FreeBSD – High-Severity OpenSSH Vulnerability [CVE-2024-7589]
CVE number = CVE-2024-7589
CVSS Score = 7.4
A signal handler in sshd(8) may call a logging function that is not async-signal-safe. The signal handler is invoked when a client does not authenticate within the LoginGraceTime seconds (120 by default). This signal handler executes in the context of the sshd(8)’s privileged code, which is not sandboxed and runs with full root privileges.
This issue is another instance of the problem in CVE-2024-6387 addressed by FreeBSD-SA-24:04.openssh. The faulty code in this case is from the integration of blacklistd in OpenSSH in FreeBSD.
As a result of calling functions that are not async-signal-safe in the privileged sshd(8) context, a race condition exists that a determined attacker may be able to exploit to allow an unauthenticated remote code execution as root.
If sshd(8) cannot be updated, this signal handler race condition can be mitigated by setting LoginGraceTime to 0 in /etc/ssh/sshd_config and restarting sshd(8). This makes sshd(8) vulnerable to a denial of service (the exhaustion of all MaxStartups connections), but makes it safe from the remote code execution presented in this advisory.
Upgrade your vulnerable system to a supported FreeBSD stable or release / security branch (releng) dated after the correction date, and restart sshd.
Further details – https://www.freebsd.org/security/advisories/FreeBSD-SA-24:08.openssh.asc
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