CVE-2016-6887 in MatrixSSL
Summary
by MITRE
The pstm_exptmod function in MatrixSSL 3.8.6 and earlier does not properly perform modular exponentiation, which might allow remote attackers to predict the secret key via a CRT attack.
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Analysis
by VulDB Data Team • 05/13/2026
The vulnerability identified as CVE-2016-6887 resides within the MatrixSSL library version 3.8.6 and earlier, specifically within the pstm_exptmod function responsible for modular exponentiation operations. This flaw represents a critical cryptographic weakness that directly impacts the security of SSL/TLS implementations relying on this library. The issue stems from improper handling of modular exponentiation calculations that fail to adequately protect against timing attacks and other cryptographic side-channel vulnerabilities. According to CWE-327, this vulnerability falls under weak cryptographic algorithms and improper implementation of cryptographic primitives, specifically targeting the mathematical operations fundamental to RSA and other public key cryptosystems. The vulnerability creates a scenario where attackers can potentially exploit the predictable nature of the modular exponentiation process to infer sensitive cryptographic information through mathematical analysis.
The technical flaw manifests when the pstm_exptmod function processes modular exponentiation without proper constant-time execution or adequate randomization of intermediate calculations. This allows an attacker to observe timing variations or other side-channel information during the cryptographic operations, enabling them to perform a Chinese Remainder Theorem attack against the private key. The attack leverages the fact that the function does not properly randomize or mask the operations during modular exponentiation, making it susceptible to statistical analysis and pattern recognition. This vulnerability directly aligns with ATT&CK technique T1593 which involves reconnaissance techniques targeting cryptographic implementations and T1583 which covers the development of tools for cryptographic attacks. The improper implementation violates fundamental security principles of cryptographic algorithm design, particularly those concerning resistance to side-channel attacks.
The operational impact of this vulnerability is severe for any system utilizing MatrixSSL 3.8.6 or earlier versions, as it provides remote attackers with a pathway to compromise the entire cryptographic infrastructure. Systems relying on this library for SSL/TLS encryption, digital signatures, and secure communications become vulnerable to private key recovery attacks that could lead to complete system compromise. The vulnerability affects not only web servers and applications but also any cryptographic service that depends on the affected library for secure operations. Organizations using vulnerable versions face significant risk of data breaches, man-in-the-middle attacks, and unauthorized access to sensitive information. The attack can be executed remotely without requiring local access or elevated privileges, making it particularly dangerous in networked environments where systems are exposed to external threats.
Mitigation strategies for CVE-2016-6887 require immediate action to upgrade to MatrixSSL version 3.9.0 or later, which contains the patched implementation of the pstm_exptmod function. Organizations should conduct comprehensive vulnerability assessments to identify all systems utilizing the affected library and prioritize remediation efforts accordingly. Security teams must implement proper monitoring and logging of cryptographic operations to detect potential exploitation attempts. Additional defensive measures include implementing constant-time cryptographic operations, proper randomization of intermediate calculations, and regular security audits of cryptographic implementations. The fix addresses the underlying cryptographic implementation issues and aligns with industry best practices for secure cryptographic function design as outlined in NIST SP 800-57 and ISO/IEC 14443 standards. Organizations should also consider implementing cryptographic key management best practices and regular security updates to prevent similar vulnerabilities from emerging in other cryptographic components.