CVE-2025-25255 is a vulnerability identified in Fortinet FortiOS versions 7.6.0 through 7.6.3 and multiple FortiProxy versions (7.6.0 through 7.6.3, 7.4.0 through 7.4.11, 7.2 all versions, and 7.0.1 through 7.0.22). The root cause is an improperly implemented security check (classified under CWE-358: Improperly Implemented Security Check for Standard) that leads to improper access control. Specifically, this flaw allows an unauthenticated proxy user to bypass the domain fronting protection feature by sending specially crafted HTTP requests. Domain fronting protection is a security mechanism designed to prevent attackers from disguising malicious traffic by manipulating domain names in HTTP headers. By bypassing this protection, attackers could potentially route traffic through the proxy in unauthorized ways, undermining security policies and controls. The vulnerability does not require any authentication or user interaction, making it easier to exploit remotely. However, the impact is limited to integrity, as there is no direct confidentiality or availability compromise reported. The CVSS v3.1 base score is 4.8 (medium severity), with the vector indicating network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), no confidentiality impact (C:N), low integrity impact (I:L), and no availability impact (A:N). No known exploits are currently reported in the wild, but the vulnerability is publicly disclosed and should be addressed promptly. The issue affects a broad range of Fortinet products widely used in enterprise and critical infrastructure environments for network security and proxy services.

For European organizations, this vulnerability poses a risk primarily to the integrity of network traffic passing through Fortinet FortiOS and FortiProxy devices. Attackers could exploit this flaw to bypass domain fronting protections, potentially allowing unauthorized traffic routing or evasion of security policies. This could facilitate further attacks such as data exfiltration, command and control communication, or lateral movement within networks. While confidentiality and availability are not directly impacted, the integrity compromise can undermine trust in network security controls and complicate incident response. Organizations in sectors with stringent security requirements—such as finance, government, telecommunications, and critical infrastructure—may face increased risk due to the reliance on Fortinet devices for perimeter and proxy security. The lack of authentication requirement increases the attack surface, enabling remote exploitation from external networks. However, the absence of known exploits in the wild and the medium severity rating suggest that immediate widespread exploitation is unlikely but should not be discounted. The vulnerability could be leveraged as part of a multi-stage attack chain by sophisticated threat actors targeting European enterprises.

1. Apply vendor patches and updates as soon as they become available for all affected FortiOS and FortiProxy versions. Monitor Fortinet advisories closely for patch releases. 2. Implement strict network segmentation and access controls to limit exposure of Fortinet proxy devices to untrusted networks. 3. Deploy advanced network traffic monitoring and anomaly detection systems to identify unusual HTTP request patterns that may indicate attempts to bypass domain fronting protections. 4. Review and tighten proxy and firewall policies to restrict allowed HTTP headers and domain names, reducing the risk of crafted requests exploiting this vulnerability. 5. Conduct regular security audits and penetration tests focusing on proxy and domain fronting controls to validate the effectiveness of mitigations. 6. Educate security operations teams about this vulnerability to improve detection and response capabilities. 7. Consider deploying additional layers of security such as web application firewalls (WAFs) and intrusion prevention systems (IPS) that can detect and block suspicious proxy traffic. 8. Maintain an inventory of all Fortinet devices and their firmware versions to ensure timely patch management and vulnerability tracking.