CVE-2025-53479: CheckUser: Reflected Cross-Site Scripting (XSS) in Special:CheckUser via unsanitized internationalized message

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-07-08 · updated 2026-06-17
Weakness: CWE-79

Description

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 1.42.X before 1.42.7, from 1.43.X before 1.43.2.

Affected products

Wikimedia Foundation Mediawiki - CheckUser extension: from 1.42.x before 1.42.7; from 1.43.x before 1.43.2 — fixed in 1.42.7, 1.43.2

How to fix

Upgrade to a fixed version: Wikimedia Foundation Mediawiki - CheckUser extension 1.42.7, 1.43.2. Check the vendor advisory for mitigations.

References

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