Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The (1) Special:Preferences and (2) Special:UserLogin pages in MediaWiki before 1.19.20, 1.22.x before 1.22.12 and 1.23.x before 1.23.5 allows remote authenticated users to conduct cross-site scripting (XSS) attacks or have unspecified other impact via crafted CSS, as demonstrated by modifying MediaWiki:Common.css.
CWE-79 - Cross Site Scripting
Cross-Site Scripting, commonly referred to as XSS, is the most dominant class of vulnerabilities. It allows an attacker to inject malicious code into a pregnable web application and victimize its users. The exploitation of such a weakness can cause severe issues such as account takeover, and sensitive data exfiltration. Because of the prevalence of XSS vulnerabilities and their high rate of exploitation, it has remained in the OWASP top 10 vulnerabilities for years.