Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Opera, when configured with the "Determine action by MIME type" option disabled, interprets an object as an HTML document even when its MIME Content-Type is text/plain, which could allow remote attackers to execute arbitrary script in documents that the user does not expect, possibly through web applications that use a text/plain type to prevent cross-site scripting attacks.
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.