Craft CMS before 4.18.6 Remote Code Execution via signed cookie

Summary

Craft CMS 4.8.0 through 4.18.5 and 5.0.0 through 5.10.12 sign an authenticated user's attacker-controlled license-shun cookie with the same key and format used to validate signed redirect parameters, because the HMAC signature is not bound to its purpose (Yii's cookieValidationKey is derived from the same Craft securityKey used for signed request parameters). An authenticated, non-administrator user (Control Panel access is not required) can set the cookie via the license-shun endpoint and transplant the signed envelope into the redirect parameter; on a successful login, Craft validates the signature and renders the authenticated bytes as an unsandboxed Twig template, where Twig's map filter accepts a string callback and allows PHP system() to execute arbitrary operating-system commands as the web-server user. Exploitation requires an account using password authentication without active 2FA, the default request configuration, and availability of PHP system(). The issue is fixed in 4.18.6 and 5.10.13.

Timeline

1. Sep 16, 2026, 9:46 PM UTCCVE published
2. Sep 17, 2026, 6:57 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Sep 17, 2026, 7:23 PM UTCRecord last updated

Affected versions

• craftcms cms

  From 4.8.0 · fixed in 4.18.6

• craftcms cms

  From 5.0.0 · fixed in 5.10.13

References 3