Fortinet Confirms Active FortiCloud SSO Bypass on Fully Patched FortiGate Firewalls

CVE-2025-59718 is a critical (CVSS 9.8) improper verification of cryptographic signature flaw (CWE-347) in the FortiCloud SSO login flow of Fortinet FortiOS, FortiProxy, and FortiSwitchManager, with the Siemens RUGGEDCOM APE1808 appliance also listed in the CVE's affected CPE entries. An unauthenticated attacker who can reach a device's FortiCloud SSO login can submit a crafted SAML response message whose cryptographic signature is not properly verified, bypassing authentication entirely. The bypass grants unauthorized access to the affected device with high impact on confidentiality, integrity, and availability, typically administrative control of the management interface. Any organization running the affected FortiOS 7.0–7.6, FortiProxy 7.0–7.6, or FortiSwitchManager 7.0–7.2 versions that uses FortiCloud SSO for administrative login is exposed. Exploitation is confirmed in the wild: the flaw was added to CISA's Known Exploited Vulnerabilities catalog on 2025-12-16, security reporting describes active attacks against FortiGate firewalls via this SAML SSO bypass, and EPSS assigns a 68.3% probability of exploitation within 30 days.

Do: Upgrade all affected products out of the vulnerable ranges — FortiOS beyond 7.6.3/7.4.8/7.2.11/7.0.17, FortiProxy beyond 7.6.3/7.4.10/7.2.14/7.0.21, and FortiSwitchManager beyond 7.2.6/7.0.5 — using the fixed builds listed in Fortinet's security advisory, and patch Siemens RUGGEDCOM APE1808 firmware per Siemens guidance. As interim mitigation, disable or restrict FortiCloud SSO-based administrative login, limit management-interface exposure to trusted networks, and review admin/SSO logs for anomalous sign-ins or forged SAML responses. Given the KEV listing, US federal agencies must apply vendor mitigations or discontinue use of affected products per BOD 22-01.

FortiWeb contains an improper verification of cryptographic signature (CWE-347) in its FortiCloud SSO login flow, allowing an unauthenticated attacker to bypass authentication by submitting a crafted SAML response whose signature is not properly validated. Because this requires no privileges or user interaction and is network-reachable, successful exploitation grants the attacker the access of a legitimate SSO-authenticated administrator to the appliance's management interface. The flaw affects FortiWeb 8.0.0, 7.6.0 through 7.6.4, and 7.4.0 through 7.4.9. Organizations running these versions are affected, particularly where the management interface is reachable and FortiCloud SSO login is enabled. As of this analysis the flaw is not in the CISA KEV catalog and no public proof-of-concept is known, but a closely related SAML SSO authentication bypass in FortiGate firewalls (CVE-2025-59718) is under active attack and Fortinet has issued urgent authentication patches, so elevated exploitation risk is plausible.

Do: Upgrade FortiWeb to a patched release per Fortinet's PSIRT advisory covering CVE-2025-59719, prioritizing internet-facing appliances on 8.0.0, 7.6.x, or 7.4.x. As interim mitigation, restrict access to the management interface, disable or limit FortiCloud SSO login in favor of local or hardened admin authentication, and review SSO login logs for successful authentications from unexpected sources. Note that the sibling FortiGate SAML bypass (CVE-2025-59718) is being actively exploited, so treat this patch as urgent.