CVE-2026-0671: MediaWiki UploadWizard XSS Vulnerability

CVE-2026-0671 Overview

CVE-2026-0671 is a Cross-Site Scripting (XSS) vulnerability affecting the Wikimedia Foundation MediaWiki UploadWizard extension. The vulnerability stems from improper neutralization of user-supplied input during web page generation, allowing attackers to inject malicious scripts that execute in the context of other users' browsers.

The UploadWizard extension is a widely-used MediaWiki component that provides a step-by-step interface for uploading multiple files to wikis, making this vulnerability particularly concerning for MediaWiki deployments that rely on this extension for file uploads.

Critical Impact

Attackers can exploit this XSS vulnerability to execute arbitrary JavaScript in victim browsers, potentially stealing session cookies, performing actions on behalf of authenticated users, or redirecting users to malicious websites.

Affected Products

• MediaWiki UploadWizard extension version 1.45
• MediaWiki UploadWizard extension version 1.44
• MediaWiki UploadWizard extension version 1.43
• MediaWiki UploadWizard extension version 1.39

Discovery Timeline

• 2026-01-08 - CVE-2026-0671 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2026-0671

Vulnerability Analysis

This vulnerability is classified as CWE-79: Improper Neutralization of Input During Web Page Generation. The UploadWizard extension fails to properly sanitize user-controlled input before rendering it within web pages, creating an attack surface for Cross-Site Scripting attacks.

The vulnerability requires user interaction to exploit successfully—an attacker must craft a malicious payload and entice a victim to interact with it. Due to the changed scope of the vulnerability, a successful exploit can impact resources beyond the vulnerable component, potentially affecting the broader MediaWiki installation and user sessions.

The attack can be initiated remotely over the network without requiring any privileges or authentication, though user interaction is necessary for successful exploitation. The vulnerability impacts both confidentiality and integrity at a limited level, as attackers can read sensitive data and modify content within the user's session context.

Root Cause

The root cause of this vulnerability lies in insufficient input validation and output encoding within the UploadWizard extension. When processing user-supplied data during the file upload workflow, the extension fails to properly escape or sanitize content before inserting it into HTML output. This allows specially crafted input containing JavaScript code to pass through unfiltered and execute when the page is rendered in a victim's browser.

Attack Vector

The attack leverages the network-accessible nature of MediaWiki installations. An attacker can craft a malicious URL or form submission containing XSS payloads targeting the UploadWizard functionality. When a victim user interacts with this crafted content—such as clicking a link or viewing a manipulated upload page—the malicious script executes within their browser session.

The vulnerability manifests in the upload workflow where user-controlled input is reflected or stored without proper sanitization. For detailed technical information about the specific vulnerable code paths and the implemented fix, refer to the Wikimedia Gerrit Commit and Wikimedia Phabricator Task T407157.

Detection Methods for CVE-2026-0671

Indicators of Compromise

• Unusual JavaScript execution or suspicious script injections in MediaWiki page source
• Unexpected redirects or pop-ups when users interact with UploadWizard functionality
• Session hijacking indicators such as users reporting unauthorized actions on their accounts
• Web server logs showing requests with encoded script tags or JavaScript event handlers in URL parameters

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect common XSS payload patterns targeting MediaWiki endpoints
• Monitor application logs for requests containing suspicious characters like <script>, javascript:, or encoded variations
• Deploy Content Security Policy (CSP) headers with report-uri to capture and alert on policy violations
• Use browser-based XSS auditors and security tools to identify reflected script content

Monitoring Recommendations

• Enable detailed logging for the UploadWizard extension and review logs for anomalous input patterns
• Configure Security Information and Event Management (SIEM) alerts for XSS-related signatures in web traffic
• Implement real-time monitoring of CSP violation reports to detect attempted exploitation
• Conduct periodic security scans of MediaWiki installations using automated vulnerability scanners

How to Mitigate CVE-2026-0671

Immediate Actions Required

• Update the UploadWizard extension to the latest patched version immediately
• Review MediaWiki access logs for signs of exploitation attempts
• Implement strict Content Security Policy headers to limit JavaScript execution sources
• Consider temporarily disabling the UploadWizard extension if patching is not immediately possible

Patch Information

Wikimedia Foundation has released a security patch addressing this vulnerability. The fix involves proper sanitization and encoding of user-supplied input to prevent script injection. Administrators should apply the patch referenced in the Wikimedia Gerrit Commit or update to a patched version of the UploadWizard extension.

For tracking and additional details about the vulnerability and remediation, refer to the Wikimedia Phabricator Task T407157.

Workarounds

• Deploy a Web Application Firewall with XSS protection rules as an additional defense layer
• Implement strict Content Security Policy headers: Content-Security-Policy: default-src 'self'; script-src 'self'
• Disable or restrict access to the UploadWizard extension until patching is complete
• Enable HttpOnly and Secure flags on session cookies to limit the impact of potential session theft

# Example LocalSettings.php configuration to add CSP headers
$wgCSPHeader = true;
$wgCSPReportOnlyHeader = [
    'default-src' => [ "'self'" ],
    'script-src' => [ "'self'" ],
    'report-uri' => '/csp-report-endpoint'
];