CVE-2025-14831: GnuTLS Denial of Service Vulnerability

CVE-2025-14831 Overview

CVE-2025-14831 is a denial-of-service (DoS) vulnerability in GnuTLS, a widely deployed open-source Transport Layer Security (TLS) library. The flaw allows a remote attacker to trigger excessive CPU and memory consumption by presenting specially crafted certificates that contain a large number of name constraints and Subject Alternative Names (SANs). During certificate validation, GnuTLS performs computationally expensive operations that scale poorly with these fields, resulting in resource exhaustion. The weakness is categorized under CWE-407: Inefficient Algorithmic Complexity. No authentication or user interaction is required, and the attack can be delivered over the network to any service performing GnuTLS-based certificate validation.

Critical Impact
Remote attackers can exhaust CPU and memory on any server or client using GnuTLS for certificate validation, disrupting TLS-terminating services without credentials.

Affected Products

GnuTLS (upstream library, tracked in GnuTLS GitLab Issue #1773)
Red Hat Enterprise Linux distributions shipping GnuTLS, per Red Hat CVE Reference CVE-2025-14831
Siemens industrial products referenced in Siemens Security Advisory SSA-032379

Discovery Timeline

2026-02-09 - CVE-2025-14831 published to the National Vulnerability Database (NVD)
2026-06-30 - Last updated in NVD database

Technical Details for CVE-2025-14831

Vulnerability Analysis

GnuTLS validates X.509 certificates by checking Subject Alternative Names (SANs) against name constraints embedded in issuer certificates. When a certificate presents a large number of SAN entries combined with many name constraint entries, the validation routine performs a cross-product of comparisons. The algorithm exhibits inefficient complexity, causing runtime and memory allocations to grow non-linearly with input size. An attacker who supplies such a certificate during a TLS handshake or certificate import operation forces the target to consume disproportionate CPU cycles and memory. Repeated requests amplify the effect, degrading or halting service availability. The issue affects any application that links against GnuTLS and processes attacker-controlled certificate data, including TLS servers requesting client authentication and clients connecting to malicious endpoints.

Root Cause

The root cause is an algorithmic complexity weakness [CWE-407] in the name-constraint verification logic. GnuTLS iterates over every SAN entry and compares it against every constraint, without imposing sane upper bounds or early-exit heuristics. Attackers weaponize this by embedding thousands of entries into a single certificate, turning routine validation into a resource-exhaustion primitive. Full technical discussion is available in GnuTLS GitLab Issue #1773 and Red Hat Bugzilla Entry #2423177.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. A malicious peer presents a crafted certificate during a TLS handshake. Servers configured for mutual TLS (mTLS) are particularly exposed because they accept and validate client certificates from untrusted peers. Clients that connect to attacker-controlled TLS endpoints, PKI utilities that import certificates, and email or VPN gateways that process S/MIME or IKE certificates are also affected. The vulnerability produces availability impact only, with no confidentiality or integrity consequences.

No verified public proof-of-concept code is available. The vulnerability manifests during name-constraint processing when validating certificates with pathological SAN and constraint counts. Refer to GnuTLS GitLab Issue #1773 for reproduction details.

Detection Methods for CVE-2025-14831

Indicators of Compromise

Sustained high CPU utilization on processes linked against libgnutls, such as httpd, nginx, postfix, openldap, or cups, correlated with inbound TLS handshakes.
Memory growth in TLS-terminating services followed by out-of-memory (OOM) kills recorded in dmesg or journalctl.
TLS handshake latency spikes or timeouts reported by monitoring probes against services using GnuTLS.
Inbound X.509 certificates containing unusually large SAN lists or name constraint extensions captured in packet traces.

Detection Strategies

Inspect TLS traffic with a network sensor and flag client or server certificates that exceed reasonable SAN or name-constraint entry counts.
Correlate CPU and memory anomalies on TLS-facing services with concurrent inbound handshake events from the same source IP.
Enable audit logging in TLS-terminating applications to capture certificate fingerprints and peer addresses for post-incident triage.

Monitoring Recommendations

Track resource utilization metrics (CPU, RSS, handshake duration) for every service linking libgnutls and alert on deviations from baseline.
Rate-limit inbound TLS handshakes per source IP at load balancers or reverse proxies to reduce amplification.
Aggregate TLS handshake failure logs into a central platform for anomaly detection and retention.

How to Mitigate CVE-2025-14831

Immediate Actions Required

Inventory all systems and applications that link against GnuTLS and prioritize internet-facing TLS terminators for patching.
Apply vendor-supplied updates using the appropriate Red Hat Security Advisory, such as RHSA-2026:3477, RHSA-2026:4188, or RHSA-2026:13812, matching your distribution.
Restart all services that load libgnutls after upgrading to ensure the patched library is in memory.
Review mTLS-enabled endpoints and consider temporarily disabling client-certificate authentication where operationally acceptable.

Patch Information

Red Hat has released fixes across multiple product streams, tracked in advisories including RHSA-2026:3477, RHSA-2026:4188, RHSA-2026:4655, RHSA-2026:5585, RHSA-2026:6618, RHSA-2026:7329, RHSA-2026:8746, RHSA-2026:13812, RHSA-2026:16008, RHSA-2026:25096, and RHSA-2026:33125. Siemens customers should consult Siemens Security Advisory SSA-032379. Upstream tracking is available at Red Hat CVE Reference CVE-2025-14831.

Workarounds

Place a reverse proxy or TLS-terminating load balancer that does not rely on GnuTLS in front of vulnerable services until patches are applied.
Enforce connection concurrency and rate limits per source IP to blunt resource-exhaustion attempts.
Where feasible, restrict which certificate authorities (CAs) are trusted for client authentication so that only issuers producing well-formed certificates are accepted.
Monitor process memory limits with systemd unit MemoryMax or cgroup controls to contain runaway allocations.

bash

# Example: constrain a GnuTLS-linked service via systemd overrides
sudo systemctl edit httpd.service
# Add the following in the override editor:
[Service]
MemoryMax=2G
CPUQuota=200%
TasksMax=512
# Reload and restart
sudo systemctl daemon-reload
sudo systemctl restart httpd.service