### 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