CVE-2026-107850: Contao Improper Access Control Vulnerability

MODERATE
Advisory Identifier: GHSA-q6wp-fr43-gm9v
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CVSS 4.3
CWE-639 CWE-862
Published
Advisory Summary

CVE-2026-107850 is a MODERATE (CVSS 4.3) improper access control vulnerability (CWE-639: Authorization Bypass Through User-Controlled Key) affecting Contao libraries in the Packagist ecosystem. The issue involves improper access control in the preview links module. Remediation is available by upgrading affected packages to version 5.7.12.

CVE-2026-107850 at a Glance

CVE Identifier CVE-2026-107850
GHSA Identifier GHSA-q6wp-fr43-gm9v
Severity MODERATE
CVSS v3.1 Base Score 4.3
CWE Classification CWE-639: Authorization Bypass Through User-Controlled Key
Affected Product Contao
Package Ecosystem Packagist
Published Date
Last Updated

Security Impact

CVSS: 4.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-107850

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

contao/core-bundle fixed: 5.7.12 
Upgrade contao/core-bundle to 5.7.12
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 contao/core-bundle

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-107850, inspect your project dependencies for Contao:

inspect dependencies for contao/core-bundle

Analyze the terminal output to locate any version of Contao in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 5.7.12), 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-639: Authorization Bypass Through User-Controlled Key, where the system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data. In Contao, 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

`core-bundle/config/services.yaml` registers the preview access voter with the class `Contao\CoreBundle\Security\Voter\DataContainer\PreviewAccessVoter`, but the class shipped on disk is named `PreviewVoter`. Symfony resolves the class with `getReflectionClass($class, false)`, gets `null`, silently skips the interface based autoconfiguration that would have tagged it as a voter, and the now untagged private service is removed when the container is compiled. The application boots without a warning, and the ownership rule in `PreviewVoter::hasAccess()` is never executed. `tl_preview_link` has no compensating SQL filter on `createdBy`, so any non administrator back end user whose group holds the `preview_link` module reads every preview link on the installation and lifts the already signed share URLs straight out of the list. Following one grants front end preview of the target page, with `showUnpublished` and with no page permission check, to anyone at all. ### Impact A non administrator back end user reads every preview link on the installation, including links created by administrators, and lifts the already signed share URLs out of the list. Following one grants front end preview of the target page with `showUnpublished` and with no page permission check at all, so unpublished or embargoed content becomes readable by a user who has no rights on those pages, and by anyone that user forwards the URL to. **Honesty caveat, stated because it bounds the severity.** `UpdateAction` on a record owned by someone else was denied in our lab by `DcaPermissionVoter`, so we do not claim that a foreign preview link can be edited or deleted.

CVE-2026-107850 Affected Contao Versions

1 component(s)
Fixed Versions:
5.7.12
Product / Package Ecosystem Affected Versions Fixed Version
contao/core-bundle
 Packagist
≥ 5.7.1
< 5.7.12
5.7.12

References

GitHub Advisory / Wiki ADVISORY
https://github.com/contao/contao/security/advisories/GHSA-q6wp-fr43-gm9v
Security Fix Commit FIX COMMIT
https://github.com/contao/contao/commit/6b483d380a4b2dc61432c4c697e411508f93bf91
Contao Source Repository REPOSITORY
https://github.com/contao/contao
Contao Release RELEASE
https://github.com/contao/contao/releases/tag/5.7.12

Frequently Asked Questions

What is CVE-2026-107850?
CVE-2026-107850 is a security vulnerability in Contao involving Improper access control in the preview links module under CWE-639: Authorization Bypass Through User-Controlled Key.
Which Contao versions are affected?
Affected versions include releases prior to 5.7.12 across affected packages: contao/core-bundle.
What version fixes CVE-2026-107850?
Upgrading to 5.7.12 (or the latest patched release for your respective branch) resolves the vulnerability.
Is contao/core-bundle affected?
Yes, contao/core-bundle is affected in releases prior to 5.7.12. Upgrade your dependency to 5.7.12 or later.
What is the CVSS score of CVE-2026-107850?
CVE-2026-107850 has a CVSS v3.1 base score of 4.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.
What CWE is associated with CVE-2026-107850?
CVE-2026-107850 is classified under CWE-639: Authorization Bypass Through User-Controlled Key (The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.).
How can I check Packagist dependencies for CVE-2026-107850?
Run the dependency tree command: 'inspect dependencies for contao/core-bundle'. 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-q6wp-fr43-gm9v",
  "modified" : "2026-10-09T20:53:46Z",
  "published" : "2026-10-09T20:53:46Z",
  "aliases" : [ "CVE-2026-107850" ],
  "summary" : "Contao: Improper access control in the preview links module",
  "details" : "`core-bundle/config/services.yaml` registers the preview access voter with the class `Contao\\CoreBundle\\Security\\Voter\\DataContainer\\PreviewAccessVoter`, but the class shipped on disk is named `PreviewVoter`. Symfony resolves the class with `getReflectionClass($class, false)`, gets `null`, silently skips the interface based autoconfiguration that would have tagged it as a voter, and the now untagged private service is removed when the container is compiled. The application boots without a warning, and the ownership rule in `PreviewVoter::hasAccess()` is never executed. `tl_preview_link` has no compensating SQL filter on `createdBy`, so any non administrator back end user whose group holds the `preview_link` module reads every preview link on the installation and lifts the already signed share URLs straight out of the list. Following one grants front end preview of the target page, with `showUnpublished` and with no page permission check, to anyone at all.\n\n### Impact\n\nA non administrator back end user reads every preview link on the installation, including links created by administrators, and lifts the already signed share URLs out of the list. Following one grants front end preview of the target page with `showUnpublished` and with no page permission check at all, so unpublished or embargoed content becomes readable by a user who has no rights on those pages, and by anyone that user forwards the URL to.\n\n**Honesty caveat, stated because it bounds the severity.** `UpdateAction` on a record owned by someone else was denied in our lab by `DcaPermissionVoter`, so we do not claim that a foreign preview link can be edited or deleted.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "contao/core-bundle"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.7.1"
      }, {
        "fixed" : "5.7.12"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/security/advisories/GHSA-q6wp-fr43-gm9v"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/commit/6b483d380a4b2dc61432c4c697e411508f93bf91"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/contao/contao"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/releases/tag/5.7.12"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-639", "CWE-862" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-09T20:53:46Z",
    "nvd_published_at" : null
  }
}