CVE-2020-12812 — FortiOS SSL VPN : Active exploitation in the wild of this older vulnerability

CyberDefender 3 mins0

CVE-2020-12812 is a critical authentication bypass vulnerability in the SSL VPN component of FortiOS, the operating system used by firewalls from Fortinet.

Recent reports (December 2025) show active exploitation in the wild of this older vulnerability, with threat actors targeting networks where FortiGate firewalls are still unpatched or misconfigured. The flaw allows an attacker to bypass two-factor authentication (2FA) by changing the capitalization of the username during login.

How the Vulnerability Works

  • FortiOS treats usernames as case-sensitive by default.
  • Many LDAP backends (such as Active Directory) are case-insensitive.
  • If a user has 2FA enabled locally but authenticates through LDAP with altered username casing:
    • FortiOS may fail to associate the account with its 2FA policy
    • Authentication succeeds without requiring the second factor

This results in password-only access, even when 2FA is configured.

Affected FortiOS Versions

  • 6.4.0
  • 6.2.0 – 6.2.3
  • 6.0.9 and earlier

Only systems using SSL VPN with LDAP authentication and 2FA are impacted.

Severity

  • CVSS v3.1 Score: 5.2
  • Impact:
    • Unauthorized VPN access
    • Bypass of multi-factor authentication
    • Potential initial access vector for network compromise

Exploitation Status

  • The vulnerability has been actively exploited in real-world attacks, particularly against:
    • Internet-exposed FortiGate VPNs
    • Organizations running legacy or unpatched FortiOS versions

Mitigation & Remediation

✅ Patch Immediately

Upgrade to one of the following (or newer):

  • 6.0.10+
  • 6.2.4+
  • 6.4.1+

✅ Configuration Hardening

Disable username case sensitivity:

set username-case-sensitivity disable

This prevents case-manipulation from bypassing 2FA checks.

✅ Defense-in-Depth

  • Review LDAP and local user mappings
  • Restrict VPN access by IP where possible
  • Monitor VPN logs for unusual login patterns or mixed-case usernames

Quick Summary

CategoryDetails
CVECVE-2020-12812
ProductFortiOS SSL VPN
Vulnerability TypeImproper Authentication
Impact2FA bypass
Severity5.2
FixPatch + disable username case sensitivity

CyberDefender