Executive Summary

A critical two-factor authentication (2FA) bypass vulnerability in Fortinet FortiGate SSL VPNs, CVE-2020-12812, continues to pose a severe threat despite being five years old. As of January 2, 2026, the Shadowserver Foundation reports that over 10,000 firewalls remain unpatched and publicly exposed. The flaw, with a CVSS score of 9.8, is actively exploited in the wild and allows an attacker who has already obtained a user's password to completely bypass the second authentication factor (FortiToken). The exploit is trivial, requiring the attacker to simply change the case of the username during the login attempt. The vulnerability's persistence highlights a significant failure in patch management across thousands of organizations, leaving critical network gateways open to takeover. The CISA added this flaw to its Known Exploited Vulnerabilities (KEV) catalog in 2022.

Vulnerability Details

• CVE ID: CVE-2020-12812
• CVSS Score: 9.8 (Critical)
• Description: The vulnerability is an improper authentication flaw that exists in the SSL VPN login portal when it is configured to use LDAP for authentication along with FortiToken-based 2FA.
• Attack Vector: An attacker with knowledge of a valid LDAP username and password can bypass 2FA. The exploit works because the FortiGate device's authentication daemon treats usernames as case-sensitive, while the backend LDAP server (like Active Directory) typically treats them as case-insensitive. By submitting the username with a different case (e.g., Jsmith instead of jsmith), the attacker's login attempt does not match the username configured for 2FA on the FortiGate. The FortiGate therefore does not prompt for a token. However, it then passes the credentials to the LDAP server, which successfully authenticates the user because it ignores the case difference. The result is a successful login with only a single factor.

Affected Systems

• Product: Fortinet FortiGate SSL VPN
• Affected Versions: FortiOS versions before 6.4.1, 6.2.4, and 6.0.10.
• Configuration: The vulnerability only applies when the SSL VPN is configured for LDAP authentication and has 2FA enabled for some, but not all, users.

As of early January 2026, over 10,000 vulnerable systems are still online globally, with the highest concentration in the United States (>1,300).

Exploitation Status

The vulnerability is actively and widely exploited. CISA and the FBI first issued a warning in 2021 that state-sponsored actors were leveraging this flaw. Its inclusion in the CISA KEV catalog underscores its status as a persistent threat used by multiple threat actors to gain initial access to corporate and government networks.

Impact Assessment

Exploiting CVE-2020-12812 grants an attacker full access to the VPN, effectively placing them inside the target's network perimeter. This has several severe consequences:

• Network Compromise: The attacker can access internal resources, move laterally through the network, and deploy further malware, such as ransomware.
• Data Exfiltration: Once inside, the attacker can steal sensitive corporate data, intellectual property, and customer information.
• Loss of Trust in 2FA: This flaw undermines the security promise of 2FA, demonstrating that even organizations that have deployed it can be vulnerable if systems are not properly configured and maintained.
• Regulatory and Compliance Failure: For organizations subject to regulations requiring MFA, operating a system with this known flaw could be considered a compliance failure.

Cyber Observables for Detection

• VPN authentication logs showing successful logins where the submitted username case does not match the stored username.
• Network scans from external IPs targeting the SSL VPN port (e.g., TCP/443, TCP/10443) on FortiGate devices.
• Post-compromise activity originating from a new VPN client IP address shortly after a successful login.

Detection Methods

• Vulnerability Scanning: Use a vulnerability scanner (e.g., Nessus, Qualys) with up-to-date plugins to scan for CVE-2020-12812 on all public-facing FortiGate devices.
• Log Auditing: Actively audit VPN authentication logs. Create a SIEM rule that alerts when a successful LDAP authentication occurs for a user account, but the username string in the log does not exactly match the canonical username stored in Active Directory (e.g., JSmith vs jsmith). This is a direct indicator of an exploitation attempt.
• Asset Inventory: Maintain a complete and accurate inventory of all perimeter devices, including their firmware versions, to quickly identify unpatched systems.

Remediation Steps

1. Patch Immediately: The top priority is to upgrade all vulnerable FortiGate devices to a patched version of FortiOS (6.4.1, 6.2.4, 6.0.10 or newer). This is the only way to fully remediate the vulnerability. This is a critical application of D3-SU: Software Update.
2. Workaround (Temporary): If patching is not immediately possible, Fortinet provided a workaround. Create a unique LDAP group for all VPN users who require 2FA and configure the FortiGate to apply 2FA to that entire group, rather than individual users. This ensures that all members of the group are prompted for 2FA regardless of username case.
3. Hunt for Compromise: After patching, assume compromise. Review all VPN logs for the past several months for signs of suspicious logins. Hunt for any anomalous activity originating from VPN client IP pools. Consider a broad password reset for all VPN users.