A deeply embedded configuration flaw, lurking for half a decade within Fortinet’s operating system, has now become a favored tool for cybercriminals seeking to dismantle the very security layers designed to keep them out. This guide provides a comprehensive overview of the threat posed by CVE-2020-12812, an improper authentication vulnerability, and outlines the critical steps system administrators must take to secure their networks against this deceptive attack. The purpose of this document is to deconstruct the attack method, clarify the specific conditions that create risk, and offer actionable solutions to mitigate the vulnerability effectively. Understanding this threat is not just about patching a single flaw; it is about recognizing how seemingly minor configuration discrepancies can cascade into major security failures.
The Illusion of Security How a Subtle Flaw Undermines 2FA
Fortinet has issued a critical advisory concerning CVE-2020-12812, a vulnerability that is now being actively exploited in the wild. This flaw directly targets the integrity of two-factor authentication (2FA) on devices running specific versions of FortiOS. The immediate threat impacts organizations that depend on FortiGate appliances for secure access, particularly through SSL VPNs. The core of the exploit lies in a clever manipulation of how different systems handle usernames, allowing an attacker with valid credentials to bypass the second authentication factor entirely. This transforms what should be a robust security checkpoint into an unlocked door.
The advisory serves as a stark reminder that even multi-layered security architectures can harbor single points of failure. In this case, the vulnerability is not a brute-force attack or a complex cryptographic break but a logical flaw born from a mismatch in default settings. Attackers are weaponizing a simple discrepancy in case-sensitivity between the FortiGate device and an external LDAP server. By exploiting this difference, they can trigger an authentication path that circumvents the 2FA requirement, gaining unauthorized access to sensitive networks. This situation underscores the urgent need for administrators to look beyond the surface of their security controls and scrutinize the underlying configurations that govern them.
Why an Old Vulnerability Poses a New Urgent Threat
Two-factor authentication is a cornerstone of modern cybersecurity, acting as a critical barrier against unauthorized access by requiring a second form of verification beyond a simple password. Its widespread adoption is a direct response to the pervasive threat of credential theft. However, the re-emergence of CVE-2020-12812 demonstrates that the effectiveness of 2FA is entirely dependent on its proper implementation and the security of the systems that enforce it. When a flaw allows this second factor to be bypassed, the entire security model predicated on its strength collapses, reducing protection to a single, often guessable, password.
The threat actors leveraging this vulnerability are sophisticated and diverse, ranging from ransomware groups seeking to encrypt corporate networks for financial gain to state-sponsored entities conducting espionage. These groups are adept at identifying and weaponizing older, often overlooked vulnerabilities, knowing that many organizations lag in their patching cycles. The fact that a five-year-old flaw is now at the center of active campaigns highlights a persistent challenge in cybersecurity: legacy vulnerabilities can remain dormant for years before becoming a clear and present danger. This active exploitation elevates the urgency for all FortiOS users to assess their exposure and take immediate remedial action.
Deconstructing the Attack A Step by Step Analysis
Step 1 Exploiting the Case Sensitivity Mismatch
The attack begins with a subtle yet powerful manipulation of a known, valid username. An attacker who has already obtained a user’s credentials—for instance, a username like ‘jsmith’ and its corresponding password—initiates the exploit by simply changing the case of one or more letters in the username. For example, they might submit ‘JSmith’ or ‘JSMITH’ during the login attempt. This seemingly innocuous alteration is the key that unlocks the entire vulnerability, as it targets a fundamental discrepancy in how different components of the authentication chain process user identities.
This initial step succeeds because of a default configuration difference between FortiGate appliances and backend LDAP servers like Active Directory. The FortiGate device, by default, treats usernames as case-sensitive. It sees ‘jsmith’ and ‘JSmith’ as two entirely different accounts. In contrast, most LDAP servers are configured to be case-insensitive, recognizing both variations as the same user. This conflict in interpretation creates a loophole that attackers can drive a wedge through, setting the stage for the subsequent steps of the 2FA bypass.
Insight FortiGates Strictness vs LDAPs Flexibility
The root of the issue is the default strictness of the FortiGate appliance when handling usernames. When it receives a login attempt, its primary check is against its local user database, where it looks for an exact, case-sensitive match. This rigid approach is designed to prevent ambiguity. However, this strictness is at odds with the inherent flexibility of LDAP directory services, which prioritize user convenience and are typically configured to ignore case variations in usernames to avoid failed logins due to minor typing errors. This operational difference, while logical in isolation for each system, creates a critical security gap when the two are integrated without proper alignment.
Warning A Single Character Change Unlocks the Door
The simplicity of this exploit vector is what makes it so dangerous. A single, deliberate change to the capitalization of a username is all that is required to initiate the attack sequence. This action does not require sophisticated tools or deep technical knowledge beyond understanding this specific flaw. It highlights a critical lesson in security architecture: the overall strength of a system is defined by its weakest link, and sometimes that weakness is not a complex piece of code but a simple assumption about how two interconnected systems will behave. The change is minor, but its impact is profound, effectively disabling a core security control.
Step 2 Triggering the Insecure Authentication Fallback
Once the attacker submits the username with altered case, the FortiGate appliance performs its initial check. Because it is case-sensitive, it fails to find a local user matching the submitted string (e.g., ‘JSmith’ does not match the configured ‘jsmith’). This failure is a crucial moment in the attack. The device concludes that the user does not have a local account configured for 2FA and therefore does not prompt for a FortiToken or other second factor. The system’s logic dictates that if there is no matching local user, there is no 2FA requirement to enforce.
With the local check failed and the 2FA prompt bypassed, the authentication process does not terminate. Instead, it falls back to the secondary authentication method configured for the policy, which is the LDAP server. The FortiGate passes the modified username and the password to the LDAP server for validation. Since the LDAP server is case-insensitive, it successfully authenticates the credentials because ‘JSmith’ is considered the same as ‘jsmith’. At this point, the attacker has provided a valid password for a valid user, and the LDAP server confirms this without any knowledge of the bypassed 2FA step. The attacker is then granted access.
Critical Failure How the FortiToken Prompt is Skipped
The critical failure point occurs at the intersection of the local user lookup and the 2FA enforcement policy. The FortiToken prompt is directly tied to the local user object on the FortiGate device. When the case-altered username is submitted, the device’s lookup process returns no match for that specific string. Consequently, the logic that would normally trigger the 2FA challenge is never activated. The system effectively believes it is dealing with a user who is not enrolled in 2FA, even though the intended user is. This illustrates a failure in the authentication state machine, where an early exit from one check leads to an insecure pathway.
The Path of Least Resistance Why LDAP Becomes the Weak Link
In this attack scenario, the LDAP server becomes the unwitting accomplice. It performs its duty correctly by validating the provided credentials according to its own rules, which are case-insensitive. However, it operates without the full context of the FortiGate’s security policy. The FortiGate’s decision to fall back to LDAP after its own failed check creates a path of least resistance for the attacker. The LDAP authentication becomes the sole determinant of access, completely undermining the intended multi-factor security posture. This highlights the danger of creating complex authentication chains where a failure in one step can lead to a less secure alternative.
Step 3 The Perfect Storm Configuration for Vulnerability
For an organization to be susceptible to this 2FA bypass, a specific and precise set of three conditions must be met simultaneously. This configuration creates a “perfect storm” scenario where all the necessary components for the exploit are in place. The presence of just one or two of these conditions is not enough to create the vulnerability. Administrators can use this checklist to quickly assess their own environment’s risk level and determine if they are exposed to this active threat. Understanding these prerequisites is the first step toward effective mitigation.
The existence of this vulnerability is not a universal flaw across all FortiGate deployments but is instead confined to environments where a particular combination of local user settings, LDAP group memberships, and authentication policies converge. This specificity is both a blessing and a curse; it limits the attack surface but can also create a false sense of security for administrators who are unaware of the precise configuration that introduces risk. A thorough audit against these three conditions is therefore essential for any organization using FortiGate with LDAP integration.
Condition A Local User Accounts with 2FA Pointing to an LDAP Server
The first and most fundamental requirement for vulnerability is that a local user account must exist on the FortiGate appliance. This local account must be configured with 2FA enabled (e.g., requiring a FortiToken). Critically, this same user account must also be set up to perform its primary password authentication against a remote LDAP server rather than using a locally stored password. This hybrid configuration is common in enterprise environments seeking to centralize credential management while using the FortiGate’s built-in 2FA capabilities.
Condition B Required Membership in an LDAP User Group
The second condition is that the user in question must be a member of at least one user group defined on the LDAP server. This is a standard practice for managing permissions and access control within an organization. The exploit relies on the fact that the authentication policy will reference one of these LDAP groups to grant access. Without membership in such a group, the subsequent policy check would fail, even if the initial credential validation succeeded. This condition ensures the attacker can progress beyond simple authentication to actual network access.
Condition C An Active Authentication Policy Using the LDAP Group
Finally, the third and final piece of the puzzle is an active authentication policy on the FortiGate. This policy, such as one governing SSL VPN or administrative access, must be configured to use one of the LDAP groups to which the user belongs. When the fallback authentication to the LDAP server succeeds, this policy is what grants the attacker access to the network or system resources. If no such policy exists or if it references a different group, the authentication attempt would be rejected, and the attack would fail.
Anatomy of the 2FA Bypass Key Takeaways
The mechanics of this vulnerability can be distilled into a few key points. Understanding this sequence is crucial for grasping both the risk and the required remediation steps. The attack is not a brute-force effort but a surgical exploitation of misaligned system behaviors.
- Username Case Manipulation: The attacker initiates the exploit by providing a known username but with at least one character’s case altered (e.g., ‘user’ becomes ‘User’).
- 2FA Bypass: Due to its default case-sensitive nature, the FortiGate appliance fails to find a matching local user and, as a result, does not trigger the configured 2FA prompt.
- Insecure LDAP Fallback: The authentication process then falls back to the case-insensitive LDAP server, which successfully validates the user’s password without requiring a second factor.
- Specific Configuration: The attack is only successful if a precise set of conditions are met: a local user with 2FA pointing to an LDAP server, membership in an LDAP group, and an active policy using that group.
Beyond the Breach The Full Impact of a Compromise
A successful exploit of this vulnerability should not be treated as a minor incident. Fortinet strongly advises that any organization discovering evidence of a compromise should consider its entire system configuration to be breached. This is because an attacker gaining access, especially with administrative privileges, can alter policies, create backdoors, exfiltrate data, and pivot deeper into the network. The initial entry point is just the beginning of a potentially devastating security event. Therefore, the response must be comprehensive and assume the worst-case scenario.
The recommended remediation goes beyond simply patching the flaw. It necessitates a complete reset of all credentials associated with the compromised system. This includes not only the user accounts that may have been abused but also the service accounts used by the FortiGate to bind to the LDAP or Active Directory server. These service accounts often have elevated privileges and, if compromised, could allow an attacker to maintain persistent access or query the entire user directory. This incident also serves as a critical case study on the hidden dangers of default configurations and the importance of understanding how different technologies interact within a security stack.
Securing Your Defenses Immediate Actions and Recommendations
Action 1 Prioritize Software Updates
The most effective and permanent solution to eliminate the CVE-2020-12812 vulnerability is to update the FortiOS software to a patched version. Fortinet has released fixes in multiple software branches to address the issue at its source. Administrators should prioritize the deployment of these updates to ensure the underlying logical flaw is corrected, making the case-sensitivity exploit impossible. This proactive measure is the best defense against this and other known vulnerabilities.
Organizations should immediately plan to upgrade to FortiOS version 6.0.10, 6.2.4, 6.4.1, or any subsequent newer release. Applying these updates ensures that the FortiGate appliance handles the authentication process correctly, preventing the insecure fallback that enables the 2FA bypass. Relying on patches is a fundamental principle of good security hygiene and is the recommended course of action for all affected customers to ensure long-term protection against this threat vector.
Action 2 Implement the Critical Configuration Fix
For organizations that cannot immediately deploy a software update due to operational constraints or change control processes, Fortinet has provided a direct mitigation that can be applied through a configuration change. This fix addresses the root cause of the vulnerability—the case-sensitivity mismatch—by altering the FortiGate’s behavior to align with that of the LDAP server. This is a powerful, immediate step that can be taken to close the security gap while a full software update is being planned.
This workaround provides an effective stopgap measure to protect the system from exploitation. By implementing this change, administrators can ensure that their defenses are shored up against active attacks targeting this specific flaw. However, it is important to view this as a temporary solution. The long-term strategy should still involve upgrading to a fully patched version of FortiOS to benefit from all security enhancements and bug fixes.
The Primary Fix Set username sensitivity to disabled
The specific mitigation involves modifying a single setting within the FortiGate’s command-line interface (CLI). Administrators should access the relevant user LDAP configuration and execute the command to set username-sensitivity to disabled. This command instructs the FortiGate to treat all variations of a username’s case as identical, mirroring the default behavior of most LDAP servers. The command structure is config user ldap, followed by edit , set username-sensitivity disable, and end.
How This Change Aligns FortiGate with LDAP Behavior
By disabling username sensitivity, the FortiGate’s initial user lookup will no longer fail when an attacker submits a case-altered username. The device will correctly identify ‘JSmith’ as the local user ‘jsmith’, recognize that 2FA is required, and present the FortiToken prompt as intended. This simple alignment of behavior between the FortiGate and the LDAP server completely neutralizes the exploit vector. The fallback to an insecure authentication path is prevented because the initial, case-insensitive match succeeds, and the proper security policy is enforced.
Action 3 Audit and Harden Authentication Policies
Beyond immediate patching and configuration fixes, this incident should serve as a catalyst for a broader review of authentication policies. Administrators are encouraged to audit their configurations and remove any secondary LDAP group configurations that are not strictly necessary for business operations. Complex authentication chains, especially those involving fallback mechanisms, can inadvertently create security loopholes. Simplifying these policies reduces the potential attack surface and makes the system easier to manage and secure.
This proactive hardening measure helps build a more resilient security posture. By ensuring that authentication policies are as straightforward as possible, organizations can minimize the risk of unforeseen interactions between different system components. Regularly reviewing and pruning non-essential configurations is a critical aspect of security hygiene that helps prevent not only this specific vulnerability but also future, yet-to-be-discovered flaws that may exploit similar logical gaps.
