CVE-2018-8970 in LibreSSL
Summary
by MITRE
The int_x509_param_set_hosts function in lib/libcrypto/x509/x509_vpm.c in LibreSSL 2.7.0 before 2.7.1 does not support a certain special case of a zero name length, which causes silent omission of hostname verification, and consequently allows man-in-the-middle attackers to spoof servers and obtain sensitive information via a crafted certificate. NOTE: the LibreSSL documentation indicates that this special case is supported, but the BoringSSL documentation does not.
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Analysis
by VulDB Data Team • 02/24/2023
The vulnerability identified as CVE-2018-8970 represents a critical flaw in LibreSSL's certificate validation mechanism that undermines the fundamental security guarantees of SSL/TLS connections. This issue affects LibreSSL versions 2.7.0 and earlier, specifically within the int_x509_param_set_hosts function located in the lib/libcrypto/x509/x509_vpm.c source file. The flaw manifests when processing X.509 certificates with zero-length hostname entries, creating a silent bypass of hostname verification that can be exploited by attackers to conduct successful man-in-the-middle attacks against otherwise secure connections.
The technical nature of this vulnerability stems from an incomplete implementation of hostname validation logic where the function fails to properly handle a specific edge case involving zero-length hostnames in certificate subject alternative names. When a certificate contains a hostname entry with a zero length, the validation process silently omits this entry from the verification checks rather than properly rejecting the certificate or treating it as an invalid entry. This behavior creates a security gap where attackers can craft certificates containing such zero-length hostnames to bypass hostname verification entirely, allowing them to present fake certificates that appear legitimate to clients using vulnerable LibreSSL implementations.
From an operational perspective, this vulnerability poses significant risks to any system utilizing LibreSSL 2.7.0 or earlier versions for SSL/TLS operations, particularly web servers, email servers, and any application that performs certificate validation. The silent nature of the flaw means that administrators may not immediately detect compromised connections, as the system continues to operate normally while accepting potentially malicious certificates. Attackers can exploit this weakness to intercept encrypted communications, steal sensitive data, perform credential harvesting, or conduct other malicious activities that rely on establishing trust through valid SSL/TLS certificates.
The vulnerability aligns with CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues related to key management and validation. From an ATT&CK framework perspective, this weakness maps to T1573.002 (TLS Spoofing) and T1046 (Network Service Scanning) as attackers can use this flaw to establish false trust relationships with target systems. The security impact extends beyond simple certificate validation failure to encompass complete trust model compromise, as the vulnerability allows attackers to bypass one of the most critical security mechanisms in SSL/TLS implementations. Organizations should immediately upgrade to LibreSSL 2.7.1 or later versions to remediate this vulnerability, as the patch addresses the missing edge case handling and ensures proper validation of all hostname entries in certificates.
This flaw demonstrates the importance of rigorous testing for edge cases in cryptographic implementations, particularly when dealing with certificate validation logic that forms the foundation of secure communications. The discrepancy between LibreSSL's documentation claiming support for this special case and BoringSSL's documentation not supporting it highlights the need for consistent security standards across different cryptographic libraries. The vulnerability serves as a reminder that even minor implementation details in security-critical code can have severe consequences for the overall security posture of systems relying on SSL/TLS protocols.