# CVE-2026-100376: MediaWiki TemplateSandbox cross-site scripting via user namespace

Source: https://junglewise.ai/threats/cve-2026-100376-improper-neutralization-of-input-during-web-page-generation-xss  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-25. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-100376 · Severity: info · Published 2026-09-25

MediaWiki's TemplateSandbox extension allows users to preview wiki templates in a sandboxed environment. An attacker can create a malicious JavaScript file in their own user namespace, then trick another user into previewing a page with a specially crafted link and clicking a button, executing arbitrary JavaScript in the victim's browser with their full wiki privileges. This enables account compromise, data theft, or further attacks against the wiki.

Vendors: [Wikimedia Foundation](https://junglewise.ai/threats/vendors/wikimedia-foundation).

## Quick answers

**Is CVE-2026-100376 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-100376 being exploited in the wild.

**How severe is CVE-2026-100376?** Informational severity, published 25 September 2026.

**Which products and versions are affected?** Wikimedia Foundation MediaWiki TemplateSandbox Extension before 1.46.1, 1.45.5, 1.43.10.

**Is there a fix for CVE-2026-100376?** A patch was published on 2026-06-29. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-100376 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-100376?** Check whether you run Wikimedia Foundation MediaWiki TemplateSandbox Extension at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The TemplateSandbox extension improperly loads JavaScript and CSS from MediaWiki site pages (e.g., Common.js) when a sandbox prefix is set to any user's namespace, without validating that the prefix belongs to the viewing user. An attacker constructs a URL to Special:TemplateSandbox with prefix=User:Attacker/sandbox, creates a malicious User:Attacker/sandbox/MediaWiki:Common.js page, and tricks another user into clicking the View button, executing the attacker's script in the victim's security context. The fix restricts JS/CSS loading to only the current user's own namespace.

## Affected products

- Wikimedia Foundation MediaWiki TemplateSandbox Extension before 1.46.1, 1.45.5, 1.43.10

## Timeline

- 2026-09-25: disclosed
- 2026-06-29: patched
- 2026-07-10: other: Fix proposed

## References

- [https://gerrit.wikimedia.org/r/q/I6630b9915f640bf75c8570165eb8728bd7a2c07a](https://gerrit.wikimedia.org/r/q/I6630b9915f640bf75c8570165eb8728bd7a2c07a)
- [https://phabricator.wikimedia.org/T430596](https://phabricator.wikimedia.org/T430596)

## Related threats

- [Mediawiki TemplateSandbox Extension authorization bypass](https://junglewise.ai/threats/cve-2026-100240-missing-authorization-vulnerability-in-wikimedia-foundation)