Two-factor authentication is widely regarded as a critical defense in modern cybersecurity, yet a subtle discrepancy in username case sensitivity within a popular firewall platform revealed how easily this crucial security layer can be dismantled. A significant vulnerability, identified as CVE-2020-12812, demonstrated that malicious actors could bypass 2FA protections on Fortinet’s FortiGate firewalls simply by altering the case of a legitimate username during an authentication attempt. This security flaw exposed a fundamental conflict between how FortiGate’s internal user database processes usernames and how interconnected LDAP directory services handle the same information. While FortiGate’s system was configured to be case-sensitive, many LDAP implementations are case-insensitive, creating a dangerous gap in security logic. Under specific, yet not uncommon, configurations, this mismatch allowed attackers to circumvent the 2FA requirement entirely, potentially gaining unauthorized administrative or VPN access to sensitive corporate networks. This incident serves as a stark reminder that the security of a system is only as strong as its weakest link, and that complex integrations can introduce unforeseen vulnerabilities.
1. Deconstructing the Authentication Flaw
The successful exploitation of this vulnerability hinged on a precise set of configuration elements being active simultaneously, creating a perfect storm for an authentication bypass. The core issue required an organization to have local user accounts configured on the FortiGate device with 2FA enabled, where these local accounts were also linked to corresponding user accounts within an LDAP directory, such as Active Directory. Furthermore, these users needed to be members of at least one LDAP group that was, in turn, referenced in a FortiGate authentication policy for administrative access, SSL VPN, or IPsec VPN connections. The attack mechanism itself was deceptively simple. An attacker, knowing a valid username like “jsmith,” could initiate a login attempt using a case-variant such as “Jsmith” or “JSMITH.” Because the FortiGate platform was treating usernames with case sensitivity, it failed to find a match for “Jsmith” in its local user database where 2FA was enforced for “jsmith.” This failure triggered a fallback mechanism, prompting the firewall to then attempt authentication against the configured LDAP server. Since the LDAP server treated usernames as case-insensitive, it successfully validated the user’s credentials but, critically, did so based on group membership rules that did not enforce the 2FA requirement, granting the attacker access without the second factor.
Addressing the Vulnerability and Future Safeguards
Fortinet promptly addressed this critical security flaw in FortiOS versions 6.0.10, 6.2.4, and 6.4.1, which were released in July 2020, effectively closing the loophole that allowed for the 2FA bypass. For organizations that could not immediately deploy these patches, a direct mitigation strategy was provided, which involved modifying the firewall’s configuration to align its username handling with that of the LDAP directory. Administrators were advised to disable case sensitivity for local usernames via a command-line interface command, specifically set username-case-sensitivity disable or set username-sensitivity disable on newer firmware versions. This simple change ensured that usernames like “jsmith” and “Jsmith” were treated as identical, preventing the initial authentication mismatch that triggered the dangerous fallback to LDAP. Security experts also recommended a more strategic, long-term solution: auditing and removing any unnecessary secondary LDAP groups from authentication policies. By eliminating the fallback authentication path entirely, organizations could ensure that if the primary, 2FA-enabled method failed, the system would deny access rather than defaulting to a less secure alternative. The incident underscored the vital importance of not only timely patching but also conducting thorough configuration reviews to understand how different systems interact and to eliminate redundant or insecure authentication pathways.
