CVE-2026-33307 in mod_gnutls
Summary
by MITRE • 03/24/2026
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.
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Analysis
by VulDB Data Team • 03/31/2026
The vulnerability described in CVE-2026-33307 affects the mod_gnutls Apache HTTPD module, which implements TLS functionality using the GnuTLS library. This module serves as a critical component in securing web communications for Apache servers, making its security paramount to overall system integrity. The flaw exists in versions prior to 0.12.3 and 0.13.0, representing a classic buffer over-read condition that could potentially lead to denial of service or more severe consequences. The vulnerability specifically targets the client certificate verification process, which is a fundamental security mechanism in mutual TLS authentication scenarios.
The technical implementation flaw stems from improper bounds checking during certificate chain processing. When client certificates are presented to the server, the mod_gnutls module attempts to import these certificates into a fixed-size array of type `gnutls_x509_crt_t x509[]`. This array has a predetermined size that is insufficient to handle arbitrary certificate chain lengths. The module does not validate whether the number of certificates provided by the client exceeds the allocated array capacity, creating a scenario where the certificate verification code may attempt to write beyond the array boundaries. Although the data being written consists of pointer values rather than attacker-controlled content, the memory corruption potential remains significant as these pointers are written into memory locations beyond the intended array bounds, potentially causing segmentation faults or stack corruption.
The operational impact of this vulnerability is particularly concerning in environments where mutual TLS authentication is required, as it could be exploited to cause service disruption through denial of service conditions. Attackers could craft malicious client certificate chains with excessive certificate counts to trigger the buffer overflow condition, leading to server crashes or instability. However, the vulnerability does not affect configurations that do not require client certificates, specifically when the `GnuTLSClientVerify ignore` setting is used, which represents a default configuration. This distinction is important for system administrators who may be able to avoid the vulnerability by adjusting their server configurations, though this approach represents a limitation rather than a complete solution.
The fix implemented in versions 0.12.3 and 0.13.0 demonstrates a proper approach to vulnerability remediation through both immediate and comprehensive solutions. Version 0.12.3 addresses the immediate issue by implementing length validation checks that reject certificate chains exceeding the buffer capacity, providing a minimal patch for users who cannot immediately upgrade to the newer version. The more substantial fix in version 0.13.0 involves complete reimplementation of the certificate verification process using `gnutls_certificate_verify_peers()`, which eliminates the problematic buffer entirely and represents a more robust architectural solution. This approach aligns with security best practices by removing the vulnerable code pattern rather than merely patching it, and it demonstrates adherence to principles that would be referenced in standards such as CWE-129 for improper boundary checking and CWE-121 for stack buffer overflow conditions. The vulnerability's characteristics and remediation approach also relate to ATT&CK techniques involving privilege escalation through service exploitation and denial of service attacks, though the specific implementation does not involve direct code execution capabilities.