Improper CRLF Injection in Fortinet FortiOS and FortiProxy

Executive Summary

This vulnerability is an Improper Neutralization of CRLF Sequences in HTTP Headers, also known as HTTP Response Splitting (CWE-113). It affects Fortinet FortiOS and FortiProxy products.

An attacker who can intercept and modify a user's captive portal authentication request may exploit this flaw. By crafting malicious HTTP requests, the attacker can inject arbitrary headers into the HTTP response. This could allow the attacker to manipulate the HTTP response, potentially leading to further attacks such as cross-site scripting (XSS) or cache poisoning.

• Affected versions include FortiOS 7.6.0 through 7.6.4, all versions of FortiOS 7.4 and 7.2, and FortiProxy 7.6.0 through 7.6.4, as well as all versions of FortiProxy 7.4 and 7.2.

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Detection Guidance

Detecting CVE-2025-62826 (HTTP Response Splitting) on your network or system requires monitoring for unusual HTTP headers or unexpected behavior in captive portal authentication requests. Since this vulnerability involves an attacker intercepting and modifying HTTP requests to inject arbitrary headers, detection methods focus on identifying anomalous HTTP traffic patterns.

• Use network monitoring tools like Wireshark or tcpdump to capture and analyze HTTP traffic. Look for unusual CRLF sequences (carriage return and line feed characters) in HTTP headers, which may indicate an attempt to exploit this vulnerability.
• Example tcpdump command to capture HTTP traffic on port 80 or 443: tcpdump -i <interface> -A -s 0 'tcp port 80 or tcp port 443' | grep -i 'HTTP/1.1\|HTTP/2\|\r\n'. This filters for HTTP headers and CRLF sequences.
• Check Fortinet device logs (FortiOS or FortiProxy) for unusual or malformed HTTP requests, particularly those involving captive portal authentication. Look for unexpected headers or responses that deviate from normal behavior.
• Deploy an intrusion detection system (IDS) or web application firewall (WAF) to detect and alert on suspicious HTTP requests containing CRLF sequences or header injection attempts.

Since this vulnerability is specific to Fortinet FortiOS and FortiProxy, ensure your monitoring tools are configured to inspect traffic to and from these devices.

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Impact Analysis

If you are using an affected version of Fortinet FortiOS or FortiProxy, this vulnerability could have several impacts:

• An attacker could manipulate HTTP responses, leading to unauthorized actions or data exposure.
• The attacker may inject malicious content into web pages viewed by users, potentially leading to phishing attacks or the theft of sensitive information such as session cookies.
• The vulnerability could be used to bypass security controls, such as those enforced by captive portals, leading to unauthorized network access.

The CVSS v3.1 base score for this vulnerability is 4.3, indicating a medium severity level. The attack vector is network-based, and exploitation requires user interaction (e.g., a user visiting a malicious site or clicking a crafted link).

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Compliance Impact

This vulnerability could impact compliance with several standards and regulations, depending on the context in which the affected products are used:

• GDPR (General Data Protection Regulation): If the vulnerability leads to unauthorized access to or manipulation of personal data, it could result in a breach of GDPR requirements. Organizations must ensure the confidentiality and integrity of personal data, and failure to patch or mitigate this vulnerability could be seen as a violation of these obligations.
• HIPAA (Health Insurance Portability and Accountability Act): For organizations handling protected health information (PHI), this vulnerability could lead to unauthorized access or disclosure of PHI. HIPAA requires safeguards to protect the integrity and confidentiality of PHI, and exploitation of this vulnerability could result in non-compliance.
• PCI DSS (Payment Card Industry Data Security Standard): If the affected systems process or transmit payment card data, this vulnerability could lead to unauthorized access or manipulation of that data. PCI DSS requires organizations to maintain a secure network and protect cardholder data, and this vulnerability could undermine those requirements.

Organizations using the affected Fortinet products should assess their risk exposure and take appropriate actions, such as applying patches or mitigations, to maintain compliance with relevant regulations.

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Mitigation Strategies

To mitigate CVE-2025-62826, follow these immediate steps to reduce the risk of exploitation:

• Apply the latest security patches or updates provided by Fortinet for FortiOS and FortiProxy. Since the vulnerability affects multiple versions, check Fortinet's official security advisories for the appropriate patch.
• If patching is not immediately possible, restrict access to captive portal authentication endpoints to trusted networks or IP ranges. This limits the attack surface for potential exploitation.
• Enable and configure a web application firewall (WAF) to block HTTP requests containing CRLF sequences or other signs of header injection attacks.
• Monitor network traffic for signs of exploitation, such as unusual HTTP headers or unexpected responses, and investigate any suspicious activity.
• Review and harden the configuration of FortiOS and FortiProxy devices to ensure they are not unnecessarily exposing captive portal authentication endpoints to untrusted networks.

For long-term mitigation, consider upgrading to a supported version of FortiOS or FortiProxy that is not affected by this vulnerability, as listed in Fortinet's advisories.

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