CVE-2026-103439 in MediaWiki
Summary
by MITRE • 09/30/2026
Improper neutralization of Script-Related HTML tags in a web page (basic XSS) vulnerability in The Wikimedia Foundation MediaWiki Wikbase extension allows Cross-Site Scripting (XSS).
This issue affects MediaWiki Wikbase extension: 1.46, 1.45, and 1.43.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 09/30/2026
The identified vulnerability represents a classic instance of improper neutralization of script-related HTML tags within the Wikimedia Foundation's MediaWiki platform, specifically affecting the Wikibase extension in versions 1.46, 1.45, and 1.43. This flaw is categorized under Cross-Site Scripting (XSS), which falls broadly under CWE-79: Improper Neutralization of Input During Web Page Generation. The core technical issue stems from the application's failure to adequately sanitize or encode user-supplied input before rendering it in an HTML context. When a web page processes data that contains executable script content, such as JavaScript code embedded within HTML tags like <script> or event handlers like onerror, and fails to neutralize these elements, the browser interprets them as active code rather than static text. In the specific case of the Wikibase extension, this likely occurs when user-provided metadata, labels, descriptions, or other data fields are rendered directly into the page structure without sufficient validation against a whitelist of safe characters or proper encoding mechanisms.
The operational impact of this vulnerability is significant because it allows an attacker to inject malicious client-side scripts that execute in the context of the victim's browser session while interacting with the affected MediaWiki instance. Since Wikibase is often used for managing structured data, particularly in environments where trust levels may vary among contributors, a successful exploitation could lead to unauthorized access to sensitive information stored within the wiki pages or user sessions. Attackers can leverage this flaw to steal cookies containing authentication tokens, session identifiers, or other credentials that grant access to privileged areas of the site. Furthermore, the injected script can perform actions on behalf of the victim without their knowledge, such as modifying page content, redirecting users to phishing sites, or defacing the website interface. This undermines the integrity and confidentiality guarantees provided by the platform.
From a threat modeling perspective aligned with the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Client-Side Scripting Injection (T1059) and potentially Credential Access via Browser Session Hijacking if session tokens are exfiltrated. The attack vector typically involves crafting a malicious URL or input field that contains the payload, which is then delivered to victims through social engineering or by exploiting other vulnerabilities in conjunction with this XSS flaw. Because MediaWiki is widely deployed across numerous public-facing wikis and internal knowledge bases, the potential blast radius extends beyond individual users to potentially affect entire communities relying on accurate data representation.
Mitigation strategies must focus on implementing robust input validation and output encoding practices consistent with industry standards for web application security. Developers should ensure that all user-supplied data is strictly validated against a whitelist of allowed characters before processing. More critically, when rendering this data in HTML contexts, it must be properly encoded to prevent the browser from interpreting special characters as executable code. This includes converting entities such as < and > into their corresponding HTML entity representations like < and >. Additionally, implementing Content Security Policy (CSP) headers can provide an additional layer of defense by restricting the sources from which scripts are allowed to load or execute, thereby mitigating the impact even if a script injection occurs. Upgrading to patched versions of the Wikibase extension where this issue has been resolved is the most effective immediate remediation step for administrators managing affected MediaWiki installations.