Bug 2450580 (CVE-2026-33307)

Summary: CVE-2026-33307 mod_gnutls: mod_gnutls: Denial of Service via malicious client certificate chain
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in mod_gnutls, a TLS module for Apache HTTPD. A remote attacker could exploit this vulnerability by sending a specially crafted client certificate chain to a server configured to use client certificates. This could lead to a buffer overflow due to the module not properly checking the length of the certificate chain against a fixed-size array. Successful exploitation would typically result in a denial of service (DoS) due to a server crash, and could theoretically lead to stack corruption.
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Bug Depends On: 2450682    
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Description OSIDB Bzimport 2026-03-24 02:01:49 UTC 
Mod_gnutls is a TLS module for Apache HTTPD based on GnuTLS. In versions prior to 0.12.3 and 0.13.0, code for client certificate verification imported the certificate chain sent by the client into a fixed size `gnutls_x509_crt_t x509[]` array without checking the number of certificates is less than or equal to the array size. `gnutls_x509_crt_t` is a `typedef` for a pointer to an opaque GnuTLS structure created using with `gnutls_x509_crt_init()` before importing certificate data into it, so no attacker-controlled data was written into the stack buffer, but writing a pointer after the last array element generally triggered a segfault, and could theoretically cause stack corruption otherwise (not observed in practice). Server configurations that do not use client certificates (`GnuTLSClientVerify ignore`, the default) are not affected. The problem has been fixed in version 0.12.3 by checking the length of the provided certificate chain and rejecting it if it exceeds the buffer length, and in version 0.13.0 by rewriting certificate verification to use `gnutls_certificate_verify_peers()`, removing the need for the buffer entirely. There is no workaround. Version 0.12.3 provides the minimal fix for users of 0.12.x who do not wish to upgrade to 0.13.0 yet.