CVE-2025-59719: Fortinet FortiWeb Auth Bypass Vulnerability

CVE-2025-59719 Overview

An improper verification of cryptographic signature vulnerability exists in Fortinet FortiWeb, a web application firewall (WAF) solution. This vulnerability allows an unauthenticated attacker to bypass FortiCloud SSO login authentication through a crafted SAML response message. The flaw stems from inadequate validation of cryptographic signatures during the SAML authentication process, enabling attackers to forge authentication assertions and gain unauthorized access to protected systems.

Critical Impact

Unauthenticated attackers can bypass FortiCloud SSO authentication entirely, potentially gaining full administrative access to FortiWeb management interfaces without valid credentials.

Affected Products

• Fortinet FortiWeb 8.0.0
• Fortinet FortiWeb 7.6.0 through 7.6.4
• Fortinet FortiWeb 7.4.0 through 7.4.9

Discovery Timeline

• December 9, 2025 - CVE-2025-59719 published to NVD
• December 9, 2025 - Last updated in NVD database

Technical Details for CVE-2025-59719

Vulnerability Analysis

This vulnerability is classified under CWE-347 (Improper Verification of Cryptographic Signature). The core issue resides in the SAML (Security Assertion Markup Language) authentication handler within FortiWeb's FortiCloud SSO integration. When processing SAML response messages from identity providers, FortiWeb fails to properly verify the cryptographic signatures that authenticate the integrity and origin of these assertions.

SAML authentication relies on XML-based assertions that are digitally signed to prevent tampering and forgery. When signature verification is improperly implemented or can be bypassed, attackers can craft malicious SAML responses that assert any identity they choose, including administrative accounts. This allows complete authentication bypass without requiring any prior access or credentials.

The network-based attack vector with no required user interaction or privileges makes this vulnerability particularly dangerous for internet-exposed FortiWeb deployments utilizing FortiCloud SSO for centralized authentication management.

Root Cause

The vulnerability originates from improper validation logic in the SAML signature verification routine. The affected FortiWeb versions do not adequately verify that the cryptographic signature on SAML response messages corresponds to a trusted identity provider's certificate. This may include scenarios where signature validation can be bypassed entirely through specially crafted SAML messages, unsigned assertions are accepted, or signature wrapping attacks succeed due to improper canonicalization handling.

Attack Vector

The attack is conducted over the network by an unauthenticated attacker targeting the FortiCloud SSO authentication endpoint. The attacker crafts a malicious SAML response message that either lacks a valid signature, contains a self-signed signature, or exploits signature verification weaknesses. When FortiWeb processes this crafted response, it incorrectly validates the assertion, granting the attacker access as the impersonated user.

The attack flow typically involves:

1. Attacker identifies a FortiWeb instance using FortiCloud SSO authentication
2. Attacker initiates an SSO login flow to receive a SAML authentication request
3. Instead of authenticating with the legitimate identity provider, the attacker crafts a malicious SAML response with forged assertions
4. The crafted response bypasses signature verification
5. FortiWeb accepts the forged identity, granting unauthorized access

For detailed technical information, refer to the Fortinet Security Advisory FG-IR-25-647.

Detection Methods for CVE-2025-59719

Indicators of Compromise

• Unusual SAML authentication requests with malformed or missing signature elements in web server logs
• Authentication successes from unexpected source IP addresses or geolocations that bypass normal identity provider flows
• Administrative access events without corresponding FortiCloud authentication records
• SAML assertions containing unexpected NameID values or attributes not matching legitimate user accounts

Detection Strategies

• Monitor FortiWeb authentication logs for SSO login events that lack corresponding FortiCloud authentication records
• Implement network intrusion detection rules to identify anomalous SAML response traffic patterns
• Enable detailed SAML debugging logs temporarily to identify signature verification bypass attempts
• Deploy web application firewall rules upstream to detect malformed SAML XML structures

Monitoring Recommendations

• Configure alerting on administrative login events, especially those occurring outside normal business hours
• Monitor for configuration changes following any SSO authentication events
• Implement centralized logging to correlate FortiWeb authentication events with FortiCloud identity provider logs
• Review authentication patterns for accounts that typically use alternative authentication methods

How to Mitigate CVE-2025-59719

Immediate Actions Required

• Upgrade affected FortiWeb installations to patched versions as specified in the Fortinet security advisory
• If immediate patching is not possible, consider disabling FortiCloud SSO authentication temporarily and using alternative authentication methods
• Restrict network access to FortiWeb management interfaces to trusted administrative networks only
• Review authentication logs for signs of exploitation prior to patching

Patch Information

Fortinet has released security updates to address this vulnerability. Administrators should consult the Fortinet Security Advisory FG-IR-25-647 for specific patched version information and upgrade instructions. Apply updates to all affected FortiWeb versions (8.0.0, 7.6.0-7.6.4, and 7.4.0-7.4.9) as soon as possible given the critical nature of this authentication bypass.

Workarounds

• Disable FortiCloud SSO integration and use local authentication or alternative identity provider configurations until patches can be applied
• Implement network-level access controls (ACLs, firewall rules) to restrict access to FortiWeb management interfaces from untrusted networks
• Enable multi-factor authentication for administrative access where supported as an additional security layer
• Deploy upstream security controls to filter potentially malicious SAML traffic before it reaches FortiWeb

# Example: Restrict management interface access via firewall (adjust for your environment)
# Limit access to FortiWeb management ports from trusted admin subnet only
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP