Qxss

Qxss, often written as QXSS, is a term used in web security to describe a class of cross-site scripting flaws that originate from user-supplied data in URL query strings and are reflected into a web page without proper sanitization. The term emphasizes the role of query parameters in triggering the vulnerability, though Qxss can involve server- or client-side code that inserts these values into HTML, JavaScript, or attribute contexts.

Attackers exploit Qxss by crafting a URL containing a malicious payload in a query parameter, which the application then includes in the response in an unsafe manner. If the payload is interpreted by the browser as code or as part of the page structure, the attacker may execute scripts in the victim’s session. Qxss is typically considered a form of reflected XSS, and defenses overlap with general XSS protections, but focusing on query-string entry points helps auditors identify specific weaknesses.

Prevention relies on defense-in-depth: encode or escape all data derived from query parameters according to the destination context (HTML body, attribute, JavaScript, URL); use safe templating libraries that auto-escape; avoid dynamic code evaluation (eval, new Function); validate inputs and enforce strict content security policies; and suppress sensitive data in error messages. Regular testing should include common XSS payloads in query strings and cross-context validation.

See also: Cross-site scripting, Reflected XSS, Content Security Policy, URL encoding.