CVE-2025-32988: GNU GnuTLS Buffer Overflow Vulnerability

CVE-2025-32988 Overview

A double-free vulnerability has been discovered in GnuTLS, a widely-used open source TLS library. The flaw exists in the export logic for Subject Alternative Name (SAN) entries containing an otherName type. When the type-id OID is invalid or malformed, GnuTLS incorrectly calls asn1_delete_structure() on an ASN.1 node it does not own. This leads to a double-free condition when the parent function or caller later attempts to free the same structure.

This vulnerability is particularly concerning because it can be triggered using only public GnuTLS APIs, making exploitation more accessible. The impact ranges from denial of service to potential memory corruption, depending on the memory allocator's behavior.

Critical Impact

Network-accessible double-free vulnerability in GnuTLS that can cause denial of service or memory corruption through malformed X.509 certificate SAN entries.

Affected Products

• GNU GnuTLS (all versions prior to patch)
• Red Hat OpenShift Container Platform 4.0
• Red Hat Enterprise Linux 6.0, 7.0, 8.0, 9.0, 10.0

Discovery Timeline

• July 10, 2025 - CVE-2025-32988 published to NVD
• December 1, 2025 - Last updated in NVD database

Technical Details for CVE-2025-32988

Vulnerability Analysis

The vulnerability stems from improper ownership handling in the GnuTLS certificate export functionality. When processing X.509 certificates with Subject Alternative Name extensions that contain otherName entries, the library must parse and export ASN.1 structures representing the OID type identifiers.

The flaw manifests specifically when an otherName SAN entry contains an invalid or malformed type-id OID. During the export process, GnuTLS calls asn1_delete_structure() to clean up an ASN.1 node. However, due to incorrect ownership semantics, the node being deleted is actually owned by a parent structure. When the parent function subsequently attempts to free its own structures (including the node already freed), a double-free condition occurs.

Double-free vulnerabilities in memory allocation can lead to heap corruption, potentially allowing attackers to manipulate memory allocator metadata. While the immediate impact is typically denial of service through application crashes, sophisticated exploitation could potentially achieve more severe outcomes depending on the heap implementation and exploit conditions.

Root Cause

The root cause is incorrect memory ownership handling in the GnuTLS SAN export logic. The code assumes ownership of an ASN.1 node during error handling for malformed OIDs when it should not. This violates the memory ownership contract between the export function and its callers.

Specifically, when asn1_delete_structure() is called on a node that belongs to the caller's ASN.1 tree rather than a locally-allocated structure, it breaks the assumption that each allocated object has exactly one owner responsible for freeing it. This is a classic double-free pattern (CWE-415).

Attack Vector

The vulnerability is network-exploitable because it can be triggered through the processing of malicious X.509 certificates. An attacker can craft a certificate containing a Subject Alternative Name extension with an otherName entry featuring a malformed type-id OID.

Attack scenarios include:

1. TLS Server Attack: A malicious client presents a crafted client certificate during mutual TLS authentication
2. TLS Client Attack: A malicious server presents a crafted server certificate to connecting clients
3. Certificate Processing: Any application using GnuTLS to parse untrusted X.509 certificates

The vulnerability requires no authentication and can be exploited remotely with no user interaction required, making it a significant concern for internet-facing services using GnuTLS.

Detection Methods for CVE-2025-32988

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using GnuTLS during TLS handshake or certificate processing
• Memory corruption errors in system logs related to GnuTLS library calls
• Heap corruption detected by memory sanitizers (ASan, Valgrind) during certificate validation operations
• Application instability when processing certificates with unusual Subject Alternative Name extensions

Detection Strategies

• Monitor for repeated crashes in TLS-enabled services that correlate with incoming connection attempts
• Deploy memory safety tools (AddressSanitizer, Valgrind) in testing environments to detect double-free conditions
• Implement certificate validation logging to identify certificates with malformed otherName SAN entries
• Use GnuTLS debugging output (GNUTLS_DEBUG_LEVEL) to trace certificate parsing failures

Monitoring Recommendations

• Enable crash reporting and core dump analysis for services using GnuTLS
• Monitor system logs for SIGSEGV or SIGABRT signals from GnuTLS-dependent applications
• Implement network traffic analysis to detect potentially malicious certificates with unusual SAN structures
• Track GnuTLS library versions across infrastructure to identify vulnerable deployments

How to Mitigate CVE-2025-32988

Immediate Actions Required

• Update GnuTLS to the latest patched version available from your distribution
• Apply Red Hat security updates via yum update gnutls or dnf update gnutls for affected RHEL systems
• Review and update all containers and images using vulnerable GnuTLS versions
• Implement certificate filtering at network perimeter devices where possible to reject malformed certificates

Patch Information

Red Hat has released multiple security advisories addressing this vulnerability:

• RHSA-2025:16115
• RHSA-2025:16116
• RHSA-2025:17181
• RHSA-2025:17348
• RHSA-2025:17361
• RHSA-2025:17415
• RHSA-2025:19088
• RHSA-2025:22529

Debian users should apply updates from the Debian LTS announcement.

For additional technical details, refer to the Red Hat CVE Report and Red Hat Bug #2359622.

Workarounds

• Limit exposure by restricting which certificates are accepted (e.g., pinning trusted CAs that don't issue certificates with otherName SAN entries)
• Deploy application-layer firewalls capable of inspecting TLS certificates before they reach vulnerable applications
• Temporarily disable client certificate authentication if not strictly required until patches can be applied
• Consider network segmentation to limit the attack surface for services processing untrusted certificates

# Update GnuTLS on Red Hat Enterprise Linux
sudo dnf update gnutls

# Verify installed GnuTLS version
rpm -q gnutls

# Check for vulnerable library in use by running processes
lsof | grep libgnutls

# Restart services after update to load patched library
sudo systemctl restart httpd nginx postfix