CVE-2020-12812

Fortinet FortiOS SSL VPN improperly validates the second factor of authentication (FortiToken), allowing a user with valid primary credentials to log in without being prompted for their one-time token. The bypass is triggered by changing the letter case of the username (e.g., typing 'Admin' instead of 'admin'), a case-sensitivity handling flaw (CWE-178) in the authentication logic (CWE-287). An attacker who has stolen, phished, or reused a user's primary credentials can therefore gain full SSL VPN access without the multi-factor protection defenders assume is enforced. Any organization running an affected FortiOS SSL VPN portal with FortiToken two-factor authentication for users is exposed. The flaw is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) with known ransomware use, and EPSS assigns a 49.3% probability of exploitation within 30 days (99th percentile); no public PoC is known.

What to do: Apply the FortiOS update per Fortinet's advisory, as required by CISA's KEV listing. Until patched, audit SSL VPN logs for successful logins in which no FortiToken second-factor prompt occurred (especially sessions authenticating with altered username case), and consider restricting the SSL VPN portal to trusted source addresses or disabling it temporarily. Any accounts found with second-factor-free VPN logins should have credentials rotated and be checked for follow-on compromise, given known ransomware use.