libssh Vulnerable to Authentication Bypass (CVE-2018-10933)

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A newly announced vulnerability in libssh, a multiplatform library that supports the Secure Shell (SSH) protocol, allows attackers to bypass authentication and gain full control over vulnerable servers.


On October 16, the libssh team published an important security update for a vulnerability in libssh versions 0.6 and above. libssh is a multiplatform library written in C that supports the SSH protocol and can be used to implement client and server applications. The security update addresses CVE-2018-10933, an authentication bypass vulnerability. Tenable confirms our products are not vulnerable to CVE-2018-10933.

Impact assessment

Using Shodan, a search engine for internet-connected devices, we found nearly 2,000 devices are running libssh versions 0.6 and above. However, the full scope of libssh use is unclear. Additionally, it has been reported that vulnerable instances of libssh need to be running in server mode, not client mode, which may limit the impact of this vulnerability.

Vulnerability details

When authenticating to a server using libssh, it expects to receive a “SSH2_MSG_USERAUTH_REQUEST” message to initiate authentication. However, servers running vulnerable versions of libssh can be presented with a “SSH2_MSG_USERAUTH_SUCCESS” message, which would allow anyone to access the server without providing any credentials.

Below is a screenshot of Tenable Research confirming this vulnerability on a test system in our lab:

Tenable Research confirmed the libssh vulnerability on a test system in our lab

Urgently required actions

This vulnerability has been addressed in libssh versions 0.8.4 and 0.7.6, so it is important to update servers once server distributions release patches. Additionally, if software creators implement the libssh library in server mode, they should update to the latest version of the library.

Identifying affected systems

A list of Nessus plugins to identify this vulnerability can be found here.

The output of plugin 118154, the exploit plugin, can be seen below.

Output of Tenable plugin 118154, the exploit plugin

This image shows the output of plugin 118155, which checks the libssh version. Please note this will only report when the Accuracy is set to “Show potential false alarms.”

Output of Tenable plugin 118155, which checks the libssh version

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About Tenable
Tenable™, Inc. is the Cyber Exposure company. Over 24,000 organizations of all sizes around the globe rely on Tenable to manage and measure their modern attack surface to accurately understand and reduce cyber risk. As the creator of Nessus®, Tenable built its platform from the ground up to deeply understand assets, networks and vulnerabilities, extending this knowledge and expertise into™ to deliver the world’s first platform to provide live visibility into any asset on any computing platform. Tenable customers include over 50 percent of the Fortune 500, large government agencies and organizations across the private and public sectors. Learn more at
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