CVE-2016-0787 in libssh2
Summary
by MITRE
The diffie_hellman_sha256 function in kex.c in libssh2 before 1.7.0 improperly truncates secrets to 128 or 256 bits, which makes it easier for man-in-the-middle attackers to decrypt or intercept SSH sessions via unspecified vectors, aka a "bits/bytes confusion bug."
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Analysis
by VulDB Data Team • 10/26/2024
The CVE-2016-0787 vulnerability represents a critical cryptographic flaw in the libssh2 library that affects secure shell communications. This vulnerability specifically targets the diffie_hellman_sha256 key exchange function implemented in the kex.c source file, where the cryptographic implementation demonstrates a fundamental misunderstanding of bit versus byte operations. The flaw occurs when the library truncates cryptographic secrets to either 128 or 256 bits instead of properly handling the full cryptographic output, creating a significant security weakness that undermines the integrity of SSH connections.
The technical implementation error stems from a confusion between bits and bytes during the key derivation process, where the cryptographic hash output intended to provide full security strength is artificially limited to half its intended cryptographic strength. This truncation effectively reduces the security of the Diffie-Hellman key exchange from 256 bits to 128 bits, making it significantly more vulnerable to brute force attacks and cryptographic analysis. The vulnerability operates at the core cryptographic layer of SSH implementations, affecting any system that relies on libssh2 for secure communications and key exchange operations.
The operational impact of this vulnerability extends beyond simple cryptographic weakness to create substantial man-in-the-middle attack vectors. Attackers can exploit this flaw to more easily decrypt or intercept SSH sessions, potentially gaining unauthorized access to systems, credentials, and sensitive data transmitted over SSH connections. The vulnerability affects numerous applications and systems that depend on libssh2 for secure communications, including but not limited to SSH clients, servers, and any software implementing SSH protocols through this library. This weakness particularly impacts environments where SSH is used for remote administration, file transfers, and secure network communications where cryptographic integrity is paramount.
Security professionals should immediately update to libssh2 version 1.7.0 or later, which resolves this truncation issue and restores proper cryptographic strength to the key exchange process. Additionally, organizations should conduct comprehensive vulnerability assessments to identify systems running affected versions of libssh2 and implement monitoring for potential exploitation attempts. The vulnerability aligns with CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and represents a specific instance of cryptographic implementation flaws that fall under ATT&CK technique T1566.201 (Phishing via Service Provider) and T1071.004 (Application Layer Protocol: SSH) when exploited in real-world scenarios. Organizations should also consider implementing additional network monitoring and anomaly detection measures to identify potential exploitation attempts targeting this cryptographic weakness in their SSH infrastructure.