CVE-2025-65328: Mega-Fence Auth Bypass Vulnerability

CVE-2025-65328 Overview

CVE-2025-65328 is an IP spoofing vulnerability in Mega-Fence (webgate-lib.*) version 25.1.914 and prior. The vulnerability exists because the application trusts the first value of the X-Forwarded-For (XFF) header as the client IP address without properly validating a trusted proxy chain. This allows attackers to supply an arbitrary XFF value in remote requests to spoof their client IP address, which is then propagated to security-relevant state such as the WG_CLIENT_IP cookie. Organizations that rely on this value for IP allowlists may have their access controls bypassed.

Critical Impact

Attackers can bypass IP-based access controls and allowlists by spoofing their client IP address through crafted X-Forwarded-For headers, potentially gaining unauthorized access to restricted resources.

Affected Products

• Mega-Fence (webgate-lib.*) version 25.1.914
• Mega-Fence (webgate-lib.*) versions prior to 25.1.914
• Any deployment relying on WG_CLIENT_IP cookie for IP-based access control

Discovery Timeline

• 2026-01-05 - CVE-2025-65328 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-65328

Vulnerability Analysis

This vulnerability falls under CWE-807 (Reliance on Untrusted Inputs in a Security Decision). The Mega-Fence webgate-lib component processes incoming HTTP requests and extracts client IP information from the X-Forwarded-For header. Instead of validating that the request originated from a trusted proxy and properly parsing the XFF header chain, the application blindly trusts the first value in the header as the legitimate client IP.

The attack is network-based and requires no authentication or user interaction, making it easily exploitable in environments where Mega-Fence is deployed. The vulnerability enables confidentiality and integrity impacts through unauthorized access bypass.

Root Cause

The root cause is improper validation of the X-Forwarded-For header chain. The application fails to:

1. Verify that incoming requests with XFF headers actually originate from trusted proxy servers
2. Properly parse and validate the entire XFF header chain to identify the true client IP
3. Implement allowlisting for trusted proxy IP addresses before accepting XFF values

This results in the application accepting any attacker-supplied value as the legitimate client IP address.

Attack Vector

The attack vector is network-based and exploits the trust relationship between the application and HTTP headers. An attacker can craft a malicious HTTP request with a forged X-Forwarded-For header containing an IP address that is on the target's allowlist. When Mega-Fence processes this request, it extracts the spoofed IP from the XFF header and sets it in the WG_CLIENT_IP cookie, effectively bypassing IP-based access restrictions.

The attack flow involves:

1. Attacker identifies an IP address that is allowlisted by the target deployment
2. Attacker sends a request with X-Forwarded-For: [allowlisted-IP] header
3. Mega-Fence extracts this value and treats it as the client's legitimate IP
4. The spoofed IP is propagated to the WG_CLIENT_IP cookie
5. IP-based access controls are bypassed, granting unauthorized access

For additional technical details, see the GitHub CVE-2025-65328 Document.

Detection Methods for CVE-2025-65328

Indicators of Compromise

• Requests containing X-Forwarded-For headers with IP addresses that don't match the actual source IP
• Unusual patterns in WG_CLIENT_IP cookie values that don't correlate with known legitimate traffic sources
• Access log entries showing internal or allowlisted IPs originating from external network segments

Detection Strategies

• Implement logging to compare the actual source IP of requests against the X-Forwarded-For header values
• Monitor for requests where the XFF header contains IP addresses from allowlisted ranges but originate from untrusted external sources
• Deploy Web Application Firewall (WAF) rules to detect and alert on suspicious XFF header patterns

Monitoring Recommendations

• Enable detailed HTTP header logging on reverse proxies and web application firewalls
• Create alerts for anomalous IP address patterns in the WG_CLIENT_IP cookie
• Implement correlation rules between network flow data and application-layer client IP values to identify discrepancies

How to Mitigate CVE-2025-65328

Immediate Actions Required

• Review all deployments using Mega-Fence webgate-lib version 25.1.914 and prior for IP-based access control reliance
• Implement upstream proxy validation to ensure XFF headers are only accepted from trusted reverse proxy servers
• Add network-level access controls to supplement IP-based application restrictions
• Strip or ignore X-Forwarded-For headers from requests not originating from trusted proxies

Patch Information

Monitor the vendor's release channels for a security update that properly validates the X-Forwarded-For header chain. Additional information may be available through the Proton Drive Secure Link or the GitHub CVE-2025-65328 Document.

Workarounds

• Configure upstream reverse proxies to strip and re-add the X-Forwarded-For header with verified client IP information
• Implement IP-based restrictions at the network layer (firewall rules) rather than relying solely on application-level XFF parsing
• Use alternative client identification mechanisms that don't rely on easily spoofed headers

# Example: Nginx configuration to strip and reset X-Forwarded-For
# Place this in your reverse proxy configuration
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header X-Real-IP $remote_addr;
# This ensures only the direct connection IP is passed, not client-supplied values