CVE-2018-16253 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 properly verify the ASN.1 metadata. 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 an even more permissive variant of CVE-2006-4790 and CVE-2014-1568.
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Analysis
by VulDB Data Team • 06/05/2023
The vulnerability described in CVE-2018-16253 represents a critical flaw in the cryptographic signature verification implementation within the axTLS library version 2.1.3 and earlier. This issue specifically affects the x509.c module's sig_verify() function, which is responsible for validating digital signatures on X.509 certificates. The flaw stems from improper handling of ASN.1 metadata during PKCS#1 v1.5 signature verification processes, creating a fundamental weakness that undermines the integrity of the certificate validation mechanism. This vulnerability is particularly concerning because it affects the core cryptographic operations that ensure trust in digital certificates, making it a significant concern for any system relying on axTLS for secure communications.
The technical root cause of this vulnerability lies in the insufficient validation of ASN.1 structure elements within the PKCS#1 v1.5 signature verification process. When the sig_verify() function processes signatures, it fails to properly validate the ASN.1 metadata that should accompany the signature data, specifically the padding structure and encoding format. This incomplete validation allows attackers to craft forged signatures that appear valid to the verification routine while actually containing malicious content. The vulnerability becomes particularly exploitable when systems use small public exponents, which are common in certain cryptographic implementations. According to CWE-327, this represents a weakness in cryptographic implementation that directly impacts the security of digital signatures and certificate validation processes.
The operational impact of CVE-2018-16253 extends far beyond simple signature forgery, as it enables sophisticated impersonation attacks through the creation of fake X.509 certificates. Attackers can leverage this vulnerability to generate certificates that will be accepted by systems using vulnerable axTLS implementations, potentially allowing them to intercept encrypted communications, perform man-in-the-middle attacks, or impersonate legitimate services. This vulnerability aligns with ATT&CK technique T1552.004, which covers the use of certificate manipulation for privilege escalation and access. The consequences are particularly severe in environments where certificate-based authentication is critical, such as enterprise networks, cloud services, and any infrastructure relying on secure TLS communications. The vulnerability's relationship to CVE-2006-4790 and CVE-2014-1568 demonstrates a recurring pattern of insufficient ASN.1 validation in cryptographic libraries, highlighting the persistent nature of these implementation flaws.
Mitigation strategies for CVE-2018-16253 require immediate action to upgrade to axTLS version 2.1.4 or later, which contains the necessary patches to properly validate ASN.1 metadata during signature verification. Organizations should also implement comprehensive certificate monitoring and validation procedures to detect any forged certificates that may have been deployed prior to the patch installation. The fix addresses the specific ASN.1 parsing issue by ensuring that all required metadata elements are properly validated before signature acceptance, thereby preventing the exploitation of the padding oracle vulnerability. Security teams should conduct thorough vulnerability assessments of all systems using axTLS, particularly those handling sensitive data or implementing certificate-based authentication. Additionally, implementing certificate pinning mechanisms and regular security audits can provide additional layers of protection against potential exploitation of this and similar cryptographic vulnerabilities.