Craft CMS vulnerable to potential authenticated Remote Code Execution via Twig SSTI

For this to work, users must have administrator access to the Craft Control Panel, and allowAdminChanges must be enabled for this to work, which is against Craft CMS' recommendations for any non-dev environment.

https://craftcms.com/knowledge-base/securing-craft#set-allowAdminChanges-to-false-in-production

Alternatively, a non-administrator account with allowAdminChanges disabled can be used, provided access to the System Messages utility is available.

It is possible to craft a malicious payload using the Twig map filter in text fields that accept Twig input under Settings in the Craft control panel or using the System Messages utility, which could lead to a RCE.

Users should update to the patched versions (5.8.21 and 4.16.17) to mitigate the issue.

References:

https://github.com/craftcms/cms/commit/d82680f4a05f9576883bb83c3f6243d33ca73ebe

https://github.com/craftcms/cms/blob/5.x/CHANGELOG.md#5821---2025-12-04