CVE-2025-53478 is a reflected Cross-Site Scripting (XSS) vulnerability identified in the CheckUser extension of the Wikimedia Foundation's Mediawiki software. Specifically, the vulnerability exists in the Special:Investigate interface, within the “IPs and User agents” tab. The root cause is improper escaping of certain internationalized system messages, which allows malicious input to be reflected back in the web page without proper neutralization. This flaw affects multiple versions of the CheckUser extension: from 1.39.x before 1.39.13, from 1.42.x before 1.42.7, and from 1.43.x before 1.43.2. The vulnerability is classified under CWE-79, which pertains to improper neutralization of input during web page generation, leading to XSS attacks. The CVSS v3.1 base score is 5.4 (medium severity), with vector AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N. This indicates that the attack can be performed remotely over the network with low attack complexity, requires privileges (PR:L) and user interaction (UI:R), and impacts confidentiality and integrity with a scope change (S:C). No known exploits are currently reported in the wild. The vulnerability allows an attacker to inject malicious scripts that execute in the context of the victim’s browser when they access the vulnerable interface, potentially leading to session hijacking, credential theft, or unauthorized actions within the Mediawiki environment. Since the CheckUser extension is typically used by trusted administrators to investigate user IPs and agents, exploitation could undermine administrative trust and lead to privilege escalation or data leakage within Mediawiki deployments.

For European organizations using Mediawiki with the CheckUser extension, this vulnerability poses a moderate risk. Mediawiki is widely used for collaborative documentation, knowledge bases, and internal wikis across public institutions, universities, and private enterprises in Europe. Exploitation could allow attackers to execute scripts in the browsers of privileged users (e.g., administrators), potentially compromising sensitive administrative functions and user data. This could lead to unauthorized access to user investigation data, manipulation of user records, or further lateral movement within the organization’s network. The reflected XSS nature means that attackers would typically need to trick an administrator into clicking a crafted link, which may limit large-scale automated exploitation but still represents a significant targeted threat. The confidentiality and integrity of user investigation data could be compromised, which is critical for organizations relying on Mediawiki for audit trails and user accountability. Additionally, compromised administrative accounts could be leveraged to disrupt services or leak sensitive organizational information. Given the scope change in the CVSS vector, the impact could extend beyond the immediate application, affecting broader organizational security posture.

1. Immediate upgrade of the CheckUser extension to the fixed versions: 1.39.13 or later for the 1.39.x branch, 1.42.7 or later for the 1.42.x branch, and 1.43.2 or later for the 1.43.x branch. 2. Implement strict Content Security Policy (CSP) headers to restrict the execution of inline scripts and reduce the impact of XSS attacks. 3. Limit access to the Special:Investigate interface to only highly trusted administrators and enforce multi-factor authentication (MFA) to reduce the risk of compromised credentials. 4. Conduct regular security training for administrators to recognize and avoid phishing attempts that could deliver malicious URLs exploiting this vulnerability. 5. Monitor web server logs and Mediawiki audit logs for unusual access patterns or suspicious query parameters targeting the Special:Investigate interface. 6. If immediate patching is not possible, consider disabling the CheckUser extension temporarily or restricting its access via network-level controls (e.g., IP whitelisting). 7. Review and sanitize all internationalized system messages and user-generated content rendered in administrative interfaces to ensure proper escaping and encoding.