CVE-2015-8805 in Nettle
Summary
by MITRE
The ecc_256_modq function in ecc-256.c in Nettle before 3.2 does not properly handle carry propagation and produces incorrect output in its implementation of the P-256 NIST elliptic curve, which allows attackers to have unspecified impact via unknown vectors, a different vulnerability than CVE-2015-8803.
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Analysis
by VulDB Data Team • 07/08/2022
The vulnerability identified as CVE-2015-8805 affects the Nettle cryptographic library version 3.1 and earlier, specifically within the ecc_256_modq function located in ecc-256.c. This flaw represents a critical implementation error in the P-256 NIST elliptic curve arithmetic that fundamentally undermines the security guarantees of elliptic curve cryptography. The issue manifests in the improper handling of carry propagation during modular arithmetic operations, which is a foundational requirement for secure cryptographic implementations. When examining the mathematical foundations of elliptic curve cryptography, the P-256 curve operates over a finite field with specific modulus operations that must be executed with absolute precision to maintain security properties.
The technical flaw in ecc_256_modq stems from inadequate carry propagation handling during modular reduction operations on the P-256 curve's prime field. This type of error falls under CWE-681, which specifically addresses incorrect use of potentially dangerous functions or operations that can lead to incorrect results. The carry propagation issue means that when performing arithmetic operations on large integers representing points on the elliptic curve, the function fails to correctly propagate carries from lower-order bits to higher-order bits, resulting in mathematically incorrect intermediate values. This error is particularly insidious because it occurs during the fundamental modular arithmetic operations that are essential for elliptic curve point multiplication and other cryptographic primitives.
The operational impact of this vulnerability extends beyond simple incorrect computation, as it creates potential attack vectors that could compromise the security of systems relying on Nettle's cryptographic functions. While the exact attack vectors remain unspecified in the CVE description, such implementation flaws in elliptic curve arithmetic typically enable attackers to perform invalid curve attacks, side-channel attacks, or potentially recover private keys through mathematical inconsistencies in the computation. The vulnerability is distinct from CVE-2015-8803, indicating that multiple flaws exist within the same cryptographic library, highlighting the complexity of securing elliptic curve implementations. According to ATT&CK framework, this vulnerability could be leveraged as part of a broader attack chain under techniques such as T1593 (Vulnerability Scanning) and T1071.004 (Application Layer Protocol: DNS) when attackers attempt to exploit cryptographic weaknesses in network communications.
Mitigation strategies for CVE-2015-8805 require immediate upgrading to Nettle version 3.2 or later, where the carry propagation issue has been corrected. System administrators should also consider implementing additional cryptographic validation checks and monitoring for anomalous cryptographic behavior in applications using the affected library. Security teams must conduct comprehensive vulnerability assessments of systems using Nettle to identify potential exposure, particularly in environments where elliptic curve cryptography is utilized for TLS/SSL certificates, digital signatures, or key exchange protocols. The fix addresses the core mathematical implementation issue by ensuring proper carry handling during modular arithmetic operations, thereby restoring the mathematical correctness required for secure elliptic curve cryptography. Organizations should also consider implementing cryptographic agility measures to facilitate rapid response to similar vulnerabilities in other cryptographic libraries and protocols.