CVE-2024-6409 in Red Hat
Summary
by MITRE • 07/08/2024
A signal handler race condition vulnerability was found in OpenSSH's server (sshd), where a client does not authenticate within LoginGraceTime seconds (120 by default, 600 in old OpenSSH versions), then sshd's SIGALRM handler is called asynchronously. However, this signal handler calls various functions that are not async-signal-safe, for example, syslog(). This issue leaves it vulnerable to a signal handler race condition on the cleanup_exit() function, which introduces the same vulnerability as CVE-2024-6387 in the unprivileged child of the SSHD server.
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Analysis
by VulDB Data Team • 10/29/2024
The vulnerability identified as CVE-2024-6409 represents a critical signal handler race condition within OpenSSH's server implementation that fundamentally compromises the security posture of affected systems. This flaw exists specifically within the sshd daemon's handling of authentication timeouts, where the server's response to failed authentication attempts creates an exploitable window for malicious actors. The vulnerability manifests when a client fails to authenticate within the designated LoginGraceTime period, which defaults to 120 seconds but was previously set to 600 seconds in older versions. The asynchronous invocation of the SIGALRM signal handler during this timeout scenario creates a dangerous concurrency situation that violates fundamental security principles of signal handling in multi-threaded environments.
The technical root cause of this vulnerability lies in the improper handling of asynchronous signals within the sshd process, specifically within the cleanup_exit() function that is invoked by the SIGALRM handler. When the signal handler executes, it calls functions that are not async-signal-safe, including syslog(), which can lead to memory corruption and undefined behavior. This race condition occurs because the signal handler operates in an environment where it may interrupt critical sections of code, potentially corrupting internal data structures or causing the process to behave unpredictably. The vulnerability is particularly concerning because it affects the unprivileged child processes of the SSHD server, making it exploitable across a broader attack surface than initially apparent. According to CWE-362, this represents a classic race condition vulnerability where concurrent execution paths can lead to unpredictable behavior and security exploits.
The operational impact of CVE-2024-6409 extends beyond simple authentication failures, as it can potentially enable attackers to execute arbitrary code within the context of the sshd process or cause denial of service conditions that disrupt legitimate remote access capabilities. The vulnerability's exploitation could allow an attacker to bypass authentication mechanisms or manipulate the server's internal state during the timeout period, creating opportunities for privilege escalation or persistent access to systems. This risk is exacerbated by the fact that the affected functionality is core to SSH operations, meaning that any exploitation attempt could compromise the integrity of the entire remote access infrastructure. The vulnerability aligns with ATT&CK technique T1078.004, which covers legitimate credentials for lateral movement, as successful exploitation could lead to unauthorized access through compromised SSH sessions.
Mitigation strategies for CVE-2024-6409 should focus on both immediate patching and operational hardening measures. The primary solution involves updating to OpenSSH versions that address this specific race condition in signal handler implementation, ensuring that all affected systems receive the appropriate security patches. Organizations should also implement monitoring for unusual authentication patterns or timeout behaviors that might indicate exploitation attempts. Network segmentation and additional authentication layers such as two-factor authentication can provide defense-in-depth protection against potential exploitation attempts. The vulnerability demonstrates the critical importance of async-signal-safe function usage in server-side applications, as outlined in POSIX standards for signal handling, and emphasizes the need for comprehensive security testing of signal handlers in production environments. Regular security audits should specifically examine signal handler implementations to prevent similar race conditions from emerging in other components of the system infrastructure.