Two critical vulnerabilities in OpenSSH—CVE-2024-6387 (dubbed “regreSSHion”) and CVE-2024-6409—pose significant risks to Linux systems running vulnerable versions. These flaws represent the first major remote code execution vulnerabilities in OpenSSH in nearly two decades, with CVE-2024-6387 allowing unauthenticated attackers to potentially gain root privileges. Security teams should prioritize patching and mitigation given the active exploitation observed in the wild.
Technical Analysis of the Vulnerabilities
The CVE-2024-6387 vulnerability stems from a signal handler race condition in OpenSSH’s login timeout mechanism. When a client fails to authenticate within the default 120-second window, the SIGALRM handler executes unsafe functions that can lead to memory corruption. This flaw affects OpenSSH versions 8.5p1 through 9.7p1 on glibc-based Linux systems, with a CVSS score of 8.1 indicating high severity.
CVE-2024-6409 was discovered during the review of regreSSHion and affects a different component—the privsep child process—due to a similar race condition in signal handling. While less severe with a CVSS score of 7.0, it still enables remote code execution in the unprivileged child process of the SSH daemon on specific Red Hat Enterprise Linux 9 systems running OpenSSH 8.7p1 and 8.8p1.
Detection and Mitigation Strategies
Organizations should immediately scan their environments for vulnerable OpenSSH installations. The following detection methods can help identify at-risk systems:
# For general OpenSSH version checking
name:"OpenBSD OpenSSH" (version:>8.4 AND version:<9.8)
# For service detection
protocol:ssh (_service.product:="OpenBSD:OpenSSH:9%" OR _service.product:="OpenBSD:OpenSSH:8%") NOT (os:OpenBSD OR banner:"OpenSSH_8.0" OR banner:"OpenSSH_8.1" OR banner:"OpenSSH_8.2" OR banner:"OpenSSH_8.3" OR banner:"OpenSSH_8.4" OR banner:"OpenSSH_9.8" OR banner:"OpenSSH_for_Windows" OR banner:"Ubuntu-3ubuntu13.3" OR banner:"Ubuntu-3ubuntu0.10" OR banner:"Ubuntu-1ubuntu3.6" OR banner:"Debian-2+deb12u3" OR banner:"FreeBSD-20240701")Recommended mitigation steps include upgrading to OpenSSH 9.8p1 or later, applying vendor-specific patches, and implementing temporary workarounds such as setting LoginGraceTime to 0 where immediate patching isn't feasible. Network-level controls restricting SSH access should be considered for critical systems.
Operational Impact and Response
These vulnerabilities present significant challenges for security teams across different functions. Red teams should assess potential new attack vectors while blue teams must prioritize patching and monitoring of SSH services. Threat intelligence indicates active exploitation attempts, primarily targeting systems in China, underscoring the need for urgent action.
The Broadcom VMware Cloud Foundation Division has assessed the vulnerability as Important/High severity, noting that while their 64-bit products are potentially impacted, exploitation has only been demonstrated on 32-bit systems. Organizations should monitor for multiple failed authentication attempts from single sources and unusual process execution following SSH connections.
Conclusion and Recommendations
The OpenSSH vulnerabilities CVE-2024-6387 and CVE-2024-6409 represent critical risks given OpenSSH's widespread use in secure communications. While exploit complexity currently limits immediate widespread attacks, the potential for weaponization remains high. Security teams should implement a three-phase response: immediate patching of vulnerable systems, implementation of compensating controls where patching isn't possible, and enhanced monitoring for exploitation attempts.
These flaws serve as a reminder of the importance of maintaining rigorous patch management processes and secure service configurations, particularly for foundational security components like OpenSSH that often operate with elevated privileges.
