CVE-2018-16150 in axTLS
Summary
by MITRE
In sig_verify() in x509.c in axTLS version 2.1.3 and before, the PKCS#1 v1.5 signature verification does not reject excess data after the hash value. Consequently, a remote attacker can forge signatures when small public exponents are being used, which could lead to impersonation through fake X.509 certificates. This is a variant of CVE-2006-4340.
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Analysis
by VulDB Data Team • 06/05/2023
The vulnerability identified as CVE-2018-16150 represents a critical flaw in the axTLS library's implementation of PKCS#1 v1.5 signature verification within the x509.c file. This weakness exists in versions 2.1.3 and earlier, creating a security gap that directly impacts the integrity of X.509 certificate validation processes. The flaw stems from insufficient validation of signature structures, specifically failing to properly reject extraneous data that follows the hash value within the signature payload.
The technical nature of this vulnerability lies in the improper handling of PKCS#1 v1.5 formatted signatures during the verification process. When the sig_verify() function processes signatures, it does not adequately check for trailing data beyond the expected hash value portion of the signature structure. This behavior creates a scenario where attackers can append additional data to valid signatures without detection, effectively allowing signature forgery under specific conditions. The vulnerability is particularly dangerous when small public exponents are employed, as the mathematical properties of such keys make the forgery attack more feasible.
The operational impact of this vulnerability extends beyond simple signature validation failures, creating potential for serious impersonation attacks through the creation of fake X.509 certificates. An attacker who successfully exploits this vulnerability could generate valid-looking certificates that would pass signature verification checks, enabling them to perform man-in-the-middle attacks or impersonate legitimate services. This weakness directly undermines the trust model that X.509 certificates provide in secure communications, potentially compromising entire certificate-based security infrastructures.
This vulnerability aligns with CWE-209, which addresses improper handling of signature verification data, and represents a variant of CVE-2006-4340, indicating a recurring pattern in cryptographic implementation flaws. The ATT&CK framework categorizes this as a signature verification bypass technique, where adversaries exploit implementation weaknesses to circumvent security controls. Organizations utilizing axTLS versions prior to 2.1.4 should immediately implement mitigations including upgrading to patched versions, implementing additional signature validation layers, and monitoring for suspicious certificate activity. The vulnerability demonstrates the critical importance of rigorous cryptographic implementation review and the potential consequences of seemingly minor validation oversights in security-critical systems.