Microsoft, Samba Badlock flaw not critical, but serious enough

The vulnerability, covered by Microsoft in its MS16-047 security bulletin published Tuesday, was also fixed in Samba 4.4.2, 4.3.8 and 4.2.11. It could allow a man-in-the-middle attacker to impersonate an authenticated user and execute arbitrary network calls to the server, possibly with administrative privileges.

Badlock's existence was announced on March 22 by a company called SerNet, which  offers Samba consulting, support and development services. It employs the person who found the flaw: a Samba development team member named Stefan Metzmacher.

SerNet was criticized by some members of the security community at the time because it created a special name, logo and website for the vulnerability and revealed its existence three weeks before the patch, giving hackers ample time to find it on their own, even in the absence of technical details.

The company argued that the vulnerability was severe enough to warrant this approach, which is debatable now that the flaw's details are out and it appears to be less serious than most people expected.

Microsoft rates the impact of CVE-2016-0128 -- Badlock's tracking ID in the Common Vulnerabilities and Exposures database -- as important, not critical. The company noted in its assessment that exploitation is unlikely.

Based on the Common Vulnerability Scoring System (CVSS), the flaw has a severity score of 7.1, out of a maximum of 10.

Badlock's potential impact is "certainly a concern and admins should patch their systems as early as possible," said Karl Sigler, threat intelligence manager at Trustwave in a blog post. "However I can't say that this vulnerability rises to any level that deserves the focus that a dedicated website and three weeks of buildup have given Badlock."

The fact that exploiting the flaw requires an attacker to be in a position to intercept RPC traffic, specifically sessions that use SMB to authenticate a system or to manage users or policies on a remote system using the SAMR or LSAD protocols, limits its severity, Sigler said. "Any effective attack requires the attacker to be in the right place at the right time."

There have been more serious and more direct remote code execution flaws using RPC and SMB/CIFS over the years. Potential attacks against Badlock will likely happen inside local networks, because running SMB/CIFS severs directly on the Internet is generally considered bad security practice.

"While we do recommend you roll out the patches as soon as possible -- as we generally do for everything -- we don't think Badlock is the Bug To End All Bugs," said Tod Beardsley, security research manager at Rapid7, in a blog post. "In reality, an attacker has to already be in a position to do harm in order to use this, and if they are, there are probably other, worse (or better depending on your point of view) attacks they may leverage."

If you're using Samba, the most popular implementation of SMB/CIFS for Linux systems, it's worth knowing that all versions between 3.6.x and 4.4.0 are affected by this flaw, but patches are only available for the 4.2.x and higher branches. That's because versions older than 4.2 are no longer supported, so if you're running any of them, you should consider upgrading to a newer release.