vulnerability / NVI-00004921 / Revision 3
Craft CMS: Authenticated "assets/preview-thumb" discloses signed fallback transform preview link to CP users without asset-view permission
GitHub lists this reviewed advisory for craftcms/cms, craftcms/cms. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
0.331%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.26303. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
craftcms/cms
source listed>= 4.0.0-RC1, <= 4.17.7
### 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
craftcms/cms
source listed>= 5.0.0-RC1, <= 5.9.13
### 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
craftcms/cms
source listedECOSYSTEM: introduced 4.0.0-RC1; fixed 4.17.8
### 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
craftcms/cms
source listedECOSYSTEM: introduced 5.0.0-RC1; fixed 5.9.14
### 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
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
- vendor patch
The original advisory lists fixed boundaries 4.17.8 for craftcms/cms. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
- vendor patch
The original advisory lists fixed boundaries 5.9.14 for craftcms/cms. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-x76w-8c62-48mg ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- GHSA-x76w-8c62-48mg ↗
GitHub Advisory Database via OSV · vendor advisory
Observed . GitHub-origin advisory, CC-BY-4.0; OSV is an aggregate copy, not independent corroboration.
- CVE-2026-56384 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 3 ·
Source facts changed; compare the preserved prior revision.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.