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CVE-2026-100240 – Authorization Bypass / Information Disclosure – MediaWiki TemplateSandbox Extensio

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Summary

CVE-2026-100240 is a missing authorization vulnerability (CWE-862) in the TemplateSandbox extension for MediaWiki. The extension fails to check read permissions on the page being previewed, allowing a user who should be blocked from viewing a restricted page (e.g., via the Lockdown extension) to nonetheless read its contents through TemplateSandbox’s preview functionality. The issue carries a CVSS v3.1 base score of 9.1 (Critical) and requires no privileges or user interaction to exploit.

Technical details

  • Root cause: TemplateSandbox provides page-preview functionality via Special:TemplateSandbox (a "Render page" field) and a "Preview page with this template" field in the edit interface. Neither interface validated that the requesting user holds read (authorizeRead) permission on the target page before rendering its content.
  • Trigger conditions: An attacker simply needs network access to a MediaWiki instance with TemplateSandbox installed and must supply the title of a page restricted by access-control mechanisms (such as the Lockdown extension) to the render/preview field.
  • Attack vector: Network (remote, unauthenticated or low-privileged depending on wiki configuration) — no special user interaction is required.
  • Impact: Unauthorized disclosure of content from pages that are supposed to be access-restricted, bypassing ACL enforcement. The vulnerability affects confidentiality and integrity of access-controlled content; availability is not impacted.

Affected software

  • Wikimedia Foundation MediaWiki – TemplateSandbox Extension, all versions before:
    • 1.46.1
    • 1.45.5
    • 1.43.10

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: Upgrade the TemplateSandbox extension to version 1.46.1, 1.45.5, or 1.43.10 (depending on your MediaWiki release track), which adds the missing read-permission check before previewing a page.
  • If no patch can be applied immediately: Restrict or disable access to Special:TemplateSandbox and the template-preview feature for untrusted or low-privileged user groups, particularly on wikis that rely on extensions such as Lockdown to enforce page-level access restrictions, until the patch can be deployed.

References

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