CVE-2024-7589 in FreeBSD
Summary
by MITRE • 08/12/2024
A signal handler in sshd(8) may call a logging function that is not async-signal-safe. The signal handler is invoked when a client does not authenticate within the LoginGraceTime seconds (120 by default). This signal handler executes in the context of the sshd(8)'s privileged code, which is not sandboxed and runs with full root privileges.
This issue is another instance of the problem in CVE-2024-6387 addressed by FreeBSD-SA-24:04.openssh. The faulty code in this case is from the integration of blacklistd in OpenSSH in FreeBSD.
As a result of calling functions that are not async-signal-safe in the privileged sshd(8) context, a race condition exists that a determined attacker may be able to exploit to allow an unauthenticated remote code execution as root.
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Analysis
by VulDB Data Team • 08/13/2024
The vulnerability described in CVE-2024-7589 represents a critical security flaw within the OpenSSH implementation on FreeBSD systems, specifically affecting the sshd daemon's signal handling mechanism. This issue arises from the improper use of non-async-signal-safe functions within a signal handler that operates in a privileged context, creating a potential pathway for remote code execution with root privileges. The vulnerability is particularly concerning because it occurs during the authentication grace period, when a client fails to authenticate within the default 120-second window, triggering the problematic signal handler execution.
The technical flaw stems from the integration of blacklistd functionality within OpenSSH on FreeBSD systems, which introduces a signal handler that invokes logging functions that are not designed to be safely called from within a signal context. According to the posix standard and common security practices, functions called from signal handlers must be async-signal-safe to prevent undefined behavior and potential exploitation. When a signal handler executes in the privileged context of sshd running with root privileges, any function that is not async-signal-safe can lead to memory corruption, stack corruption, or other exploitable conditions. This particular implementation violates the fundamental principle that signal handlers should only call async-signal-safe functions, creating a dangerous execution environment where arbitrary code execution becomes possible.
The operational impact of this vulnerability is severe and directly relates to the attacker's ability to exploit a race condition during the authentication process. The vulnerability exists because the signal handler executes in the context of a privileged sshd process that has full root privileges, making the potential attack surface extremely dangerous. The race condition occurs when the signal handler attempts to perform logging operations that are not async-signal-safe, potentially allowing an attacker to manipulate the execution flow and gain unauthorized access to the system with root privileges. This aligns with the attack pattern described in the ATT&CK framework under privilege escalation techniques, specifically targeting the exploitation of process memory and signal handling mechanisms to achieve unauthorized system access.
The vulnerability is closely related to CVE-2024-6387, which addressed similar issues in OpenSSH's handling of asynchronous signals within privileged contexts. Both vulnerabilities fall under the CWE category of CWE-121, which deals with stack-based buffer overflow conditions, and more specifically relate to CWE-362, which addresses race conditions in concurrent programming. The FreeBSD security advisory FreeBSD-SA-24:04.openssh provides the official mitigation for this issue, emphasizing the need to properly handle signal contexts within privileged processes. The recommended mitigations include updating to the patched version of OpenSSH, implementing proper signal handler design that avoids calling non-async-signal-safe functions, and ensuring that logging operations occur outside of signal contexts to prevent potential exploitation.
Security practitioners should implement immediate monitoring for unauthorized access attempts during authentication grace periods, as this vulnerability could be exploited by attackers attempting to establish persistent access to systems running vulnerable versions of OpenSSH. The fix requires careful attention to signal handler implementation, ensuring that all functions called from signal contexts are explicitly async-signal-safe according to POSIX standards. This vulnerability demonstrates the critical importance of proper signal handling in privileged processes and highlights the need for thorough security reviews of code that executes in high-privilege contexts where race conditions and memory corruption can lead to complete system compromise. Organizations should prioritize patching affected systems and implementing additional monitoring to detect potential exploitation attempts, as the combination of the privileged execution context and the race condition creates a particularly dangerous security exposure.