Craft CMS: Authenticated "assets/preview-thumb" discloses signed fallback transform preview link to CP users without asset-view permission

Summary

A user with Control Panel access but without permission to view a target private asset can call assets/preview-thumb and receive preview HTML that contains a signed fallback transform link for that private asset.

Details

Root-cause analysis:

1. The endpoint accepts an attacker-controlled assetId.
2. Asset is resolved, and thumbnail HTML is returned.
3. No explicit asset-view permission check is performed before preview generation.

Impact

Type:

1. Missing authorization
2. Unauthorized preview-link disclosure

Affected deployments:

1. Craft sites with control panel users who have partial permissions and private assets.

Security consequence:

1. A control panel user without asset-view permission can still obtain signed preview transform link data for private assets.
2. This may increase private asset exposure risk depending on deployment and endpoint chaining.

Resources

https://github.com/craftcms/cms/commit/d30df3112220db1ffd6726a3ed11857014c7fb27