CVE-2025-62826: 
FortiOS vulnerability analysis and mitigation

CVE-2025-62826 is an HTTP Response Splitting vulnerability (CWE-113) in Fortinet FortiOS and FortiProxy captive portal authentication that allows an attacker who can intercept and modify a user's authentication request to inject arbitrary HTTP headers via crafted requests. Affected products include FortiOS 7.6.0–7.6.4, FortiOS 7.4 all versions, FortiOS 7.2 all versions, FortiProxy 7.6.0–7.6.4, FortiProxy 7.4 all versions, and FortiProxy 7.2 all versions; FortiOS 8.0 is not affected. FortiPAM across multiple versions (1.0.x–1.7.x) is also listed as affected. The vulnerability was publicly disclosed on July 14, 2026, with a CVSSv3 score of 3.1 (Low) per Fortinet's advisory, though NVD assigns a base score of 4.3 (Medium) (FortiGuard Advisory).

The root cause is improper neutralization of CRLF sequences (carriage return \r and line feed \n) in HTTP headers within the captive portal authentication form, classified as CWE-113 (HTTP Response Splitting). An attacker positioned to intercept and modify a user's captive portal authentication request — a man-in-the-middle scenario — can embed CRLF sequences in crafted HTTP requests to inject arbitrary headers into the server's HTTP response. This technique can be used to manipulate response content, set malicious cookies, or facilitate cache poisoning. Exploitation requires user interaction (the victim must be performing a captive portal authentication) and network-level access to intercept the request (FortiGuard Advisory).

Successful exploitation allows an attacker to inject arbitrary HTTP headers into responses delivered to captive portal users, primarily affecting response integrity. The impact is limited to integrity (no confidentiality or availability impact per CVSS scoring), but could enable secondary attacks such as session hijacking via cookie injection, cross-site scripting via header manipulation, or cache poisoning affecting other users sharing the same proxy or cache. The attack scope is unchanged, meaning impact is confined to the targeted user's session rather than the broader system (FortiGuard Advisory).

1. Positioning: Establish a man-in-the-middle position on the network segment where captive portal users authenticate — for example, via ARP spoofing, rogue access point, or compromised network device.
2. Intercept authentication request: Capture an HTTP authentication request submitted by a user to the FortiOS/FortiProxy captive portal endpoint.
3. Inject CRLF payload: Modify the intercepted request to include CRLF sequences (%0d%0a or \r\n) within a header field or parameter that is reflected back in the HTTP response, such as a redirect URL or form parameter.
4. Deliver manipulated response: Forward the crafted request to the FortiOS/FortiProxy server; the server processes the unsanitized input and includes the injected headers in its HTTP response to the victim.
5. Achieve objective: Use the injected headers to set malicious Set-Cookie headers (session hijacking), inject a second HTTP response body (cache poisoning), or redirect the user to a phishing page (FortiGuard Advisory).

• Network: Unusual HTTP requests to captive portal endpoints containing URL-encoded CRLF sequences (%0d%0a, %0a, %0d) in header fields or query parameters; unexpected HTTP responses with duplicate or anomalous headers.
• Logs: FortiOS/FortiProxy access logs showing requests with encoded newline characters in parameter values targeting captive portal authentication paths; unexpected Set-Cookie or Location headers in responses not matching normal application behavior.
• Network: Traffic from unexpected intermediate hosts between clients and the FortiGate/FortiProxy captive portal, potentially indicating a man-in-the-middle position (ARP anomalies, duplicate MAC addresses).

Fortinet has released patches addressing this vulnerability: upgrade FortiOS to 7.6.5 or above, and FortiProxy to 7.6.5 or above. Users on FortiOS 7.4 or 7.2 (all versions) and FortiProxy 7.4 or 7.2 (all versions) should migrate to a fixed release, as no in-branch fix is available for those branches. Fortinet recommends using the official upgrade path tool at https://docs.fortinet.com/upgrade-tool. As a network-level mitigation, deploying TLS/SSL encryption for captive portal traffic and implementing network segmentation can reduce the risk of an attacker intercepting and modifying authentication requests (FortiGuard Advisory).

The vulnerability was reported to Fortinet by Vang3lis from VARAS@IIE under responsible disclosure and was published as part of a broader Fortinet patch release addressing seven vulnerabilities across FortiOS, FortiProxy, FortiPAM, and FortiSandbox. Security news outlets including CyberSecurityNews, GBHackers, and CyberPress covered the patch release, generally characterizing CVE-2025-62826 as a lower-severity issue within the batch. No significant independent researcher commentary or social media controversy has been observed regarding this specific CVE (FortiGuard Advisory).