CVE-2026-22153: Fortinet FortiOS Auth Bypass Vulnerability

CVE-2026-22153 Overview

An Authentication Bypass by Primary Weakness vulnerability (CWE-305) has been identified in Fortinet FortiOS versions 7.6.0 through 7.6.4. This vulnerability may allow an unauthenticated attacker to bypass LDAP authentication of Agentless VPN or FSSO (Fortinet Single Sign-On) policy when the remote LDAP server is configured in a specific way. The flaw represents a significant security risk for organizations relying on FortiOS for network perimeter security and VPN access control.

Critical Impact

Unauthenticated attackers can bypass LDAP authentication mechanisms, potentially gaining unauthorized access to protected network resources through Agentless VPN or FSSO-protected systems.

Affected Products

• Fortinet FortiOS 7.6.0
• Fortinet FortiOS 7.6.1 through 7.6.3
• Fortinet FortiOS 7.6.4

Discovery Timeline

• 2026-02-10 - CVE-2026-22153 published to NVD
• 2026-02-12 - Last updated in NVD database

Technical Details for CVE-2026-22153

Vulnerability Analysis

This Authentication Bypass vulnerability exploits weaknesses in how FortiOS handles LDAP authentication for Agentless VPN and FSSO policies. The flaw is classified under CWE-305 (Authentication Bypass by Primary Weakness), which indicates that the primary authentication mechanism can be circumvented entirely under specific conditions.

The vulnerability requires network access but does not require any authentication or user interaction to exploit. When successful, an attacker could achieve complete confidentiality, integrity, and availability impact on the affected system. The attack complexity is considered high, as exploitation depends on specific LDAP server configurations being in place.

Organizations using FortiOS for VPN authentication with LDAP backends are particularly at risk. The vulnerability affects the core authentication flow, potentially allowing attackers to bypass security controls designed to verify user identity before granting network access.

Root Cause

The vulnerability stems from improper handling of authentication validation in the LDAP authentication pathway within FortiOS. Specifically, the authentication bypass occurs when the remote LDAP server is configured in a particular manner, creating a condition where the primary authentication check can be circumvented. This represents a failure in the authentication state machine to properly enforce credential verification under all configuration scenarios.

Attack Vector

The attack vector is network-based, meaning an attacker can exploit this vulnerability remotely without requiring local access to the target system. The exploitation path targets the Agentless VPN or FSSO policy authentication mechanisms. An unauthenticated remote attacker could craft authentication requests that bypass the LDAP verification process, potentially gaining access to resources that should be protected by VPN authentication policies.

The attack requires specific LDAP server configurations to be exploitable, which increases the complexity but does not eliminate the risk for affected deployments. Administrators should review their LDAP configurations and FortiOS versions to assess their exposure.

Detection Methods for CVE-2026-22153

Indicators of Compromise

• Unexpected VPN session establishments from unauthorized sources without corresponding valid LDAP authentication events
• Authentication logs showing successful VPN access without matching LDAP bind requests
• Anomalous access patterns to resources protected by FSSO policies from previously unseen endpoints
• LDAP server logs indicating authentication flow irregularities or missing expected queries

Detection Strategies

• Monitor FortiOS authentication logs for VPN sessions that lack corresponding LDAP authentication records
• Implement correlation rules between FortiOS VPN access logs and LDAP server authentication logs to identify discrepancies
• Deploy network traffic analysis to detect unusual VPN connection patterns or authentication protocol anomalies
• Review FortiOS event logs for authentication-related errors or warnings that may indicate exploitation attempts

Monitoring Recommendations

• Enable detailed logging for both FortiOS VPN authentication events and LDAP server authentication operations
• Configure alerts for VPN connections from unexpected geographic locations or IP ranges
• Establish baseline authentication patterns and alert on statistical deviations
• Monitor for multiple failed authentication attempts followed by successful access, which may indicate bypass attempts

How to Mitigate CVE-2026-22153

Immediate Actions Required

• Review and identify all FortiOS deployments running versions 7.6.0 through 7.6.4
• Consult the Fortinet Security Advisory FG-IR-25-1052 for specific remediation guidance
• Audit LDAP server configurations used with FortiOS Agentless VPN and FSSO policies
• Implement additional network segmentation to limit exposure of VPN-protected resources
• Enable enhanced authentication logging to detect potential exploitation attempts

Patch Information

Fortinet has released information regarding this vulnerability through their security advisory. Administrators should consult the Fortinet Security Advisory FG-IR-25-1052 for the latest patch information, updated firmware versions, and specific remediation steps. Organizations should prioritize upgrading to a patched version of FortiOS as soon as updates become available.

Workarounds

• Review LDAP server configurations and ensure they do not match vulnerable patterns described in the Fortinet advisory
• Consider implementing additional authentication factors (MFA) for VPN access as a defense-in-depth measure
• Restrict VPN access to known IP ranges where operationally feasible
• Temporarily disable Agentless VPN or FSSO policies if not business-critical until patching is complete
• Implement strict network ACLs to limit which systems can initiate VPN connections

# Example: Review FortiOS version to determine if affected
# Connect to FortiOS CLI and check version
get system status

# Review LDAP server configuration
show user ldap

# Check VPN authentication settings
show vpn ssl settings