CVE-2025-13086 - OpenVPN Fix Released for HMAC State Exhaustion

CVE-2025-13086 affects OpenVPN 2.6.16 and earlier, enabling attackers to overwhelm TLS handshake processing by exploiting a flawed memcmp check in the HMAC verification routine. This vulnerability weakens the state exhaustion protections within OpenVPN’s three-way handshake, potentially allowing spoofed handshake packets to bypass rate-limiting controls. The vulnerability maps to MITRE ATT&CK technique T1499 (Endpoint Denial of Service) and is especially relevant to organizations with high-traffic VPN gateways. Keywords such as "OpenVPN vulnerability", "state exhaustion attack", and "TLS handshake flaw" naturally support SEO relevance. Slackware’s security advisory :contentReference[oaicite:3]{index=3} confirms that the bug renders HMAC-based handshake protections inefficient, enabling adversaries to degrade or temporarily deny access to OpenVPN servers. Although exploitation does not grant remote code execution or authentication bypass, it enables targeted disruption and increases the feasibility of broader denial-of-service chains. The flaw impacts both Slackware 15.0 and -current branches, with patched packages released across x86, x86_64, and i686 architectures. From a business risk perspective, this vulnerability threatens remote-work infrastructure, site-to-site tunnels, and operational continuity for organizations dependent on OpenVPN. Successful exploitation can disrupt authentication, slow down or block legitimate connections, and trigger cascading service degradation. Compliance frameworks such as PCI-DSS and ISO 27001 require availability protections, making unpatched VPN endpoints a potential compliance violation. Mitigation is straightforward: upgrade to the patched OpenVPN 2.6.16 packages provided in Slackware's advisory or apply equivalent vendor patches where applicable. Organizations should harden VPN endpoints by enabling rate-limiting, monitoring handshake anomalies, and evaluating DDoS protections around VPN ingress points.