Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The default configuration of ASP.NET in Mono before 2.6.4 has a value of FALSE for the EnableViewStateMac property, which allows remote attackers to conduct cross-site scripting (XSS) attacks, as demonstrated by the __VIEWSTATE parameter to 2.0/menu/menu1.aspx in the XSP sample project.
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.