A proof-of-concept (PoC) exploit for the critical OpenSSH vulnerability CVE-2024-6387, also known as “regreSSHion,” has been released, raising alarms across the cybersecurity community.
The flaw, which affects millions of OpenSSH servers globally, allows unauthenticated, remote attackers to execute arbitrary code with root privileges under specific conditions.
The Vulnerability: A Regression Of A 2006 Issue
CVE-2024-6387 arises from a signal handler race condition in OpenSSH’s server (sshd). This issue occurs when an unauthenticated client fails to log in within the `LoginGraceTime` limit (120 seconds by default).
The server’s SIGALRM handler, triggered in this scenario, calls non-async-signal-safe functions such as `syslog()`, creating a race condition that can be exploited to achieve remote code execution (RCE).
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This vulnerability is particularly critical because it reintroduces a flaw first patched in 2006 (CVE-2006-5051), making it a regression issue. The vulnerability was uncovered by Qualys Threat Research Unit.
It impacts OpenSSH versions 8.5p1 through 9.8p1 on glibc-based Linux systems. OpenBSD systems remain unaffected due to their different signal-handling mechanisms.
Exploitation Challenges And Risks
While the vulnerability is severe, exploiting it is not straightforward. Security researchers have described it as a “statistical exploit,” requiring numerous attempts to win the race condition.
In controlled environments, successful exploitation takes between several hours and one week, depending on system configurations and mitigations like Address Space Layout Randomization (ASLR). Despite these challenges, the release of PoC code significantly lowers the barrier for attackers.
Reports indicate that exploit tools targeting CVE-2024-6387 are already circulating on underground forums, and at least one IP address has been observed attempting exploitation in the wild.
The vulnerability affects over 14 million internet-exposed OpenSSH servers globally, according to scans conducted using tools like Shodan and Censys.
Approximately 700,000 of these are confirmed vulnerable based on Qualys’ data. Exploitation could lead to full system compromise, allowing attackers to install malware, create backdoors, manipulate data, and propagate within networks.
Notably, exploitation has only been demonstrated on 32-bit Linux systems with glibc. While exploitation on 64-bit systems or non-glibc environments is theoretically possible, it has not been confirmed.
Mitigation Measures
To address this critical vulnerability:
- Upgrade to OpenSSH 9.8 or Later: The latest version includes patches that resolve the race condition.
- Temporary Workaround: Set `LoginGraceTime` to `0` in the sshd configuration file. While this prevents exploitation of the vulnerability, it may expose systems to denial-of-service risks.
- Restrict Access: Use network-based controls to limit SSH access.
- Monitor for Indicators of Compromise (IoCs): Organizations should deploy intrusion detection systems and monitor logs for unusual activity.
The release of the PoC code has sparked widespread concern among cybersecurity experts. While some researchers have struggled to achieve successful exploitation outside laboratory settings, others warn that attackers could refine these methods over time.
Organizations are urged to act swiftly to patch affected systems and implement additional security measures where immediate upgrades are not feasible.
The release of a PoC exploit for CVE-2024-6387 underscores the urgency of addressing this critical vulnerability in OpenSSH servers.
While mass exploitation remains unlikely due to technical barriers, the potential impact of successful attacks is severe. Organizations must prioritize patching and adopt layered security measures to mitigate risks associated with this flaw.
