Despite patches being available for more than five years and repeated warnings from cybersecurity agencies, over 10,000 Fortinet firewalls across the globe continue to harbor a critical multi-factor authentication bypass vulnerability that is being actively exploited by threat actors. This persistent exposure to a flaw, tracked as CVE-2020-12812, underscores a troubling and persistent gap between the availability of security fixes and their widespread implementation within enterprise networks. The issue is not one of a new, sophisticated zero-day attack, but rather the failure to address a well-documented weakness in perimeter defenses. As attackers continue to successfully leverage this bug to circumvent crucial security controls, organizations are being forcibly reminded that unpatched legacy vulnerabilities can pose a threat as significant as any novel exploit, leaving sensitive corporate data and critical infrastructure at substantial risk of unauthorized access and compromise.
1. An Old Bug With New Consequences
The vulnerability itself is rooted in a subtle yet critical logic flaw within the SSL VPN component of FortiOS, the operating system at the core of Fortinet’s popular FortiGate firewalls. Originally disclosed in mid-2020, the bug allows an attacker to completely bypass secondary authentication measures, such as FortiToken MFA, through a surprisingly simple trick: manipulating the case sensitivity of a target username during the login process. For example, if a legitimate username is “jsmith,” an attacker could attempt to log in as “JSmith.” This seemingly minor alteration is the key to unlocking the flaw. The FortiOS authentication system, when encountering this case variation, fails to match the login attempt to the locally configured user account where MFA is enforced. Instead of rejecting the login, the system falls back to authenticating against a remote LDAP directory, such as Active Directory, which is a common configuration in enterprise environments for centralizing user management.
The successful exploitation of CVE-2020-12812 hinges on a specific, yet common, set of configuration parameters. The flaw becomes active when an organization has a local user account configured with two-factor authentication, simultaneously delegates authentication to a remote LDAP directory, and includes that user account within an LDAP group that is authorized for VPN or administrative access. Because most LDAP implementations, including Active Directory, are case-insensitive by default, they treat “jsmith” and “JSmith” as the same user. Consequently, when FortiOS passes the case-altered username to the LDAP server, it receives a successful authentication response. The firewall then grants access based on the user’s group membership without ever triggering the mandatory second-factor prompt associated with the local account. Fortinet addressed this logic error in FortiOS releases starting in mid-2020, and the U.S. Cybersecurity and Infrastructure Security Agency (CISA) added the vulnerability to its Known Exploited Vulnerabilities catalog in 2021 after it was observed being used in ransomware attacks.
2. The Scope of Active Exploitation
The continued threat posed by this aging vulnerability is not merely theoretical. In a late 2025 advisory, Fortinet’s own Product Security Incident Response Team (PSIRT) confirmed that CVE-2020-12812 is being actively abused in the wild. The advisory highlighted that threat actors are specifically targeting devices that remain unpatched or are improperly configured, demonstrating a clear understanding of where these security gaps are most likely to exist. The primary objective of these attacks is to bypass MFA controls to gain unauthorized VPN access into corporate networks or to seize administrative control over the firewalls themselves. These externally facing security appliances are prime targets because they represent the very gateways designed to protect an organization’s most sensitive internal resources, making a successful compromise a high-impact event for any victim. Once inside, attackers can move laterally, escalate privileges, and exfiltrate data, all while appearing as a legitimate, authenticated user.
Independent monitoring provides a stark picture of the global scale of this exposure. The Shadowserver Foundation, a nonprofit organization that scans the internet for vulnerable systems, reports that over 10,000 Fortinet firewalls remain susceptible to this five-year-old MFA bypass flaw. The geographic distribution of these exposed devices is widespread, with the United States having the largest concentration at approximately 1,300 systems, followed by significant numbers in Thailand, Taiwan, Japan, and China. However, these figures likely represent only the tip of the iceberg, as they only account for internet-visible services that can be identified through external scanning. Many more vulnerable devices may exist within networks shielded behind cloud providers or other layers of security, remaining hidden from public view but still representing a significant internal risk. The continued presence of this flaw across industrial, educational, and government networks highlights the persistent challenge of ensuring consistent patching and secure configuration across diverse and complex IT environments.
3. Mitigation and Lasting Lessons
Fortinet’s official guidance for addressing CVE-2020-12812 focuses on two fundamental actions that organizations must take to close this dangerous security gap. The most critical step is to upgrade vulnerable devices to a fixed version of the FortiOS firmware. Patched releases, including versions 6.0.10, 6.2.4, 6.4.1, and all subsequent major releases, contain the necessary logic corrections to prevent the case-sensitivity exploit. Alongside patching, security administrators must conduct a thorough review of their authentication configurations to ensure that local user accounts with MFA enabled are not combined with LDAP group policies in a way that could trigger the dangerous fallback behavior. Additional hardening measures are also strongly recommended to reduce the overall attack surface. These include disabling SSL VPN services on any internet-facing interfaces where they are not strictly necessary, enforcing principles of least-privilege access for all VPN and management interfaces, and actively monitoring system logs for unusual login attempts, particularly those involving case-variant usernames that could indicate probing or active exploitation attempts.
The prolonged exposure of thousands of critical security appliances to this known flaw illustrated the broader, systemic challenges that continued to plague enterprise cybersecurity. This incident highlighted the severe impact of “patch fatigue,” where overburdened IT teams struggle to keep pace with the constant stream of vulnerability disclosures. Furthermore, it demonstrated how configuration complexity, operational constraints, and legitimate concerns about service disruption often led to delays in applying necessary fixes, leaving critical infrastructure vulnerable for extended periods. The successful and continued exploitation of CVE-2020-12812 served as a powerful lesson: the age of a vulnerability did not diminish its potential for harm. Opportunistic attackers consistently proved their willingness to exploit any available weakness, new or old. Ultimately, this situation underscored that foundational security practices, such as vigilant configuration management and diligent maintenance, remained as crucial to a strong defense as the adoption of any new security technology.
