A recently discovered vulnerability, tracked as CVE-2024-6387, affects the OpenSSH server (sshd) when a client fails to authenticate within the LoginGraceTime window. By default, this period is set to 120 seconds (600 seconds in earlier OpenSSH versions). If the client does not authenticate within the allotted time, sshd's SIGALRM signal handler is called asynchronously, potentially leading to a race condition.

This vulnerability is a result of the signal handler calling various functions, such as syslog(), which are not considered async-signal-safe by default. This article examines the core issue in depth, provides a code snippet illustrating the problem, and offers links to original references and exploit details.

Exploit Details

This vulnerability could potentially be exploited by sending crafted signals to the OpenSSH server while also causing the signal handler to be called. Although no known exploit exists for CVE-2024-6387, it is still crucial to understand the underlying issue to mitigate similar attacks in the future.

The following code snippet demonstrates the signal handler race condition in the OpenSSH server

sigalarm_handler(int signo)
    syslog(LOG_CRIT, "Timeout before authentication completed.");
    cleanup_exit(255); // vulnerable: race condition

The signal handler sigalarm_handler() is invoked when the specified LoginGraceTime is reached before authentication is completed. Here, the syslog() function is called, which is not async-signal-safe. If another async-signal occurs before the syslog() call completes, it could lead to a race condition.

Original References

1. OpenSSH - Open source implementation of the SSH protocol: https://www.openssh.com/
2. CVE details - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-6387

Mitigations and Recommendations

Although the risk associated with CVE-2024-6387 is low, it is important to remain cautious and vigilant. OpenSSH administrators should consider the following recommendations:

1. Use the latest version of OpenSSH server, which can be found on the official website: https://www.openssh.com/


Understanding vulnerabilities such as CVE-2024-6387 and their potential impact is crucial for maintaining a secure OpenSSH server environment. Armed with this knowledge, system administrators can take appropriate precautions and implement the necessary controls to ensure their infrastructure remains secure.


Published on: 07/01/2024 13:15:06 UTC
Last modified on: 07/02/2024 03:55:36 UTC