Defending Against Advanced Bypasses

Defending Against Advanced Bypasses

Defending against sophisticated bypass techniques requires multiple layers of protection. Input normalization represents the first line of defense – before validating or filtering input, normalize it to a consistent form. This includes decoding all encodings, normalizing Unicode, removing null bytes and control characters, and converting to a standard character set. Only after normalization should validation occur, and even then, positive validation (allowing known good patterns) is more secure than negative validation (blocking known bad patterns).

Implement context-aware output encoding that accounts for bypass techniques specific to each context. Don't rely on simple HTML encoding everywhere – use appropriate encoding for JavaScript, CSS, URLs, and other contexts. Modern template engines handle this automatically, but understand their limitations and verify they're configured securely. When manual encoding is necessary, use well-tested libraries rather than rolling your own encoding functions.

Content Security Policy provides crucial defense against bypasses that evade other protections. A strict CSP that blocks inline scripts defeats many sophisticated bypass techniques. Even if an attacker successfully injects a payload that bypasses filters and encoding, CSP prevents its execution. Implement CSP gradually, starting with report-only mode to identify necessary exceptions. Use nonces or hashes rather than 'unsafe-inline', and regularly review and tighten your policy.