Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Mozilla Firefox before 22.214.171.124 and 2.x before 126.96.36.199, and SeaMonkey before 1.0.8 ignores trailing invalid HTML characters in attribute names, which allows remote attackers to bypass content filters that use regular expressions.
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.