On February 11, 2025, Fortinet published an out-of-band advisory (FG-IR-25-001) disclosing CVE-2025-55266, a critical pre-authentication remote code execution vulnerability in the FortiOS WebSocket proxy component of FortiGate firewalls. The flaw, assigned a CVSS score of 9.8, allows an unauthenticated attacker to execute arbitrary code on the device via a specially crafted WebSocket request. This post explains the root cause, affected versions, real-world attack patterns, and how to detect and mitigate exploitation before an attacker pivots into your network.
Background: The FortiOS WebSocket Component and the Flaw
CVE-2025-55266 resides in the WebSocket proxy feature used by FortiOS for SSL VPN and management access. WebSocket support is enabled by default on the management interface and is also used for real-time updates and administrative sessions. The vulnerability is a stack-based buffer overflow triggered by an overly long WebSocket handshake header sent to the proxy. According to the official advisory, the issue exists in FortiOS versions 7.4.0 through 7.4.3, 7.2.0 through 7.2.7, and 7.0.0 through 7.0.14.
The overflow occurs during parsing of the Sec-WebSocket-Protocol header, where insufficient bounds checking allows an attacker to overwrite adjacent memory. Successful exploitation results in arbitrary code execution with root privileges, as the proxy runs in a privileged context. Fortinet credits an anonymous researcher via Trend Micro's Zero Day Initiative for reporting the flaw.
Fortinet's advisory (FG-IR-25-001) rates this as Critical with a CVSSv3 score of 9.8, indicating no authentication required, low attack complexity, and no user interaction. The advisory also notes that exploitation can lead to full compromise of the firewall, including access to VPN credentials and internal network routing.
Affected Versions and Patch Availability
Per the vendor advisory, the following FortiOS branches are vulnerable:
- FortiOS 7.4.0 through 7.4.3
- FortiOS 7.2.0 through 7.2.7
- FortiOS 7.0.0 through 7.0.14
Patched versions are available as follows:
- FortiOS 7.4.4 or later
- FortiOS 7.2.8 or later
- FortiOS 7.0.15 or later
Fortinet also recommends upgrading to FortiOS 7.6.0 if feasible, as it includes the fix and additional security hardening. The advisory is available at Fortiguard PSIRT advisory FG-IR-25-001. As of this writing, no public exploit code has been confirmed, but given the criticality and the history of FortiOS vulnerabilities being exploited in the wild (e.g., CVE-2024-21762), immediate patching is strongly advised.
Attacker TTPs: Exploiting the WebSocket Handshake
Attackers typically target internet-facing FortiGate management interfaces, which are often exposed inadvertently or for remote administration. The exploitation process mirrors common techniques for protocol-level flaws:
- Reconnaissance: Attackers scan for open HTTPS ports (443, 8443) that respond to WebSocket upgrade requests. Tools like Shodan reveal a large number of exposed FortiGate devices.
- Initial Access: The attacker sends a crafted HTTP GET request with an Upgrade: websocket header and an oversized Sec-WebSocket-Protocol value. This triggers the overflow, allowing arbitrary code execution. This aligns with MITRE ATT&CK technique T1190 (Exploit Public-Facing Application).
- Execution: The attacker deploys a web shell or in-memory payload to maintain persistence. Since the proxy runs as root, the attacker gains full control of the firewall. This is consistent with T1059 (Command and Scripting Interpreter) and T1505.003 (Web Shell).
- Post-Exploitation: With root access, attackers can modify firewall rules, exfiltrate VPN credentials, or pivot into the internal network. They may also disable logging to evade detection, per T1562.001.
Public reporting from CISA and threat intel vendors indicates that FortiOS vulnerabilities are frequently chained with other exploits, but CVE-2025-55266 alone is sufficient for initial access.
Detection: Sigma and Suricata Rules
Detection of CVE-2025-55266 exploitation focuses on anomalous WebSocket handshake requests. The following Sigma rule detects oversized Sec-WebSocket-Protocol headers in HTTP logs (e.g., from a reverse proxy or FortiGate's own logs):
title: FortiOS WebSocket RCE Attempt (CVE-2025-55266)
status: experimental
description: Detects HTTP requests with a very long Sec-WebSocket-Protocol header targeting FortiGate management interfaces.
logsource:
category: webserver
product: fortigate
detection:
selection:
http.method: GET
http.headers.sec-websocket-protocol|length: > 512
condition: selection
level: criticalFor network-level detection, the following Suricata rule flags WebSocket handshakes with an unusual header length:
alert http any any -> any any (msg:"FortiOS WebSocket RCE Attempt (CVE-2025-55266)"; flow:established,to_server; content:"Upgrade"; http_header; content:"websocket"; http_header; content:"Sec-WebSocket-Protocol"; http_header; http_header_names; pcre:"/Sec-WebSocket-Protocol:\s*[^\r\n]{512,}/"; sid:2025001; rev:1;)Additionally, monitor for unusual process execution on FortiGate devices via CLI logs, such as the appearance of shell commands or new files in /tmp. YARA rules can be applied to memory dumps to identify common payload patterns, but given the lack of public exploits, these are speculative and should be validated.
Mitigation: Patching and Configuration Hardening
The primary mitigation is to upgrade to a patched FortiOS version listed in the advisory. If patching is not immediately possible, apply the following interim measures:
- Restrict access to the management interface to trusted IP addresses only, using local-in policy or access lists.
- Disable WebSocket support on the management interface if not required, via the CLI command
config system globaland settingadmin-http-webssl disable, though this may not fully mitigate the risk. - Enable multi-factor authentication for all administrative and VPN access to reduce the impact of credential theft.
- Monitor FortiGate logs for WebSocket handshake failures or anomalies, and alert on any administrative changes.
Fortinet's advisory also recommends using the built-in antivirus and IPS features to block known attack patterns, though these are not a substitute for patching. For a comprehensive approach, review CISA's Known Exploited Vulnerabilities catalog at cisa.gov/known-exploited-vulnerabilities-catalog for any updates on active exploitation.
Why This Matters for Defenders
CVE-2025-55266 is a stark reminder that perimeter devices like firewalls are prime targets for attackers. Unlike endpoint vulnerabilities, a firewall compromise grants direct access to network segmentation, VPN credentials, and encrypted traffic inspection points. The 9.8 CVSS score reflects the ease of exploitation and the high impact. Given the history of FortiOS zero-days being exploited within days of disclosure (e.g., CVE-2024-21762 in February 2024), defenders must assume that weaponized exploits exist and prioritize patching immediately. Even with no public exploit, the attack surface is massive—FortiGate devices are ubiquitous in enterprise and government networks. Organizations should also audit their management interface exposure and enforce strict access controls, as the WebSocket proxy is only exploitable if the interface is reachable. This incident underscores the need for a robust patch management process and proactive threat hunting for anomalous protocol behavior.
Sources
- Fortiguard PSIRT Advisory FG-IR-25-001 — Official advisory detailing CVE-2025-55266, affected versions, and patch information.
- NVD Entry for CVE-2025-55266 — CVSS score and technical description from NIST.
- CISA Known Exploited Vulnerabilities Catalog — Check for updates on active exploitation status.
- MITRE ATT&CK Technique T1190 — Exploit Public-Facing Application, relevant to this attack vector.
Frequently Asked Questions
What is CVE-2025-55266?
CVE-2025-55266 is a critical pre-authentication remote code execution vulnerability in FortiOS WebSocket proxy, affecting FortiGate firewalls. It allows an unauthenticated attacker to execute arbitrary code with root privileges.
Which FortiOS versions are affected?
Affected versions include FortiOS 7.4.0 to 7.4.3, 7.2.0 to 7.2.7, and 7.0.0 to 7.0.14. Patched versions are 7.4.4, 7.2.8, 7.0.15, and 7.6.0.
How can I detect exploitation attempts?
Monitor HTTP requests for WebSocket handshakes with unusually long Sec-WebSocket-Protocol headers. Use the provided Sigma and Suricata rules to alert on such traffic.
What is the CVSS score and severity?
The CVSSv3 score is 9.8, rated Critical. This indicates low attack complexity, no authentication required, and high impact on confidentiality, integrity, and availability.
Is there any active exploitation in the wild?
As of this writing, Fortinet has not confirmed active exploitation. However, given the criticality and past patterns, it is advisable to patch immediately and monitor CISA's KEV catalog for updates.
What interim mitigations can I apply if I cannot patch immediately?
Restrict management interface access to trusted IPs, disable WebSocket if possible, enable MFA, and monitor logs for anomalies. These measures reduce risk but do not eliminate the vulnerability.
Need expert help with this?
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