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

Satnam Narang

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.

Updated on Oct. 19, 2018: Several distributions have released patches to address this vulnerability, including Debian, Ubuntu and SUSE. Additionally, Cisco and F5 Networks have released security advisories for their products. Cisco is currently investigating its product line to determine whether any are affected by the libssh vulnerability. F5 Networks released an advisory for BIG-IP application delivery controllers and lists the Advanced Firewall Manager (AFM) as vulnerable but the company has not yet provided a patch.

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

Get more information

Learn more about, the first Cyber Exposure platform for holistic management of your modern attack surface. Get a free 60-day trial of Vulnerability Management.

Read more >

Published on Oct 17, 2018

People also viewed

Technical Support Engineer

Columbia Maryland United States Columbia Gateway Drive, Columbia, Maryland, United States, 21046 Technical Support Technical Support
Your Role:Tenable is looking for motivated individuals to join our Technical Support Team. The Technical Support Engineer (TSE) is the first contact for all customers when they require technical assistance.  In a time when the next Spectre and Mel...

Customer Success Manager - APAC Nessus

Singapore Singapore Singapore North Bridge Road, Parkview Square, Singapore, 188788 Customer Success Sales
Your Role:Tenable has an immediate need for a Customer Success Manager who will be responsible for establishing and driving sales activities for our software products within a designated geography. You will be expected to penetrate and prospect na...

Sales Development Representative

Singapore Singapore Singapore North Bridge Road, Parkview Square, Singapore, 188788 Lead Generation Sales
Your Role:The Sales Development Representative supports remote sales teams in and is responsible for generating new business via inbound and outbound phone opportunity qualification. Your Opportunity: Perform outbound calling to generate new sales...

Security Engineer - Mexico

Mexico City Mexico Mexico Presidenta Masarik, Polanco V Seccion, Mexico City, Mexico, 21045 Sales Engineering Sales
Your Role:Tenable is seeking a Security Engineer to support demos and evaluations of enterprise opportunities across the territory. Assist in developing business value propositions around Tenable's products and solutions. Drives sales with technic...

Director of Engineering - Shared Services Infrastructure

Columbia Maryland United States Columbia Gateway Drive, Columbia, Maryland, United States, 21046 Sensors Engineering
Your Role:Tenable is looking for a talented and energetic Director of Software Engineering to lead the Shared Services Infrastructure (SSI) development team. The SSI team helps organizations address key challenges of traditional vulnerability scan...

Principal UX Designer

San Jose California United States E Santa Clara St., San Jose, California, United States, 95113 Cloud Platforms Engineering
Your Role:Tenable is looking for an extraordinary Principal UX Designer to join our team. Our group is chartered with creating the next generation of security products while at the same time pioneering unprecedented user experience in the digital ...

We have big plans for continued global growth, and we’re looking for people who are creative, flexible and dedicated to helping us build something great – something that matters.