The CheckUser extension’s Special:CheckUser interface is vulnerable to reflected XSS via the rev-deleted-user message. This message is rendered without proper escaping, making it possible to inject JavaScript through the uselang=x-xss language override mechanism.
This issue affects Mediawiki - CheckUser extension: from 1.39.X before 1.39.13, from 1.42.X before 1.42.7, from 1.43.X before 1.43.2.
CVE ID: CVE-2025-53479
Vendor: Wikimedia Foundation
Product: Mediawiki - CheckUser extension
EPSS Score: 0.03% (probability of being exploited)
EPSS Percentile: 6.3% (scored less or equal to compared to others)
EPSS Date: 2025-07-10 (when was this score calculated)