Bypassing Security Measures

Modern web applications often implement various security measures to prevent XSS, but skilled attackers have developed techniques to bypass these defenses. Input validation might block certain characters or patterns, but alternative encodings or context-specific tricks can evade these filters. For instance, if an application blocks single quotes, attackers might use double quotes or template literals. If script tags are filtered, event handlers or other JavaScript execution contexts provide alternatives.

Content Security Policy (CSP) headers represent a powerful defense against XSS, but misconfigurations or overly permissive policies can still allow attacks. An CSP that allows 'unsafe-inline' for scripts negates much of its protective value. Attackers might also exploit CSP bypasses through JSONP endpoints, AngularJS libraries with unsafe-eval, or base tag injection to control script sources. Understanding these bypass techniques helps developers implement more robust defenses and avoid common security pitfalls.