A default setting in both Windows 7 and 8.1 could allow local users to elevate privileges and in some situations, escape application sandboxes.
The issue, something that leaves all current Windows client installations vulnerable, lies in the way the operating system handles authentication. In some instances it could be possible for a user to use a reflection attack in NT LAN Manager, a collection of security protocols found in Windows systems, to leverage WebDAV (Web Distributed Authoring and Versioning) and carry out an attack.
“It’s possible to abuse cross-protocol NTLM reflection to attack the local SMB server by forcing a local system process to access a WebDAV UNC path,” warned James Forshaw, the Google Project Zero security researcher who found the issue, on Monday.
Forshaw discovered the issue last year and reported it to Microsoft’s Security Response Center on Dec. 18 but the time that Project Zero gives to vendors to fix bugs – 90 days – elapsed last week, so the Google Security Research post and its proof of concept were opened to the public.
According to Microsoft however the issue doesn’t merit a fix as the company has implemented mitigations for it, like Extended Protection for Authentication, in the past. According to Forshaw’s disclosure timeline, the company informed him in January that undoing the mitigations could cause “application compatibility concerns.”
When reached Wednesday a Microsoft spokesperson confirmed that users should implement EPA to avoid reflection attacks using the NTLM as a vector.
“Extended Protection for Authentication (EPA) is a security feature built-in to Windows 8 and 8.1, and available for older versions of Windows via knowledge base article 2345886, that helps protect our customers against this technique. We encourage customers to follow the guidance outlined in the article to enable EPA, which is off by default as it may cause some application compatibility concerns.”
As EPA doesn’t come enabled by default however, Forshaw is stressing that users looking to avoid reflection attacks follow a different set of precautions, including enabling SMB signing or enabling SMB Server SPN verification.
Forshaw points out that users can also disable their Webclient service, something that would make it trickier to elevate to the local system, but that this wouldn’t prevent attacks like sandbox escapes, which require user level permissions. It also might be possible to stage the exploit in another fashion, including via a DCE/RPC call.
As Forshaw acknowledges in his write-up, this is far from a new issue for Microsoft – the company actually addressed a similar issue way back in 2008 (MS08-068) that could have let attackers use NTLM to mirror authentication from one machine back to the same machines. The patch disallowed NTLM sessions in flight but failed to address cross-protocol attacks like the one Project Zero found.