CVE-2025-3110: OpenVPN Access Server Smuggling Flaw

CVE-2025-3110 Overview

CVE-2025-3110 affects OpenVPN Access Server versions 2.7.2 through 3.1.0. The server accepts bare line-feed (LF) sequences inside HTTP header values instead of enforcing carriage-return line-feed (CRLF) delimiters. Remote attackers can exploit this parsing weakness to perform HTTP request smuggling when the server is deployed behind a reverse proxy. The flaw is classified as CWE-444, Inconsistent Interpretation of HTTP Requests.

Critical Impact
Attackers can smuggle malicious HTTP requests past front-end reverse proxies, potentially bypassing security controls, poisoning caches, or accessing restricted endpoints on OpenVPN Access Server.

Affected Products

OpenVPN Access Server 2.7.2
OpenVPN Access Server versions 2.7.2 through 3.1.0
OpenVPN Access Server deployments behind reverse proxies

Discovery Timeline

2026-07-08 - CVE-2025-3110 published to NVD
2026-07-08 - Last updated in NVD database

Technical Details for CVE-2025-3110

Vulnerability Analysis

CVE-2025-3110 is an HTTP request smuggling vulnerability in the HTTP parser of OpenVPN Access Server. RFC 7230 requires HTTP headers to terminate with CRLF sequences. The affected OpenVPN Access Server versions relax this requirement and accept bare LF characters as valid header terminators. When a reverse proxy sits in front of the server, the two components can disagree on where one HTTP request ends and the next begins. This desynchronization is the core condition required for request smuggling attacks.

Attackers craft a single TCP stream containing embedded bare LF characters inside header values. The front-end proxy treats the stream as one request, while OpenVPN Access Server interprets it as two. The smuggled second request executes with attributes derived from the trusted proxy connection, bypassing controls applied at the perimeter.

Root Cause

The root cause is permissive HTTP header parsing that violates strict CRLF framing. Accepting bare LF inside header values creates inconsistent interpretation between upstream and downstream HTTP processors, matching the [CWE-444] pattern.

Attack Vector

Exploitation requires network access to the reverse proxy fronting OpenVPN Access Server. No authentication or user interaction is required. The attacker sends a specially formed HTTP request containing bare LF sequences within header values. The proxy forwards the request as a single message, while the OpenVPN parser splits it, resulting in a smuggled request. Consequences include cache poisoning, request routing manipulation, and bypass of proxy-enforced authorization or filtering rules.

Refer to the OpenVPN AS 3.2 Release Notes for the vendor's technical description of the fix.

Detection Methods for CVE-2025-3110

Indicators of Compromise

HTTP requests containing bare LF (0x0A) bytes inside header values without a preceding CR (0x0D).
Unexpected duplicate or malformed request lines in OpenVPN Access Server access logs.
Discrepancies between the number of requests logged by the reverse proxy and by the OpenVPN Access Server.

Detection Strategies

Inspect raw HTTP traffic at the proxy edge for bare LF byte sequences inside header sections and alert on any occurrence.
Correlate request counts and request identifiers across the reverse proxy and OpenVPN Access Server logs to surface desynchronization.
Deploy web application firewall rules that reject any HTTP request whose headers are not strictly CRLF-terminated.

Monitoring Recommendations

Enable verbose HTTP logging on both the reverse proxy and OpenVPN Access Server for post-incident correlation.
Monitor for anomalous request patterns targeting the OpenVPN Access Server web UI and API endpoints.
Track the OpenVPN Access Server version and configuration drift across all deployments to confirm patch status.

How to Mitigate CVE-2025-3110

Immediate Actions Required

Upgrade OpenVPN Access Server to version 3.2 or later, which enforces strict CRLF header parsing.
Inventory all OpenVPN Access Server instances deployed behind reverse proxies and prioritize them for patching.
Review reverse proxy configurations to ensure they normalize or reject malformed HTTP framing.

Patch Information

OpenVPN addressed CVE-2025-3110 in OpenVPN Access Server 3.2. The release enforces strict CRLF header termination and rejects bare LF characters in HTTP header values. Review the OpenVPN AS 3.2 Release Notes for upgrade instructions and version-specific guidance.

Workarounds

Configure the reverse proxy to reject or rewrite any HTTP request containing bare LF characters in header values.
Restrict network access to OpenVPN Access Server administrative interfaces to trusted management networks until patching is complete.
Enable strict HTTP protocol validation on any WAF or API gateway positioned in front of OpenVPN Access Server.

bash

# Example NGINX hardening: reject requests with bare LF in headers
# Requires a module or Lua rule to inspect raw request bytes
http {
    # Enforce strict HTTP parsing where supported
    ignore_invalid_headers off;
    underscores_in_headers off;
    large_client_header_buffers 4 8k;

    server {
        listen 443 ssl;
        server_name vpn.example.com;

        # Drop connections with malformed framing
        if ($http_upgrade ~* "\n") { return 400; }

        location / {
            proxy_pass https://openvpn-as-backend;
            proxy_http_version 1.1;
        }
    }
}