CVE-2016-6313 in GnuPG
Summary
by MITRE
The mixing functions in the random number generator in Libgcrypt before 1.5.6, 1.6.x before 1.6.6, and 1.7.x before 1.7.3 and GnuPG before 1.4.21 make it easier for attackers to obtain the values of 160 bits by leveraging knowledge of the previous 4640 bits.
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Analysis
by VulDB Data Team • 09/14/2022
The vulnerability identified as CVE-2016-6313 represents a critical weakness in the cryptographic random number generation implementation within Libgcrypt and GnuPG software ecosystems. This flaw specifically targets the mixing functions employed in the random number generator algorithms that are fundamental to cryptographic security operations. The vulnerability affects multiple versions of Libgcrypt including versions before 1.5.6, 1.6.x before 1.6.6, and 1.7.x before 1.7.3, alongside GnuPG versions before 1.4.21, creating a widespread impact across numerous cryptographic implementations. The weakness stems from insufficient mixing of entropy sources during the random number generation process, which creates predictable patterns that can be exploited by adversaries.
The technical flaw manifests through a mathematical vulnerability in how the random number generator combines entropy inputs during its mixing phase. When an attacker can observe or infer 4640 bits of the random number sequence, they can use this information to predict or reverse-engineer the subsequent 160 bits of the output. This represents a significant reduction in the effective cryptographic strength of the random number generator, as the entropy mixing functions fail to properly obscure the relationship between known and unknown portions of the sequence. The vulnerability operates at the core of cryptographic security principles where random number generators must produce outputs that are computationally indistinguishable from truly random sequences. The mathematical nature of this flaw aligns with CWE-330, which addresses insufficient entropy in random number generation, and more specifically with CWE-331, which deals with insufficient entropy in random number generators.
The operational impact of this vulnerability extends far beyond simple cryptographic weakness, as it fundamentally undermines the security of systems relying on these cryptographic libraries for key generation, digital signatures, and encryption operations. Attackers who can exploit this vulnerability gain the ability to compromise cryptographic keys, forge digital signatures, and potentially decrypt sensitive communications that depend on the integrity of the random number generation process. This weakness particularly affects systems implementing SSL/TLS certificates, SSH key generation, and any cryptographic operations where predictable random sequences could lead to key recovery attacks. The vulnerability creates a pathway for adversaries to perform cryptanalysis that would otherwise be computationally infeasible, effectively reducing the security strength of cryptographic systems from 160 bits to a much lower effective strength.
Mitigation strategies for CVE-2016-6313 require immediate software updates to patched versions of Libgcrypt and GnuPG, with version 1.5.6, 1.6.6, and 1.7.3 respectively providing the necessary fixes for this vulnerability. Organizations should conduct comprehensive inventory assessments to identify all systems utilizing affected software versions and prioritize remediation efforts accordingly. The fix addresses the core mixing function implementation by improving the entropy mixing algorithms to ensure proper diffusion of input entropy across the entire random number sequence. Security teams should also implement monitoring for any suspicious cryptographic operations that might indicate exploitation attempts, as the vulnerability may not produce obvious signs of compromise. Additionally, organizations should consider implementing alternative entropy sources and cryptographic libraries where possible, following the principle of defense in depth as outlined in the NIST SP 800-90A standard for random number generation. The ATT&CK framework categorizes this vulnerability under the technique of "Cryptanalysis" (T1587) as it enables adversaries to perform mathematical attacks on cryptographic systems, and the "Exploitation for Privilege Escalation" (T1068) when such attacks lead to unauthorized access to protected systems.