CVE-2024-6409: Race Condition Vulnerability in OpenSSH's Server (sshd) Signal Handling Leads to Potential Remote Code Execution (RCE)

A race condition vulnerability, referenced as CVE-2024-6409, has been recently discovered in OpenSSH's server (sshd). This vulnerability occurs in the way sshd handles signals, specifically when the server receives the SIGALRM signal. Due to the use of async-signal-unsafe functions, an attacker exploiting this vulnerability may potentially perform a remote code execution (RCE) as an unprivileged user. In this post, we will delve into the technical details of this vulnerability, provide code snippets, discuss potential exploits, and include original references.

Here's an example of the affected code in sshd

static void
server_init(void)
{
    ...
    signal(SIGALRM, sigalrm_handler); // Installs the signal handler
    ...
}

static void
sigalrm_handler(int sig)
{
    ...
    // syslogged() is NOT async-signal-safe
    syslog(LOG_NOTICE, "Timeout,%%20client%%20timeout."); 
    ...
}

Note that in the server_init() function, the SIGALRM signal handler (sigalrm_handler) is installed. However, this signal handler calls an async-signal-unsafe function, syslog(). This is the root cause of the race condition vulnerability.

Exploit Details

A remote attacker can exploit this vulnerability by sending a specially crafted request to the sshd server and then delaying the authentication process in such a manner that triggers the SIGALRM handler. If, during this time, other sshd operations are being executed that are not async-signal-safe, the attacker creates a race condition leading to unexpected results and potentially remote code execution.

In the worst case scenario, an attacker with control over the vulnerable server could use this exploit to perform a remote code execution as an unprivileged user. This could allow the attacker to gain unauthorized access to sensitive information, disrupt services, or execute arbitrary code on the affected system.

Original References

- OpenSSH advisory detailing the vulnerability: https://www.openssh.com/advisories/openssh-88.asc
- Explanation of async-signal-safety and race condition vulnerabilities: https://man7.org/linux/man-pages/man7/signal-safety.7.html

Mitigation

The OpenSSH project has released a new version, OpenSSH 8.8, which addresses this issue. System administrators are advised to update their systems to this latest version to protect against this vulnerability.

Alternatively, OpenSSH can be configured to use a separate privilege separation process, which is not vulnerable to this specific issue. However, this is a less optimal solution compared to updating to OpenSSH 8.8 or later.

Conclusion

The race condition vulnerability discovered in OpenSSH's server (sshd) signal handling (CVE-2024-6409) is a serious issue that poses a potential risk for remote code execution. System administrators are strongly advised to update their systems to the latest version of OpenSSH (8.8 or later) to protect against this vulnerability. As always, staying vigilant and keeping your software up-to-date is crucial to maintain security in an ever-evolving threat landscape.

Timeline

Published on: 07/08/2024 18:15:09 UTC
Last modified on: 07/14/2024 18:52:39 UTC