An issue was discovered in MediaWiki before 1.35.12, 1.36.x through 1.39.x before 1.39.5, and 1.40.x before 1.40.1. There is XSS in youhavenewmessagesmanyusers and youhavenewmessages i18n messages. This is related to MediaWiki:Youhavenewmessagesfromusers.
CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')