CVE-2026-56384: Craft CMS Authenticated "assets/preview-thumb" Discloses Signed Fallback Vulnerability

MODERATE
Advisory Identifier: GHSA-x76w-8c62-48mg
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CWE-862 CWE-200
Published
Advisory Summary

CVE-2026-56384 is a MODERATE authenticated "assets/preview-thumb" discloses signed fallback vulnerability (CWE-862: Missing Authorization) affecting Craft CMS libraries in the Packagist ecosystem. The issue involves authenticated "assets/preview-thumb" discloses signed fallback transform preview link to cp users without asset-view permission. Remediation is available by upgrading affected packages to version 4.17.8, 5.9.14.

CVE-2026-56384 at a Glance

CVE Identifier CVE-2026-56384
GHSA Identifier GHSA-x76w-8c62-48mg
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-862: Missing Authorization
Affected Product Craft CMS
Package Ecosystem Packagist
Published Date
Last Updated

How to Fix CVE-2026-56384

To remediate this vulnerability, upgrade all affected Craft CMS dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

craftcms/cms fixed: 4.17.8 
Upgrade craftcms/cms to 4.17.8
craftcms/cms fixed: 5.9.14 
Upgrade craftcms/cms to 5.9.14
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for craftcms/cms

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-56384, inspect your project dependencies for Craft CMS:

inspect dependencies for craftcms/cms

Analyze the terminal output to locate any version of Craft CMS in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.17.8), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-862: Missing Authorization, where the product does not perform an authorization check when an actor attempts to access a resource or perform an action. In Craft CMS, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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

CVE-2026-56384 Affected Craft CMS Versions

2 component(s)
Fixed Versions:
4.17.85.9.14
Product / Package Ecosystem Affected Versions Fixed Version
craftcms/cms
 Packagist
≥ 4.0.0-RC1
< 4.17.8
4.17.8
craftcms/cms
 Packagist
≥ 5.0.0-RC1
< 5.9.14
5.9.14

References

GitHub Advisory / Wiki ADVISORY
https://github.com/craftcms/cms/security/advisories/GHSA-x76w-8c62-48mg
NVD — CVE-2026-56384 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-56384
Security Fix Commit FIX COMMIT
https://github.com/craftcms/cms/commit/d30df3112220db1ffd6726a3ed11857014c7fb27
Craft CMS Source Repository REPOSITORY
https://github.com/craftcms/cms
www.vulncheck.com/advisories/craft-cm... WEB
https://www.vulncheck.com/advisories/craft-cms-missing-authorization-in-assets-preview-thumb-endpoint

Frequently Asked Questions

What is CVE-2026-56384?
CVE-2026-56384 is a security vulnerability in Craft CMS involving Authenticated "assets/preview-thumb" discloses signed fallback transform preview link to CP users without asset-view permission under CWE-862: Missing Authorization.
Which Craft CMS versions are affected?
Affected versions include releases prior to 4.17.8, 5.9.14 across affected packages: craftcms/cms.
What version fixes CVE-2026-56384?
Upgrading to 4.17.8, 5.9.14 (or the latest patched release for your respective branch) resolves the vulnerability.
Is craftcms/cms affected?
Yes, craftcms/cms is affected in releases prior to 4.17.8, 5.9.14. Upgrade your dependency to 4.17.8, 5.9.14 or later.
What CWE is associated with CVE-2026-56384?
CVE-2026-56384 is classified under CWE-862: Missing Authorization (The product does not perform an authorization check when an actor attempts to access a resource or perform an action.).
How can I check Packagist dependencies for CVE-2026-56384?
Run the dependency tree command: 'inspect dependencies for craftcms/cms'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-x76w-8c62-48mg",
  "modified" : "2026-08-06T21:06:17Z",
  "published" : "2026-07-06T20:28:07Z",
  "aliases" : [ "CVE-2026-56384" ],
  "summary" : "Craft CMS: Authenticated \"assets/preview-thumb\" discloses signed fallback transform preview link to CP users without asset-view permission",
  "details" : "### Summary\n\nA 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.\n\n### Details\n\nRoot-cause analysis:\n1. The endpoint accepts an attacker-controlled `assetId`.\n2. Asset is resolved, and thumbnail HTML is returned.\n3. No explicit asset-view permission check is performed before preview generation.\n\n### Impact\n\nType:\n\n1. Missing authorization\n2. Unauthorized preview-link disclosure\n\n  Affected deployments:\n\n1. Craft sites with control panel users who have partial permissions and private assets.\n\n  Security consequence:\n\n  1. A control panel user without asset-view permission can still obtain signed preview transform link data for private assets.\n  2. This may increase private asset exposure risk depending on deployment and endpoint chaining.\n\n## Resources\n\nhttps://github.com/craftcms/cms/commit/d30df3112220db1ffd6726a3ed11857014c7fb27",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "craftcms/cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0-RC1"
      }, {
        "fixed" : "4.17.8"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.17.7"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "craftcms/cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.0.0-RC1"
      }, {
        "fixed" : "5.9.14"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 5.9.13"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/craftcms/cms/security/advisories/GHSA-x76w-8c62-48mg"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-56384"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/craftcms/cms/commit/d30df3112220db1ffd6726a3ed11857014c7fb27"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/craftcms/cms"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/craft-cms-missing-authorization-in-assets-preview-thumb-endpoint"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200", "CWE-862" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-06T20:28:07Z",
    "nvd_published_at" : null
  }
}