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 possesses a valid web filter override token can exploit this vulnerability by tricking a user into clicking on a specially crafted link. This allows the attacker to inject arbitrary headers into HTTP responses.

The vulnerability exists in specific versions of FortiOS (7.6.0 through 7.6.4, all 7.4 versions, and all 7.2 versions) and FortiProxy (7.6.0 through 7.6.4, all 7.4 versions, and all 7.2 versions).

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Detection Guidance

Detecting this vulnerability requires checking for signs of HTTP response splitting attacks or unusual header injections in logs. Since this vulnerability involves tricking a user into clicking a crafted link to inject arbitrary headers, detection may focus on monitoring web traffic and logs for suspicious activity.

• Review FortiOS and FortiProxy logs for unusual HTTP headers or unexpected CRLF sequences (e.g., %0d%0a) in URLs or headers.
• Use network monitoring tools to inspect HTTP responses for split responses or injected headers, particularly in requests involving web filter override tokens.
• Check for the presence of vulnerable versions of FortiOS (7.6.0 through 7.6.4, all 7.4 and 7.2 versions) or FortiProxy (7.6.0 through 7.6.4, all 7.4 and 7.2 versions) in your environment.

No specific commands are provided in the context, but you can use tools like curl or Burp Suite to manually test for header injection by crafting requests with CRLF sequences. Example (use with caution):

• curl -v "http://<target>/path?param=value%0d%0aInjected-Header: malicious" -H "Web-Filter-Override-Token: <valid_token>"

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Impact Analysis

If exploited, this vulnerability could have several impacts:

• An attacker could manipulate HTTP responses, potentially leading to cross-site scripting (XSS) attacks or other client-side attacks.
• The attacker could inject malicious headers, which might be used to bypass security controls or mislead users.
• Since the attack requires tricking a user into clicking a crafted link, it could be used in phishing campaigns to target specific individuals or organizations.

The CVSS score of 3.4 indicates a low severity, but the impact depends on the context of the affected system and how it is used in your environment.

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Compliance Impact

This vulnerability could potentially affect compliance with standards and regulations in the following ways:

• GDPR: If the vulnerability leads to unauthorized access or manipulation of personal data, it could result in a violation of GDPR's data protection requirements. Organizations must ensure the integrity and confidentiality of personal data.
• HIPAA: For organizations handling protected health information (PHI), this vulnerability could pose a risk if it allows unauthorized access or tampering with sensitive data. HIPAA requires safeguards to protect the integrity and security of PHI.
• Other standards: Depending on the industry, this vulnerability might also impact compliance with frameworks like PCI DSS (if payment data is involved) or ISO 27001, which require secure handling of data and protection against injection attacks.

While the vulnerability itself does not directly violate these regulations, failure to mitigate it could lead to non-compliance if it results in a data breach or loss of data integrity.

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Mitigation Strategies

To mitigate this vulnerability, follow these immediate steps:

• Upgrade FortiOS and FortiProxy to a fixed version as soon as patches are available from Fortinet. The context does not specify a patched version, so monitor Fortinet's security advisories for updates.
• Restrict access to web filter override tokens to minimize the risk of exploitation. Ensure only authorized users can generate or use these tokens.
• Implement web application firewall (WAF) rules to block requests containing CRLF sequences or suspicious header patterns.
• Educate users about the risks of clicking on untrusted links, especially those that may contain unusual characters or parameters.
• Monitor network traffic for signs of exploitation, such as unexpected headers or split HTTP responses.

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