CVE-2013-0176 in libssh
Summary
by MITRE
The publickey_from_privatekey function in libssh before 0.5.4, when no algorithm is matched during negotiations, allows remote attackers to cause a denial of service (NULL pointer dereference and crash) via a "Client: Diffie-Hellman Key Exchange Init" packet.
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Analysis
by VulDB Data Team • 12/28/2024
The vulnerability identified as CVE-2013-0176 represents a critical denial of service flaw within the libssh library version 0.5.3 and earlier. This vulnerability specifically targets the publickey_from_privatekey function which handles cryptographic operations during the SSH key exchange process. The flaw manifests when the library encounters a scenario where no matching algorithm can be established during the Diffie-Hellman key exchange negotiation phase, creating a condition that leads to a NULL pointer dereference and subsequent system crash.
The technical implementation of this vulnerability occurs during the client-side Diffie-Hellman key exchange initiation packet processing within the SSH protocol stack. When the negotiation process fails to identify compatible cryptographic algorithms between client and server, the publickey_from_privatekey function attempts to access a NULL pointer reference that was not properly validated or handled. This particular weakness falls under the CWE-476 category of NULL Pointer Dereference, which is a well-documented software security flaw where an application attempts to access memory through a pointer that has not been properly initialized or validated. The vulnerability is particularly dangerous because it can be triggered remotely by any attacker who can establish an SSH connection to the affected system, making it a significant threat vector for network-based attacks.
The operational impact of this vulnerability extends beyond simple service disruption to potentially enable more sophisticated attack vectors. When exploited, the NULL pointer dereference causes the libssh library to crash and terminate the SSH service, effectively rendering the targeted system inaccessible to legitimate users. This denial of service condition can be particularly devastating in environments where SSH is used as a primary management interface, as it can prevent authorized administrators from accessing critical systems during an attack. The vulnerability aligns with ATT&CK technique T1499.004, which involves network denial of service attacks, and specifically targets the service availability aspect of the CIA triad.
Mitigation strategies for CVE-2013-0176 primarily focus on immediate software updates and system hardening measures. Organizations should prioritize upgrading to libssh version 0.5.4 or later, where the NULL pointer dereference has been properly addressed through improved input validation and error handling mechanisms. Additionally, network-level protections such as implementing SSH access controls, rate limiting, and monitoring for unusual connection patterns can help detect and prevent exploitation attempts. The vulnerability demonstrates the critical importance of proper error handling in cryptographic libraries and highlights the need for comprehensive security testing of protocol implementations. System administrators should also consider implementing intrusion detection systems that can identify malformed Diffie-Hellman key exchange packets and alert security teams to potential exploitation attempts. This vulnerability serves as a reminder of how seemingly minor implementation flaws in cryptographic libraries can have significant security implications across enterprise networks.