Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The Stop Spammers WordPress plugin before 2021.9 did not escape user input when blocking requests (such as matching a spam word), outputting it in an attribute after sanitising it to remove HTML tags, which is not sufficient and lead to a reflected Cross-Site Scripting issue.
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.