Vulnerability Note VU#456088

CWE-200: Information Exposure - CVE-2016-0777

According to the OpenSSH release notes for version 7.1p2 :

 The OpenSSH client code between 5.4 and 7.1 contains experimental support for resuming SSH-connections (roaming).

The matching server code has never been shipped, but the client code was enabled by default and could be tricked by a malicious server into leaking client memory to the server, including private client user keys.

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.

The buffer overflow, on the other hand, is present in the default configuration of the OpenSSH client but its exploitation requires two non-default options: a ProxyCommand, and either ForwardAgent (-A) or ForwardX11 (-X). This buffer overflow is therefore unlikely to have any real-world impact, but provides a particularly interesting case study.

A user that authenticates to a malicious or compromised server may reveal private data, including the user's private SSH key, or cause a buffer overflow that may lead to remote code execution in certain non-default configurations.

Apply an update

OpenSSH 7.1p2 has released to address these issues. Affected users are recommended to update as soon as possible.

If update is currently not an option, you may consider the following workaround:

Disable the 'UseRoaming' Feature

The vulnerable code in the client can be completely disabled by adding 'UseRoaming no' to the global ssh_config(5) file, or to user configuration in ~/.ssh/config, or by passing -oUseRoaming=no on the command line.