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GHSA-x76w-8c62-48mg

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

Source: GitHub Advisory Database (reviewed) Moderate CVE-2026-56384

Packages

EcosystemPackageAffectedFixed in
Packagist craftcms/cms >= 4.0.0-RC1, < 4.17.8 4.17.8
Packagist craftcms/cms >= 5.0.0-RC1, < 5.9.14 5.9.14

Details

As written in the advisory (Markdown source, shown as plain text).

### 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

References

External links from the advisory. They lead to third-party sites.