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'Critical' vulnerability in OpenSSH uncovered, affects almost all Linux systems

14 million internet-facing servers are vulnerable, says Qualys

By John Leonard  01 July 2024
https://www.computing.co.uk/news/4329906/critical-vulnerability-openssh-uncovered-affects-linux-systems


Researchers at the Qualys Threat Research Unit (TRU) have unearthed discovered 
a critical security flaw in OpenSSH's server (sshd) in glibc-based Linux 
systems.

They have named this vulnerability "regreSSHion", since it represents the 
re-emergence of a bug that was previously patched in 2006 (CVE-2006-5051). It 
is described as "critical".

The new vulnerability, assigned CVE-2024-6387, allows for unauthenticated 
remote code execution (RCE) with root privileges, posing a severe threat to 
affected systems.

An attacker could potentially gain complete control of the affected system, 
executing arbitrary code with root privileges. This could allow the 
installation of malware, the creation of backdoors and the exfiltration or 
manipulation of data.

In addition, gaining root privileges could allow the intruder to disable or 
bypass critical security systems in order to maintain a permanent presence.

The vulnerability is "a signal handler race condition in OpenSSH's server 
(sshd)", according to Bharat Jogi, senior director at Qualys TRU, in a post on 
the company's website. "This race condition affects sshd in its default 
configuration."

Fortunately, however, being a race condition means is not easy to exploit, 
requiring multiple attempts for a successful attack. "This can cause memory 
corruption and necessitate overcoming Address Space Layout Randomization 
(ASLR)."

Nevertheless, the researchers say, given the risk posed by regreSHHion, 
organisations should take immediate measures to locate and secure vulnerable 
systems applying patches, reconfiguring sshd and segmenting networks, where 
possible.

Affected systems

OpenSSH is a suite of software tools that enable secure remote login using the 
SSH encryption protocol. It is included in all glibc-based Linux systems, which 
means virtually every major distribution except for Alpine Linux, which uses 
libc. BSD systems are not affected. Qualys says it does not yet know the extent 
to which macOS or Windows operating systems may be impacted.

Using the Censys and Shodan search engines, TRU researchers identified over 14 
million potentially vulnerable OpenSSH server instances exposed to the 
internet. Among Qulays customers, there are around 700,000 such instances, 
representing 31% of the customer base.

OpenSSH versions earlier than 4.4p1 (released 2006) are vulnerable unless 
they've been patched for CVE-2006-5051 and CVE-2008-4109. Versions 8.5p1 
(released March 2021) up to, but not including, 9.8p1 (released 1st July, 2024) 
are also affected, owing to the accidental removal of a critical component. The 
vulnerability has been fixed in version 9.8p1.

Vendors are expected to release their own patches shortly. In the meantime 
there are mitigating measures that organisations can take.

"If sshd can't be updated or recompiled, set LoginGraceTime to 0 in the config 
file," the researchers recommend. "This exposes sshd to a denial of service by 
using up all MaxStartups connections, but it prevents the remote code execution 
risk."

Computing has contacted Qualys to ask whether any exploitations of regreSSHion 
have been observed in the wild.

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