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

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

Alternatively, you can have a non-administrator account with allowAdminChanges disabled, but you have access to the System Messages utility.

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:

d82680f

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