Zero-day vulnerability in FortiOS

Improper verification of cryptographic signature
CVE-2026-24858

Vulnerability details

Advisory: SB2026012392 - FortiCloud SSO login authentication bypass in Fortinet products

Vulnerable component: FortiOS

CVE-ID: CVE-2026-24858

CVSSv3 score: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:H/RL:O/RC:C

CWE-ID: CWE-347 - Improper Verification of Cryptographic Signature

Description:

The vulnerability allows a remote attacker to bypass SSO authentication.

The vulnerability exists due to improper verification of cryptographic signature when handling SAML messages. A remote non-authenticated attacker can bypass the FortiCloud SSO login authentication via a crafted SAML message. 

Successful exploitation of the vulnerability requires that the FortiCloud SSO feature is enabled on the device. However, when an administrator registers the device to FortiCare from the device's GUI, unless the administrator disables the toggle switch "Allow administrative login using FortiCloud SSO" in the registration page, FortiCloud SSO login is enabled upon registration.

This vulnerability exists due to incomplete fix for #VU119694 (CVE-2025-59718) and #VU119696 (CVE-2025-59719).

Note, the vulnerability is being actively exploited in the wild.

External links:

https://www.fortinet.com/blog/psirt-blogs/analysis-of-sso-abuse-on-fortios

About zero-day vulnerabilities

Zero-day vulnerability is an undisclosed vulnerability in software that hackers can exploit to compromise computer programs, gain unauthorized access to sensitive data, penetrate networks, etc. We consider vulnerability a zero-day when there is no solution provided from software vendor and the vulnerability is being actively exploited by malicious actors.

Zero-day candidate is a potential zero-day vulnerability in software which might have been used in targeted attacks, however there is no evidence to support this suggestion.