CVE-2026-58031: MediaWiki XSS Vulnerability

CVE-2026-58031 Overview

CVE-2026-58031 is a Cross-Site Scripting (XSS) vulnerability in Wikimedia Foundation MediaWiki. The flaw is associated with the resources/src/mediawiki.Special.Apisandbox/ApiSandboxLayout.Js client-side resource. Improper neutralization of user-supplied input during web page generation allows attackers to inject script content that executes in a victim's browser session [CWE-79].

The issue affects MediaWiki versions from 1.46.0-rc.0 before 1.46.0. Exploitation requires an authenticated user with low privileges and user interaction, limiting mass exploitation potential.

Critical Impact
Successful exploitation can execute attacker-controlled JavaScript in the context of a MediaWiki user viewing a crafted API Sandbox page, enabling session abuse and UI manipulation.

Affected Products

Wikimedia Foundation MediaWiki 1.46.0-rc.0
Wikimedia Foundation MediaWiki pre-release builds up to but not including 1.46.0
Deployments loading resources/src/mediawiki.Special.Apisandbox/ApiSandboxLayout.Js

Discovery Timeline

2026-07-01 - CVE-2026-58031 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-58031

Vulnerability Analysis

The vulnerability is a stored or reflected Cross-Site Scripting weakness in the MediaWiki Special:ApiSandbox interface. The affected resource ApiSandboxLayout.Js renders user-controllable content into the DOM without sufficient neutralization. When the browser parses the injected markup, script content executes under the origin of the MediaWiki instance.

The vulnerability is tracked under CWE-79: Improper Neutralization of Input During Web Page Generation. It is exploitable over the network, requires low-privileged authentication, and requires user interaction to trigger the malicious payload.

Root Cause

The root cause is missing or incomplete output encoding within the ApiSandboxLayout.Js client-side component. Data flowing through the ApiSandbox layout is inserted into the page without HTML-context escaping. This allows script fragments supplied through API parameters or crafted URLs to be interpreted as executable code by the browser.

Attack Vector

An attacker with a low-privilege account crafts input containing JavaScript payloads that pass through Special:ApiSandbox. The attacker then lures a victim, such as an administrator or editor, to load the crafted sandbox state. Once rendered, the payload executes in the victim's browser, allowing session cookie theft where cookies are accessible, cross-site request forgery against MediaWiki endpoints, and defacement of rendered content.

Refer to the Wikimedia Task Discussion for the vendor's technical write-up of the affected code path.

Detection Methods for CVE-2026-58031

Indicators of Compromise

Unexpected <script> tags, event handler attributes, or javascript: URIs in requests to Special:ApiSandbox.
Outbound requests from user browsers to attacker-controlled domains immediately after loading MediaWiki pages.
Unusual API actions performed under privileged accounts shortly after those users viewed sandbox URLs.

Detection Strategies

Inspect web server access logs for requests to Special:ApiSandbox containing suspicious query parameters or encoded script content.
Deploy Content Security Policy (CSP) reporting to capture inline script violations from the MediaWiki origin.
Correlate authentication events with abnormal API usage patterns to identify session abuse.

Monitoring Recommendations

Monitor the resources/src/mediawiki.Special.Apisandbox/ApiSandboxLayout.Js endpoint for anomalous request volumes.
Alert on any modification to MediaWiki JavaScript resources served to end users.
Track privileged user sessions for concurrent activity from multiple geographies or user agents.

How to Mitigate CVE-2026-58031

Immediate Actions Required

Upgrade MediaWiki to version 1.46.0 or later, which contains the fix for the ApiSandbox layout resource.
Restrict access to Special:ApiSandbox to trusted user groups until patching is complete.
Enforce a strict Content Security Policy that blocks inline script execution on MediaWiki pages.

Patch Information

Wikimedia Foundation resolved the issue in MediaWiki 1.46.0. Administrators running the 1.46.0-rc.0 release candidate or any interim build should apply the stable 1.46.0 release. Reference the Wikimedia Task Discussion for patch and commit details.

Workarounds

Disable the ApiSandbox extension or restrict its use through $wgGroupPermissions until the upgrade is applied.
Apply a Content Security Policy with script-src 'self' and no unsafe-inline directive to reduce payload execution.
Require re-authentication for privileged actions to limit the impact of stolen session state.

bash

# Configuration example: restrict ApiSandbox to trusted groups in LocalSettings.php
$wgGroupPermissions['*']['viewmyprivateinfo'] = false;
$wgGroupPermissions['user']['apisandbox'] = false;
$wgGroupPermissions['sysop']['apisandbox'] = true;

# Enforce a strict Content Security Policy header
$wgCSPHeader = [
    'script-src' => ["'self'"],
    'object-src' => ["'none'"],
    'base-uri'   => ["'self'"],
];