CVE-2026-22153: 
FortiOS 脆弱性の分析と軽減

CVE-2026-22153 is an Authentication Bypass by Primary Weakness (CWE-305) vulnerability in Fortinet FortiOS affecting the fnbamd daemon. It allows unauthenticated attackers to bypass LDAP authentication for Agentless VPN or FSSO (Fortinet Single Sign-On) policies when the remote LDAP server is configured in a specific way (i.e., permitting unauthenticated binds). The vulnerability affects FortiOS versions 7.6.0 through 7.6.4; FortiOS 7.4, 7.2, 6.4, and 8.0 are not affected. It was publicly disclosed on February 10, 2026, with a CVSSv3 score of 7.5–8.1 (High) (FortiGuard Advisory, Feedly).

The root cause lies in the fnbamd component of FortiOS, which handles authentication for Agentless VPN and FSSO policies. When the backend LDAP server is configured to allow unauthenticated (anonymous) binds, FortiOS fails to enforce proper credential validation, enabling an attacker to bypass the authentication check entirely — classified as CWE-305 (Authentication Bypass by Primary Weakness). The attack is network-based, requires no privileges or user interaction, but does depend on the specific LDAP server configuration (unauthenticated binds enabled), making it conditionally exploitable. The vulnerability was discovered externally and reported by Jort Geurts from the Actemium Cyber Security Team under responsible disclosure (FortiGuard Advisory). Public proof-of-concept exploit code has been published on GitHub (PoC-1, PoC-2).

Successful exploitation allows an unauthenticated network attacker to circumvent LDAP-based authentication controls and gain unauthorized access to resources protected by Agentless VPN or FSSO policies. This results in high confidentiality and integrity impact — attackers could access sensitive data and modify system configurations without valid credentials. Availability may also be affected depending on what resources are exposed through the bypassed authentication mechanism (FortiGuard Advisory, Feedly).

1. Reconnaissance: Identify internet-facing FortiOS appliances running versions 7.6.0–7.6.4 using tools like Shodan or Censys, filtering for FortiGate/FortiOS banners on VPN or management interfaces.
2. Verify LDAP configuration: Determine whether the target FortiOS device is configured to use LDAP authentication for Agentless VPN or FSSO, and whether the backend LDAP server permits unauthenticated (anonymous) binds — a prerequisite for exploitation.
3. Craft authentication bypass request: Send a crafted authentication request to the Agentless VPN or FSSO endpoint that exploits the fnbamd daemon's failure to enforce credential validation when the LDAP server accepts unauthenticated binds.
4. Bypass authentication: The FortiOS fnbamd component performs an unauthenticated LDAP bind against the backend server; because the server accepts it, the authentication check succeeds without valid credentials.
5. Gain unauthorized access: With authentication bypassed, the attacker gains access to resources protected by the Agentless VPN or FSSO policy, potentially enabling lateral movement, data exfiltration, or further network compromise (FortiGuard Advisory, PoC-1).

• Network: Unexpected or anomalous authentication attempts to Agentless VPN or FSSO endpoints from unknown or external IP addresses; LDAP anonymous bind requests originating from the FortiOS appliance to the backend LDAP server.
• Logs: FortiOS authentication logs (fnbamd logs) showing successful LDAP authentication events without corresponding valid user credentials; LDAP server logs showing anonymous bind operations from the FortiGate IP address.
• Authentication Events: Successful VPN or FSSO session establishments from IP addresses or user accounts not previously seen, particularly without corresponding valid LDAP credentials in directory logs.
• LDAP Server: Unusual volume of anonymous bind requests from the FortiGate management IP; Windows Active Directory event logs (Event ID 2889) indicating unsigned LDAP binds if monitoring is enabled.

Fortinet has released FortiOS 7.6.5 as the patched version; organizations running FortiOS 7.6.0 through 7.6.4 should upgrade immediately using the Fortinet upgrade path tool at https://docs.fortinet.com/upgrade-tool (FortiGuard Advisory). As an immediate workaround, disable unauthenticated (anonymous) binds on the backend LDAP server. For Windows Active Directory (Windows Server 2019 and later), this can be enforced via PowerShell:

$configDN = (Get-ADRootDSE).configurationNamingContext
$dirSvcDN = "CN=Directory Service,CN=Windows NT,CN=Services,$configDN"
Set-ADObject -Identity $dirSvcDN -Add @{'msDS-Other-Settings'='DenyUnauthenticatedBind=1'}

Organizations should also review LDAP server configurations to ensure anonymous binds are disabled as a security baseline regardless of patching status (FortiGuard Advisory).

The vulnerability received coverage from multiple cybersecurity news outlets including CyberSecurityNews, eSecurity Planet, and CyberPress shortly after disclosure on February 10, 2026 (CyberSecurityNews, eSecurity Planet). The Center for Internet Security (CIS) included it in a multi-vulnerability Fortinet advisory (CIS Advisory), and Belgium's Centre for Cybersecurity issued a warning urging immediate patching. Community discussion on LinkedIn and Bluesky noted the conditional nature of the exploit (requiring specific LDAP server configuration), which somewhat limits its severity in practice.