Critical Bug Affects OpenSSH Users

Users of OpenSSH should take immediate action to secure and patch their systems based on a newly reported bug that affects OpenSSH versions 5.4 through 7.1.  Note that this includes both the OpenBSD-specific and portable versions of OpenSSH. 

This newly revealed bug affects users due to a failed implementation of a feature called roaming, which  was intended to facilitate resuming SSH-connections.  While server-side code was never released to support this feature, on the client-side this code remained vulnerable to exploitation by a malicious host.  The flaw enables a malicious server host to access memory on the client system—which would include the ability for the host to access the private client user keys.

The patch announcement by Damien Miller notes that:

The authentication of the server host key prevents exploitation by a man-in-the-middle, so this information leak is restricted to connections to malicious or compromised servers.

It is important to note that unpatched clients are vulnerable because the malformed feature is enabled by default.  What is worse is that the “UseRoaming” option is not usually present in the configuration file, so it may not be readily apparent that a system is vulnerably by simply scanning SSH configuration files.

Patches are available, and the latest portable release of OpenSSH 7.1p2 has been released.  Users who are unable to apply the patch to their system can use one of the following changes to their SSH configuration file to prevent the flawed code from executing:

• Add “UseRoaming no” to their global SSH config file (typically /etc/ssh/ssh_config)
• Add “UseRoaming no” to their user SSH config file (typically ~/.ssh/config)
• Include –oUseRoaming=no in their command line invocation of SSH

This bug was found thanks to a report from the Qualys Security Advisory team.  Note that the 7.1p2 release includes fixes for a few other bugs, so applying it as soon as possible is recommended.  For additional details, refer to CVE-2016-0777 and CVE-2016-0778.  Qualys has shared their security advisory on Seclists.