CVE-2017-8301 in LibreSSL
Summary
by MITRE
LibreSSL 2.5.1 to 2.5.3 lacks TLS certificate verification if SSL_get_verify_result is relied upon for a later check of a verification result, in a use case where a user-provided verification callback returns 1, as demonstrated by acceptance of invalid certificates by nginx.
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Analysis
by VulDB Data Team • 12/02/2022
The vulnerability identified as CVE-2017-8301 represents a critical certificate verification flaw within LibreSSL versions 2.5.1 through 2.5.3 that fundamentally undermines the security of TLS connections. This issue arises from a specific interaction between the SSL_get_verify_result function and user-provided verification callbacks, creating a scenario where certificate validation can be bypassed even when the callback function returns a positive verification result. The vulnerability specifically affects applications that rely on SSL_get_verify_result for subsequent verification checks, such as the nginx web server which was demonstrated to accept invalid certificates under these conditions. The flaw demonstrates a classic case of improper validation where the system assumes that a positive callback return value guarantees certificate validity, when in fact the underlying verification process may have been circumvented.
The technical implementation of this vulnerability stems from how LibreSSL handles certificate verification when user-defined callback functions are employed. When a user-provided verification callback returns 1, indicating successful verification, the system should ensure that the actual certificate validation has been properly performed. However, in the affected LibreSSL versions, the verification process fails to properly enforce certificate validation even when the callback indicates success. This creates a dangerous situation where applications can accept certificates that should be rejected due to various issues such as expired certificates, incorrect subject names, or untrusted certificate authorities. The vulnerability essentially allows attackers to bypass certificate validation entirely, potentially enabling man-in-the-middle attacks where malicious certificates can be accepted without proper scrutiny.
The operational impact of CVE-2017-8301 is severe and far-reaching across any application stack that utilizes LibreSSL for TLS connections and relies on SSL_get_verify_result for verification checks. Web servers, proxy servers, and any network applications that depend on this specific verification pattern become vulnerable to certificate forgery attacks, potentially allowing attackers to establish secure connections with malicious certificates. The vulnerability particularly affects nginx and similar web servers that implement certificate verification workflows using this specific API pattern. Organizations using affected versions of LibreSSL may unknowingly accept certificates from untrusted sources, undermining the entire purpose of TLS security. This flaw can be exploited by attackers to perform session hijacking, data interception, or complete credential theft attacks, as the system no longer properly validates the authenticity of certificate chains presented during TLS handshakes.
Mitigation strategies for CVE-2017-8301 require immediate action to upgrade affected LibreSSL installations to versions 2.5.4 or later where the vulnerability has been resolved. System administrators should prioritize updating all affected applications and infrastructure components that utilize LibreSSL, particularly web servers, load balancers, and any TLS-enabled services. Organizations should conduct comprehensive inventory audits to identify all systems running affected LibreSSL versions and implement patch management procedures to ensure timely remediation. Additionally, security teams should review their certificate management policies and consider implementing additional verification layers beyond the standard SSL_get_verify_result checks. The vulnerability aligns with CWE-295 which addresses improper certificate validation, and can be mapped to ATT&CK technique T1046 for network service scanning and T1566 for credential access through man-in-the-middle attacks. Organizations should also consider implementing certificate pinning mechanisms and monitoring for unusual certificate validation behavior as additional defensive measures against this class of vulnerability.