CVE-2024-6409: OpenSSH sshd Race Condition RCE Vulnerability

CVE-2024-6409 Overview

A race condition vulnerability was discovered in how signals are handled by OpenSSH's server (sshd). If a remote attacker does not authenticate within a set time period, then sshd's SIGALRM handler is called asynchronously. However, this signal handler calls various functions that are not async-signal-safe, for example, syslog(). As a consequence of a successful attack, in the worst-case scenario, an attacker may be able to perform a remote code execution (RCE) as an unprivileged user running the sshd server.

Critical Impact

Potential remote code execution (RCE) as an unprivileged user

Affected Products

• OpenSSH

Discovery Timeline

• Not Available - Vulnerability discovered by
• Not Available - Responsible disclosure to vendor
• Not Available - CVE CVE-2024-6409 assigned
• Not Available - Vendor releases security patch
• 2024-07-08 - CVE CVE-2024-6409 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-6409

Vulnerability Analysis

The vulnerability arises due to improper handling of asynchronous signals by sshd. This can lead to calls to functions that are not designed to be async-signal-safe, creating a race condition that may be exploited by attackers.

Root Cause

The root cause is the invocation of non-async-signal-safe functions within a signal handler in sshd.

Attack Vector

This vulnerability can be exploited remotely over the network, allowing unauthenticated attackers to potentially execute code on the target server.

// Example exploitation code (sanitized)
signal(SIGALRM, alarm_handler);
...
alarm_handler() {
  // unsafe call
  syslog(LOG_NOTICE, "Timeout expired");
}

Detection Methods for CVE-2024-6409

Indicators of Compromise

• Unusual sshd server behavior
• Unexpected logs from sshd
• System instability following SSH connections

Detection Strategies

Monitor for exploitation attempts by using IDS/IPS systems to detect abnormal SSH traffic patterns and potential unauthorized code execution attempts.

Monitoring Recommendations

Implement continuous monitoring of sshd logs for unusual behavior and run anomaly detection algorithms on network traffic data.

How to Mitigate CVE-2024-6409

Immediate Actions Required

• Upgrade to the latest version of OpenSSH that addresses this vulnerability.
• Implement temporary firewall rules to limit access to the SSH service.
• Disable SSH access for untrusted hosts until patches are applied.

Patch Information

Refer to the vendor advisories and errata pages for detailed patch information and update instructions.

Workarounds

Ensure that sshd runs with the least privileges necessary and consider using alternative authentication methods to reduce exposure.

# Configuration example
PermitRootLogin no
PasswordAuthentication no
AllowUsers your_secure_user