CVE-2024-7589: FreeBSD OpenSSH RCE Vulnerability

CVE-2024-7589 Overview

CVE-2024-7589 is a race condition vulnerability in the FreeBSD implementation of OpenSSH's sshd(8) daemon. The vulnerability exists in a signal handler that may call logging functions that are not async-signal-safe. This signal handler is invoked when a client does not authenticate within the LoginGraceTime period (120 seconds by default) and executes in the context of sshd's privileged code, which runs with full root privileges without sandboxing.

This vulnerability is directly related to CVE-2024-6387 (also known as "regreSSHion") and stems from FreeBSD's integration of blacklistd functionality into OpenSSH. A determined attacker may be able to exploit the race condition to achieve unauthenticated remote code execution as root.

Critical Impact

This vulnerability allows potential unauthenticated remote code execution with root privileges on affected FreeBSD systems running OpenSSH, making it a critical security concern for internet-facing SSH servers.

Affected Products

• FreeBSD 13.3 (p1 through p4)
• FreeBSD 14.0 (beta5, rc3, rc4-p1, and p1 through p8)
• FreeBSD 14.1 (p1 and p2)

Discovery Timeline

• August 12, 2024 - CVE-2024-7589 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2024-7589

Vulnerability Analysis

This race condition vulnerability (CWE-362, CWE-364) affects the signal handler mechanism in FreeBSD's OpenSSH implementation. The core issue is that when a client fails to authenticate within the configured LoginGraceTime window, a SIGALRM signal is raised that triggers a handler running in the privileged (unsandboxed) context of sshd(8).

The vulnerability is particularly dangerous because the signal handler executes with full root privileges and calls functions that are not designed to be async-signal-safe. This creates a race condition where memory state can be inconsistent during the signal handler's execution, potentially allowing an attacker to manipulate program flow.

The faulty code originates from FreeBSD's integration of blacklistd (a connection tracking daemon) into OpenSSH, making this a FreeBSD-specific variant of the broader OpenSSH signal handler vulnerability class that includes CVE-2024-6387 and the historical CVE-2006-5051.

Root Cause

The root cause is the invocation of non-async-signal-safe logging functions from within a signal handler context. Signal handlers have strict requirements about which functions can be safely called because they can interrupt normal program execution at any point. When async-signal-unsafe functions (like many logging functions that allocate memory or use locks) are called from a signal handler, they can corrupt internal data structures or create deadlocks if the interrupted code was using the same resources.

In FreeBSD's case, the blacklistd integration added logging code paths to the signal handler that violate these async-signal-safety requirements, creating exploitable race conditions.

Attack Vector

The attack vector is network-based and does not require authentication. An attacker can exploit this vulnerability by:

1. Initiating multiple SSH connections to the target server
2. Not completing authentication within the LoginGraceTime period to trigger the SIGALRM signal handler
3. Timing the attack to hit the race condition window where the signal handler interrupts execution at a vulnerable point
4. Exploiting the resulting memory corruption or state inconsistency to achieve code execution

The attack requires high complexity due to the timing-sensitive nature of race conditions, but successful exploitation grants full root privileges on the target system.

Detection Methods for CVE-2024-7589

Indicators of Compromise

• Unusual patterns of incomplete SSH authentication attempts that timeout after 120 seconds
• Multiple simultaneous SSH connections from the same source that never complete authentication
• Unexpected crashes or restarts of the sshd(8) service
• Anomalous system behavior following SSH authentication timeout events

Detection Strategies

• Monitor SSH logs for patterns of authentication timeout events (Connection closed by authenticating user messages)
• Implement network-level detection for high volumes of SSH connections that persist for approximately 120 seconds without completing authentication
• Deploy endpoint detection solutions that monitor for anomalous behavior in privileged SSH daemon processes
• Review system logs for sshd crashes or unexpected terminations

Monitoring Recommendations

• Enable verbose SSH logging to capture authentication attempt details
• Implement rate limiting on SSH connections at the firewall level
• Configure alerting for unusual SSH connection patterns, particularly long-lived unauthenticated sessions
• Monitor system integrity and watch for signs of root-level compromise

How to Mitigate CVE-2024-7589

Immediate Actions Required

• Apply the security patches provided in FreeBSD-SA-24:08.openssh immediately
• Update FreeBSD systems to patched versions: 13.3-RELEASE-p5, 14.0-RELEASE-p9, or 14.1-RELEASE-p3 or later
• If immediate patching is not possible, implement the LoginGraceTime 0 workaround
• Review network exposure and consider restricting SSH access to trusted IP ranges

Patch Information

FreeBSD has released security patches addressing this vulnerability in advisory FreeBSD-SA-24:08.openssh. The fix ensures that only async-signal-safe functions are called from within the signal handler context. Organizations running affected FreeBSD versions should update to the patched releases as soon as possible. NetApp has also released an advisory for affected products at their security advisory page.

Workarounds

• Set LoginGraceTime 0 in /etc/ssh/sshd_config to disable the timeout mechanism entirely (note: this leaves connections open indefinitely until authentication completes)
• Implement strict firewall rules to limit SSH access to trusted networks only
• Use fail2ban or similar tools to block sources generating excessive SSH connection attempts
• Consider deploying a bastion host or VPN for SSH access to reduce attack surface

# Temporary workaround: Disable LoginGraceTime
# Edit /etc/ssh/sshd_config and add:
LoginGraceTime 0

# Restart sshd to apply changes
service sshd restart

# Verify the configuration
sshd -T | grep logingracetime