A critical authentication bypass vulnerability in FortiGate devices enables threat actors to circumvent two-factor authentication (2FA) protections through case-sensitive username manipulation.
The flaw, tracked as CVE-2020-12812, affects organizations with specific LDAP integration configurations and remains exploitable on unpatched systems.
The vulnerability stems from FortiGate’s default case-sensitive username handling conflicting with LDAP directories that treat usernames as case-insensitive.
When attackers modify the capitalization of legitimate usernames during login attempts, the firewall fails to match the entry against local 2FA-enabled accounts, triggering a fallback to less-secure LDAP group authentication.
Technical Analysis
Successful exploitation requires three configuration elements: local FortiGate user entries with 2FA enabled that reference LDAP accounts, LDAP group membership for those users, and firewall policies utilizing LDAP groups for authentication.
An attacker logging in as “Jsmith” instead of “jsmith” bypasses the local user policy entirely, forcing FortiGate to evaluate secondary authentication rules.
The system then authenticates against the LDAP server directly using only username and password, completely ignoring 2FA requirements and even disabled account statuses.
| CVE Identifier | FG-IR Reference | CVSS Score | Attack Vector | Patch Availability |
|---|---|---|---|---|
| CVE-2020-12812 | FG-IR-19-283 | 9.1 (Critical) | Network-based | FortiOS 6.0.10, 6.2.4, 6.4.1+ |
This vulnerability poses severe risks for administrative access and VPN security. Successful bypass grants attackers unauthorized entry to management interfaces or corporate networks without possessing 2FA tokens.
Organizations experiencing exploitation must treat their configurations as compromised and reset all credentials, including LDAP/AD binding accounts.
The attack leaves minimal forensic evidence since failed local authentication attempts may not trigger security alerts.
Fortinet addressed the vulnerability in July 2020 through configuration enhancements. Administrators must implement the set username-case-sensitivity disable command on all local accounts for FortiOS versions 6.0.10, 6.2.4, and 6.4.1.
For later releases (6.0.13+, 6.2.10+, 6.4.7+, 7.0.1+), use set username-sensitivity disable. This ensures FortiGate treats all username case variations as identical, preventing authentication fallback.
Additional hardening requires removing unnecessary secondary LDAP groups from authentication policies.
Organizations should audit firewall configurations to eliminate redundant LDAP group references and enforce strict local user matching.
Where LDAP groups are non-essential, their complete removal blocks the authentication bypass pathway entirely.
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