10,000+ Fortinet Firewalls Still Exposed to a 5-Year-Old MFA Bypass Vulnerability

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10,000+ Fortinet Firewalls Still Exposed to a 5-Year-Old MFA Bypass Vulnerability

The cyber security community continues to face a troubling reality: thousands of critical infrastructure systems remain exposed to known vulnerabilities long after fixes have been released. A recent report revealed that more than 10,000 Fortinet firewalls worldwide are still vulnerable to a five-year-old multi-factor authentication (MFA) bypass flaw, identified as CVE-2020-12812.

Despite being disclosed in 2020 and patched years ago, this vulnerability is once again in the spotlight due to confirmed active exploitation and persistent exposure across the internet. The situation highlights a serious and recurring problem in cyber defense—failure to apply security updates in a timely manner.

This article explores the Fortinet MFA bypass vulnerability in detail, how it works, why it remains dangerous, how attackers exploit it, and the key lessons organizations must learn to avoid similar security failures.

Understanding CVE-2020-12812

CVE-2020-12812 is an authentication bypass vulnerability affecting FortiOS SSL VPN portals on certain Fortinet firewall versions. The flaw allows attackers to bypass the second authentication factor, typically FortiToken-based MFA, under specific conditions.

The vulnerability stems from improper handling of username case sensitivity during the authentication process. In simple terms, the system fails to correctly validate usernames when letter casing is altered.

Affected FortiOS Versions

  • FortiOS 6.4.0

  • FortiOS 6.2.0 through 6.2.3

  • FortiOS 6.0.9 and earlier

Organizations running these versions without patches remain vulnerable.

Why This Vulnerability Is Still a Serious Threat ⚠️

At first glance, it may seem surprising that a vulnerability disclosed over five years ago is still relevant. However, several factors make CVE-2020-12812 particularly dangerous even today.

1. MFA Bypass Undermines Trust

Multi-factor authentication is widely regarded as one of the strongest defenses against account compromise. When attackers can bypass MFA, the security model collapses.

2. Internet-Facing VPN Services

Fortinet SSL VPN portals are often exposed directly to the internet to allow remote access. This makes them prime targets for automated scanning and exploitation.

3. Large Attack Surface

With over 10,000 exposed firewalls, attackers have a massive pool of potential targets, many of which protect enterprise and government networks.

4. Active Exploitation Confirmed

Security researchers and Fortinet itself have confirmed that this vulnerability is being actively exploited in the wild, increasing the risk of real-world breaches.

How the MFA Bypass Works 🧠

The core issue behind CVE-2020-12812 lies in case-sensitive username handling during the authentication workflow.

Simplified Explanation

  • A legitimate username exists, such as user

  • The attacker attempts login using User or USER

  • The system incorrectly validates the username

  • MFA verification is skipped or bypassed

  • Attacker gains access without the second factor

This flaw allows attackers who already know or guess a valid username to bypass MFA protections entirely.

Why Attackers Love This Vulnerability

From an attacker’s perspective, CVE-2020-12812 is extremely attractive:

  • No need to exploit memory corruption or complex bugs

  • No malware installation required

  • No user interaction needed

  • Can be automated and scaled easily

Attackers can simply script login attempts and test different username casing until access is granted.

Real-World Exploitation Scenarios 🧨

Unauthorized Network Access

Attackers can gain direct VPN access, effectively placing them inside an organization’s internal network.

Credential Abuse

Once inside, attackers can:

  • Access internal services

  • Harvest additional credentials

  • Escalate privileges

  • Move laterally across systems

Data Exfiltration

VPN access allows attackers to locate and steal sensitive data such as:

  • Customer records

  • Intellectual property

  • Financial data

  • Internal communications

Ransomware Deployment

Many ransomware attacks begin with compromised VPN credentials. This vulnerability provides an ideal entry point for ransomware operators.

Why Are So Many Systems Still Vulnerable?

This raises an important question: Why are thousands of Fortinet firewalls still unpatched after five years?

1. Poor Patch Management

Many organizations lack structured patch management processes, especially for network appliances.

2. Fear of Downtime

Administrators may delay updates due to concerns about service interruptions or configuration issues.

3. Forgotten Infrastructure

Legacy systems are often left running without regular maintenance, especially in large or decentralized environments.

4. Lack of Visibility

Some organizations may not even be aware that vulnerable versions are still deployed within their networks.

Detection Challenges 🔍

Detecting exploitation of CVE-2020-12812 can be difficult:

  • Login attempts may appear legitimate

  • No malware is required

  • MFA logs may not show failures

  • Attackers may blend in with normal VPN traffic

Without advanced logging and behavioral analysis, attacks may go unnoticed for long periods.

Mitigation and Defense Strategies 🛡️

1. Apply Security Updates Immediately

Fortinet released patches years ago to address this vulnerability. Updating to a supported and patched FortiOS version is critical.

2. Audit All VPN Gateways

Organizations should inventory all VPN endpoints and verify that none are running vulnerable firmware versions.

3. Enforce Strong Authentication Policies

In addition to MFA, enforce:

  • Strong password policies

  • Account lockout thresholds

  • Login anomaly detection

4. Monitor VPN Logs Closely

Look for unusual login patterns, including:

  • Multiple login attempts with similar usernames

  • Logins from unfamiliar locations

  • Sudden spikes in VPN activity

5. Network Segmentation

Limit what VPN users can access. Even if attackers gain VPN access, segmentation can reduce impact.

Lessons Learned From CVE-2020-12812 📘

Lesson 1: MFA Is Not a Silver Bullet

While MFA is powerful, it must be implemented correctly. Weak logic can render it ineffective.

Lesson 2: Old Vulnerabilities Never Truly Die

Attackers continuously exploit old flaws because they know many organizations fail to patch.

Lesson 3: Edge Devices Are High-Value Targets

Firewalls, VPNs, and gateways sit at the network perimeter and are frequently targeted.

Lesson 4: Visibility Is Essential

Organizations cannot protect what they do not know exists. Asset visibility is a foundational security requirement.

Lesson 5: Security Is a Process, Not a Product

Buying a security appliance does not guarantee protection. Continuous monitoring, updating, and auditing are essential.

Broader Impact on Cyber Security

The persistence of CVE-2020-12812 highlights a broader issue in cyber security: the gap between vulnerability disclosure and remediation.

Attackers exploit this gap aggressively, knowing that:

  • Many systems remain unpatched

  • Enterprises struggle with legacy infrastructure

  • Operational concerns often outweigh security priorities

This vulnerability serves as a reminder that basic cyber hygiene is just as important as advanced threat detection.

What Organizations Should Do Now

To reduce exposure to similar risks, organizations should:

  • Conduct regular firmware audits

  • Establish strict patch timelines

  • Automate vulnerability scanning

  • Review MFA implementations

  • Test authentication logic thoroughly

Security teams should also assume that perimeter defenses will be targeted and plan accordingly.

Final Thoughts

The fact that over 10,000 Fortinet firewalls remain vulnerable to a five-year-old MFA bypass flaw is a wake-up call for the entire cyber security industry. CVE-2020-12812 demonstrates how small implementation mistakes can have massive security implications when left unaddressed.

Cyber threats do not always rely on zero-day exploits. Often, attackers succeed simply because known vulnerabilities are ignored. Staying secure requires discipline, awareness, and a commitment to continuous improvement.

In today’s threat landscape, patching is not optional—it is essential.

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