CVE-2026-59845 in Red Hat
Summary
by MITRE • 07/21/2026
A flaw was found in libssh. When ProxyCommand is used, an unchecked fork() failure can be stored as process ID -1; during cleanup, signals may then be sent across the caller's accessible process tree, leading to local denial of service.
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Analysis
by VulDB Data Team • 07/21/2026
The vulnerability in libssh represents a critical security flaw that emerges when the ProxyCommand functionality is utilized within SSH connections. This issue stems from improper error handling during the fork() system call execution, where the library fails to properly validate or handle scenarios where the fork operation does not succeed as expected. The technical root cause lies in how the library manages process identifiers when fork() returns an error condition, specifically storing a process ID of -1 without adequate validation mechanisms. This oversight creates a dangerous state within the application's process management framework that can be exploited by malicious actors to disrupt normal system operations.
The operational impact of this vulnerability manifests as a local denial of service condition that can be triggered through careful manipulation of the ProxyCommand feature. When an attacker successfully exploits this flaw, they can cause the SSH client to send signals across the caller's accessible process tree, effectively disrupting legitimate processes and potentially causing system instability. The vulnerability particularly affects systems where libssh is integrated into applications that utilize proxy functionality for establishing secure connections, making it relevant to a wide range of network security tools and services. This type of vulnerability aligns with CWE-248, which addresses "Uncaught Exception" conditions that can lead to unexpected program states and potential denial of service scenarios.
From an attack perspective, this vulnerability demonstrates characteristics consistent with the ATT&CK technique T1499.004, specifically "Toggle System/Network Monitoring," where adversaries may leverage system-level flaws to disrupt normal operations. The flaw operates at the operating system interaction level, where improper handling of process creation failures creates a pathway for signal injection attacks that can cascade through the process tree. Security practitioners should note that this vulnerability does not require network access or authentication credentials to exploit, making it particularly concerning from a local attack standpoint. The issue is especially dangerous in environments where SSH clients are frequently used with proxy configurations and where multiple processes share common parent-child relationships.
Mitigation strategies for this vulnerability should include immediate patching of affected libssh versions, as the fix typically involves proper error handling for fork() failures and validation of process identifiers before signal operations. Organizations should also implement monitoring for anomalous signal activity patterns that might indicate exploitation attempts. Additionally, system administrators should consider disabling ProxyCommand functionality in environments where it is not strictly required, particularly when dealing with untrusted network connections. The vulnerability highlights the importance of robust error handling practices in cryptographic libraries and underscores the need for comprehensive testing of process management functions within security-critical software components. Regular security assessments of SSH implementations and proxy configurations should be conducted to identify similar potential issues that could lead to system disruption or unauthorized access.