CVE-2025-55018: Fortinet FortiOS Request Smuggling Flaw

CVE-2025-55018 Overview

CVE-2025-55018 is an HTTP Request Smuggling vulnerability affecting multiple versions of Fortinet FortiOS. This vulnerability arises from inconsistent interpretation of HTTP requests, allowing an unauthenticated attacker to smuggle unlogged HTTP requests through firewall policies using specially crafted headers. The flaw could enable attackers to bypass security controls and evade logging mechanisms, potentially facilitating further attacks against protected resources.

Critical Impact

Unauthenticated attackers can bypass firewall policies and evade logging by smuggling malicious HTTP requests through FortiOS firewalls using malformed headers.

Affected Products

• Fortinet FortiOS 7.6.0
• Fortinet FortiOS 7.4.0 through 7.4.9
• Fortinet FortiOS 7.2 all versions
• Fortinet FortiOS 7.0 all versions
• Fortinet FortiOS 6.4.3 through 6.4.16

Discovery Timeline

• 2026-02-10 - CVE-2025-55018 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2025-55018

Vulnerability Analysis

HTTP Request Smuggling vulnerabilities occur when front-end and back-end servers interpret HTTP request boundaries differently. In CVE-2025-55018, FortiOS exhibits inconsistent handling of HTTP headers, which creates a discrepancy in how requests are parsed. This allows an attacker to craft malicious HTTP requests that the firewall interprets as a single request while the back-end server processes them as multiple separate requests.

The core issue lies in how FortiOS handles ambiguous HTTP headers, particularly around Content-Length and Transfer-Encoding fields. By exploiting these parsing inconsistencies, attackers can inject unauthorized requests that bypass firewall policies entirely. Critically, these smuggled requests are not logged by FortiOS, making detection and forensic analysis significantly more difficult.

This vulnerability is classified under CWE-444 (Inconsistent Interpretation of HTTP Requests), which specifically addresses HTTP request smuggling scenarios where different components interpret request boundaries differently.

Root Cause

The root cause of this vulnerability is improper validation and normalization of HTTP request headers in the FortiOS HTTP parsing engine. When processing requests with ambiguous or malformed headers, FortiOS fails to consistently determine request boundaries, creating an opportunity for request smuggling attacks. The lack of strict RFC compliance in header parsing allows specially crafted requests to be interpreted differently by FortiOS compared to back-end servers.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Crafting HTTP requests with ambiguous headers that exploit parsing differences
2. Sending these requests through the FortiOS firewall to protected back-end servers
3. The smuggled request bypasses firewall policy enforcement
4. The attack remains unlogged, evading detection mechanisms

The vulnerability allows attackers to potentially access restricted resources, poison web caches, or perform request hijacking attacks against other users' sessions. The ability to evade logging significantly increases the risk as security teams may not be aware of ongoing exploitation attempts.

Detection Methods for CVE-2025-55018

Indicators of Compromise

• Unusual HTTP traffic patterns with malformed or duplicate Content-Length or Transfer-Encoding headers
• Discrepancies between firewall logs and back-end server access logs indicating unlogged requests
• Unexpected access to restricted resources without corresponding firewall log entries
• HTTP 400 or 502 errors on back-end servers that don't correlate with firewall activity

Detection Strategies

• Deploy deep packet inspection to identify HTTP requests with ambiguous or conflicting headers
• Implement log correlation between FortiOS firewalls and back-end servers to detect request discrepancies
• Monitor for HTTP requests containing both Content-Length and Transfer-Encoding headers
• Analyze network traffic for requests with malformed chunked encoding or unusual header sequences

Monitoring Recommendations

• Enable verbose HTTP logging on both firewall and back-end servers for comparison analysis
• Configure network intrusion detection systems to alert on HTTP request smuggling signatures
• Implement anomaly detection for unusual request patterns targeting protected applications
• Review Fortinet security advisories and apply detection signatures from FortiGuard

How to Mitigate CVE-2025-55018

Immediate Actions Required

• Review Fortinet Security Advisory FG-IR-25-667 for specific patch information and mitigation guidance
• Inventory all FortiOS devices running affected versions (6.4.3-6.4.16, 7.0.x, 7.2.x, 7.4.0-7.4.9, 7.6.0)
• Apply available patches from Fortinet as documented in the security advisory
• Enable strict HTTP protocol validation where available in FortiOS configuration

Patch Information

Fortinet has released security information regarding this vulnerability. Administrators should consult the official Fortinet Security Advisory FG-IR-25-667 for detailed patch availability and upgrade paths. Organizations should prioritize patching FortiOS devices exposed to untrusted networks, particularly those protecting web applications.

Workarounds

• Configure web application firewalls to normalize HTTP requests before they reach FortiOS
• Implement strict HTTP protocol validation on back-end servers to reject ambiguous requests
• Deploy reverse proxies that enforce consistent HTTP parsing to front FortiOS-protected applications
• Consider network segmentation to limit exposure of vulnerable FortiOS devices to untrusted traffic

# Recommended: Check current FortiOS version and plan upgrade
# Connect to FortiOS CLI and verify version
get system status

# Review HTTP inspection settings
config firewall profile-protocol-options
    edit "default"
        config http
            set inspect-all enable
        end
    next
end