CVE-2025-6395: GnuTLS NULL Pointer Dereference Flaw

CVE-2025-6395 Overview

A NULL pointer dereference vulnerability has been identified in GnuTLS, specifically within the _gnutls_figure_common_ciphersuite() function. This flaw can be exploited remotely over a network connection to cause denial of service conditions and potentially compromise the integrity of TLS communications. The vulnerability affects systems utilizing GnuTLS for secure communications, making it a significant concern for servers and applications relying on this library for cryptographic operations.

Critical Impact

Remote attackers can trigger a NULL pointer dereference in GnuTLS during cipher suite negotiation, potentially causing service crashes and affecting the availability of TLS-protected services.

Affected Products

• GnuTLS library (versions prior to patched releases)
• Red Hat Enterprise Linux distributions using affected GnuTLS versions
• Debian-based distributions using affected GnuTLS versions

Discovery Timeline

• 2025-07-10 - CVE-2025-6395 published to NVD
• 2025-12-01 - Last updated in NVD database

Technical Details for CVE-2025-6395

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference), occurring in the GnuTLS cryptographic library's cipher suite negotiation process. The flaw manifests within the _gnutls_figure_common_ciphersuite() function, which is responsible for determining compatible cipher suites between client and server during the TLS handshake process.

When exploited, the NULL pointer dereference causes the affected application or service to crash unexpectedly. This can lead to denial of service conditions affecting availability, and the vulnerability may also allow limited integrity impacts during the negotiation phase. The network-accessible nature of this flaw means attackers do not require local access or authentication to trigger the condition, though exploitation complexity is considered high due to the specific conditions required.

Root Cause

The root cause of this vulnerability lies in improper NULL pointer validation within the _gnutls_figure_common_ciphersuite() function. During the cipher suite negotiation phase of a TLS handshake, the function fails to adequately check for NULL values before dereferencing pointers. When crafted or malformed handshake data is processed, the function may attempt to access memory through an uninitialized or NULL pointer, triggering an immediate crash of the process utilizing the GnuTLS library.

Attack Vector

The attack vector is network-based, allowing remote exploitation without authentication. An attacker can send specially crafted TLS handshake requests to a server or client application using vulnerable versions of GnuTLS. The attack targets the cipher suite negotiation phase, where malformed data causes the vulnerable function to dereference a NULL pointer. While no user interaction is required, the attack complexity is elevated as specific conditions must be met during the handshake process to trigger the vulnerability.

The vulnerability affects the availability of services through denial of service and may have limited integrity implications during the cipher negotiation process. Confidentiality is not impacted by this flaw.

Detection Methods for CVE-2025-6395

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using GnuTLS for TLS communications
• Service interruptions correlating with incoming TLS connection attempts
• Core dumps or crash logs showing NULL pointer dereference in _gnutls_figure_common_ciphersuite() or related GnuTLS functions
• Abnormal patterns of TLS handshake failures in server logs

Detection Strategies

• Monitor system logs for segmentation faults and crashes in processes utilizing GnuTLS libraries
• Implement application-level monitoring to detect unexpected terminations during TLS handshake operations
• Review core dumps for stack traces indicating NULL pointer dereference in GnuTLS cipher suite functions
• Deploy network intrusion detection rules to identify potentially malformed TLS handshake packets

Monitoring Recommendations

• Configure crash reporting and core dump collection for applications using GnuTLS
• Implement health checks and automatic service restart mechanisms for critical TLS-dependent services
• Monitor network traffic patterns for unusual TLS handshake behavior or repeated connection failures
• Set up alerting for application crashes that correlate with incoming network connections

How to Mitigate CVE-2025-6395

Immediate Actions Required

• Update GnuTLS to the latest patched version available for your distribution
• Review and apply relevant security advisories from Red Hat and Debian
• Implement service monitoring to detect and automatically recover from crash conditions
• Consider temporarily restricting access to affected services if immediate patching is not possible

Patch Information

Security patches addressing CVE-2025-6395 are available through multiple distribution channels. Red Hat has released several security advisories including RHSA-2025:16115, RHSA-2025:16116, RHSA-2025:17181, RHSA-2025:17348, RHSA-2025:17361, RHSA-2025:17415, RHSA-2025:19088, and RHSA-2025:22529. Debian users should refer to the Debian LTS Security Announcement for applicable patches. Additional technical details are available in Red Hat Bug Report #2376755 and the Openwall OSS-Security discussion.

Workarounds

• Implement rate limiting on incoming TLS connections to reduce the impact of potential exploitation attempts
• Deploy a reverse proxy or load balancer in front of vulnerable services to provide an additional layer of protection
• Configure automatic service restart mechanisms to minimize downtime in the event of a crash
• Restrict network access to affected services using firewall rules where feasible

# Example: Update GnuTLS on Red Hat-based systems
sudo yum update gnutls

# Example: Update GnuTLS on Debian-based systems
sudo apt-get update && sudo apt-get upgrade gnutls-bin libgnutls30

# Verify installed GnuTLS version
gnutls-cli --version