Fortinet’s FG‑IR‑19‑283 (CVE‑2020‑12812) is an SSL VPN authentication flaw that lets attackers bypass Multi-Factor Authentication (MFA) under specific configurations, and roughly ten thousand FortiGate firewalls are still exposed and vulnerable on the public internet today. 

For any environment that “relies” on VPN MFA as a primary control, this turns a perimeter device into an easy initial‑access vector rather than a security boundary. Due to the severity and number of devices affected by this vulnerability, Fortinet released a new security advisory last month on December 24^th, 2025.

What FG‑IR‑19‑283 / CVE‑2020‑12812 Is

The vulnerability is an improper authentication issue in FortiOS SSL VPN where users can authenticate without completing MFA, depending on how usernames and back‑end authentication are configured. It affects FortiOS 6.4.0, 6.2.0–6.2.3, 6.0.9 and earlier and is tied to the way FortiGate handles case sensitivity for usernames, especially when combined with LDAP or other remote authentication sources.

How the MFA Bypass Works

FortiGate treats usernames as case‑sensitive by default, while many LDAP and Active Directory back‑ends do not treat them as case‑sensitive. This means that a user like “Alice” and “alice” may map differently on the firewall than in LDAP. Under vulnerable configurations, changing the case of the username can cause FortiOS to accept a login and skip the second‑factor challenge (FortiToken), giving full VPN access without any MFA prompt to the user.

Scale of Exposure: ~10,000 Devices

Recent internet‑wide scans by Shadowserver and other watchdogs found over 10,000 Fortinet firewalls exposed online that remain unpatched and vulnerable to this MFA bypass despite fixes available since 2020. Thousands of these devices are concentrated in the U.S., but they span multiple regions and sectors, including enterprises, service providers, and public‑sector networks that still depend on these VPNs for remote access.

Why This Matters for Defenders

A successful exploit on an SSL VPN gateway provides an authenticated VPN session, often with direct access to internal networks, so attackers can move laterally, steal data, and stage ransomware while completely bypassing what admins think is enforced MFA. Because this has been actively abused in the wild, CISA added CVE‑2020‑12812 to the Known Exploited Vulnerabilities catalog and multiple advisories explicitly tie it to real ransomware and state‑sponsored campaigns.

Patch Status and Mitigation Steps

Fortinet fixed the flaw in FortiOS 6.0.10, 6.2.4, and 6.4.1 and later and introduced additional configuration options such as the username-case-sensitivity / username-sensitivity setting to align behavior and prevent the bypass. Recommended actions include:

• Upgrading to a safe FortiOS version
• Disabling username case‑sensitivity where appropriate
• Reviewing VPN auth policies
• Watching logs for successful VPN logins that lack corresponding MFA events

Take a look at Fortinet’s recent blog for more details.