CVE-2016-0739 in libssh
Summary
by MITRE
libssh before 0.7.3 improperly truncates ephemeral secrets generated for the (1) diffie-hellman-group1 and (2) diffie-hellman-group14 key exchange methods to 128 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 • 07/24/2022
The vulnerability identified as CVE-2016-0739 represents a critical cryptographic flaw in the libssh library affecting versions prior to 0.7.3. This issue stems from improper handling of ephemeral secrets during the Diffie-Hellman key exchange process, specifically impacting the diffie-hellman-group1 and diffie-hellman-group14 methods. The flaw manifests as a truncation error where cryptographic values are incorrectly limited to 128 bits instead of their proper bit length, creating a significant weakness in the security infrastructure that protects SSH communications. The vulnerability falls under the category of cryptographic implementation errors and can be classified as a CWE-327 weakness, which specifically addresses the use of insecure cryptographic algorithms or improper implementation of cryptographic functions.
The technical nature of this vulnerability creates a dangerous condition where attackers can exploit the bits/bytes confusion bug to perform man-in-the-middle attacks against SSH sessions. When the ephemeral secrets are truncated to 128 bits, the effective cryptographic strength is significantly reduced, making it substantially easier for adversaries to perform brute force attacks or other cryptographic analysis techniques. The impact extends beyond simple key exchange manipulation as it fundamentally undermines the security guarantees that SSH protocols are designed to provide. This truncation creates a situation where the cryptographic keys used for session encryption become vulnerable to attack vectors that would normally require exponentially more computational resources to overcome. The vulnerability represents a fundamental misunderstanding of cryptographic bit handling and demonstrates poor implementation practices that compromise the entire security model of the SSH protocol.
The operational impact of CVE-2016-0739 is severe and far-reaching across organizations relying on libssh for secure communications. Systems using affected versions of the library become vulnerable to passive eavesdropping and active man-in-the-middle attacks that can decrypt or intercept sensitive data transmitted over SSH connections. This includes but is not limited to configuration files, credentials, network commands, and sensitive business data that flows through SSH sessions. The vulnerability affects any system that utilizes the specified Diffie-Hellman key exchange methods, which are commonly implemented in various SSH server and client implementations. Organizations may experience unauthorized access to critical systems, data breaches, and potential compromise of entire network infrastructures. The attack surface is particularly concerning because SSH is widely used for administrative access to servers, network devices, and cloud infrastructure, making this vulnerability a prime target for cybercriminals and nation-state actors.
Mitigation strategies for CVE-2016-0739 require immediate action to upgrade affected libssh installations to version 0.7.3 or later, which contains the necessary fixes to properly handle ephemeral secret lengths. Organizations should also consider implementing additional security measures such as disabling the vulnerable Diffie-Hellman group1 and group14 methods in their SSH configurations, while enabling stronger cryptographic algorithms like diffie-hellman-group16 and diffie-hellman-group18. Network monitoring should be enhanced to detect potential exploitation attempts, and security teams should conduct comprehensive audits of all systems using libssh to identify and remediate affected components. The fix addresses the underlying implementation error by ensuring that cryptographic values are properly handled according to their intended bit lengths rather than being incorrectly truncated. Security professionals should also consider implementing certificate pinning and other authentication mechanisms to provide additional layers of protection against potential exploitation attempts. This vulnerability serves as a reminder of the critical importance of proper cryptographic implementation and the potential consequences of seemingly minor implementation errors in security-critical software components.