CVE-2008-4989 in gnutls
Summary
by MITRE
The _gnutls_x509_verify_certificate function in lib/x509/verify.c in libgnutls in GnuTLS before 2.6.1 trusts certificate chains in which the last certificate is an arbitrary trusted, self-signed certificate, which allows man-in-the-middle attackers to insert a spoofed certificate for any Distinguished Name (DN).
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Analysis
by VulDB Data Team • 05/25/2025
The vulnerability described in CVE-2008-4989 represents a critical flaw in the GnuTLS library's certificate verification process that fundamentally undermines the security of SSL/TLS communications. This issue affects versions of GnuTLS prior to 2.6.1 and specifically targets the _gnutls_x509_verify_certificate function located in lib/x509/verify.c. The flaw stems from an improper certificate chain validation mechanism that fails to properly enforce the hierarchical trust structure inherent in public key infrastructure. When processing certificate chains, the vulnerable implementation allows an attacker to construct a malicious chain where the final certificate in the sequence is a trusted self-signed certificate of their choosing, thereby bypassing normal certificate validation procedures that should prevent such arbitrary trust delegation.
The technical nature of this vulnerability can be categorized under CWE-295, which addresses improper certificate validation, and more specifically aligns with CWE-310, concerning cryptographic issues related to certificate handling. The operational impact of this flaw is severe as it enables man-in-the-middle attacks by allowing attackers to insert spoofed certificates that appear valid to systems using vulnerable GnuTLS versions. The vulnerability specifically permits attackers to create certificate chains where any Distinguished Name (DN) can be spoofed, effectively undermining the identity verification mechanisms that SSL/TLS protocols rely upon for secure communication. This occurs because the verification function does not properly validate that the final certificate in the chain is appropriately signed by a trusted certificate authority, instead accepting any trusted self-signed certificate as a valid end-entity certificate.
From an adversarial perspective, this vulnerability maps directly to techniques described in the ATT&CK framework under T1552, which covers credential access through the manipulation of cryptographic systems. Attackers can exploit this flaw by generating certificate chains that terminate with their own trusted self-signed certificates, making their malicious certificates appear legitimate to vulnerable systems. The implications extend beyond simple certificate forgery to encompass complete compromise of secure communication channels, as the trust model that SSL/TLS relies upon becomes fundamentally broken. This vulnerability affects any application or system that depends on GnuTLS for secure communications, including web servers, email clients, and any software implementing SSL/TLS protocols through this library.
The recommended mitigation strategy involves immediate upgrading to GnuTLS version 2.6.1 or later, where the certificate verification logic has been corrected to properly enforce certificate chain validation. System administrators should also implement additional monitoring to detect potential certificate manipulation attempts and consider implementing certificate pinning mechanisms where possible. Organizations using vulnerable versions should conduct comprehensive vulnerability assessments to identify all systems relying on affected GnuTLS implementations and establish a remediation timeline that accounts for the critical nature of this vulnerability. The fix implemented in version 2.6.1 addresses the core validation flaw by ensuring proper chain verification that prevents arbitrary trust delegation through self-signed certificates at the end of certificate chains, thereby restoring the expected security properties of the SSL/TLS protocol stack.