sydney-based software company that develops Crowd, the product is used by around 1,000 organizations in 55 countries, including large banks, car manufacturers, government agencies, telecommunication companies, software firms, online services providers, universities and others.Crowd can be used to link identities between Active Directory, LDAP and other directory services; Atlassian's popular bug tracking, collaboration, project management and code repository tools; third party services like Google Apps, Apache or Subversion, and custom in-house developed Web applications.The newly patched vulnerability stems from the way in which Crowd parses external XML entities defined in Document Type Definition (DTD) headers and is a variation of a vulnerability known as CVE-2012-2926 that was reported and patched back in 2012, researchers from security consultancy firm Command Five said Friday in a security advisory.An attacker can exploit the vulnerability by sending requests with specially crafted entity URLs in order to trick the server into returning any file from the internal network that it has access to, including its own configuration files that contain unencrypted credentials, or to initiate a denial-of-service attack that would make the server inaccessible to users.The 2012 vulnerability, for which an exploit module already exists in the Metasploit penetration testing tool, was fixed in Crowd 2.4.1. However, that patch only blocks external entities defined in requests sent to Crowd URLs that end in "\/services," the Command Five researchers said.Versions of Crowd up to and including 2.6.2 continue to process entities defined in DTD headers for requests that are sent to URLs ending in "\/services\/2" or "\/services\/latest," which re-enables the exploit, they said. "With a two character change to the targeted URL the Metasploit module is again 'fully armed and operational'."The new issue has been assigned the CVE-2013-3925 identifier and was fixed in the latest stable version of the product, Crowd 2.6.3, that was released on June 24. According to the corresponding entry in Atlassian's bug tracker, the vulnerability has also been fixed in versions 2.5.4 and 2.7."Successful exploitation of this vulnerability can (but does not necessarily) lead to a hacker taking full control of an organization's single sign-on service, potentially resulting in a catastrophic security event," the Command Five researchers said in their advisory. At the very least, successful exploitation is likely to enable attackers to expand their unauthorized access within the targeted organization, they said.Organizations that expose their Crowd installations to the Internet in order to enable remote authentication for employees or affiliates are at increased risk of suffering a security breach, the researchers said.Aside from this patched vulnerability, Command Five is also aware of at least another critical vulnerability in Atlassian Crowd that hasn't been fixed yet. That vulnerability could be classified as a backdoor and allows unauthenticated attackers to take full control of any Crowd server they can access over the network, the researchers said.Successful exploitation of the yet-to-be-patched vulnerability "invariably results" in the compromise of all active Crowd application credentials, user credentials, accessible data storage, configured directories and dependent secure systems, they said.Atlassian patched the first vulnerability -- identified as CVE-2013-3925 -- in a maintenance release of Crowd in June, but was unable to substantiate the existence of the second alleged vulnerability, designated CVE-2013-3926, Atlassian spokeswoman Catherine Norman said Monday via email. "The author of the report has not contacted Atlassian, making it difficult to validate the claim.""While we've been unable to confirm the existence of the second vulnerability, we take it seriously and have reached out to the author directly for more details," Norman said. "If we can confirm there is a vulnerability, a patch will be issued and all Crowd customers will be emailed details for how to update."