CWE-692 — Vulnerability Class 5

CWE-692 represents a critical input validation weakness where developers rely on an incomplete denylist to mitigate Cross-Site Scripting (XSS) attacks. This flaw occurs when security controls attempt to block malicious payloads by filtering out known bad characters or tags, yet fail to account for the vast array of encoding techniques, browser-specific parsing quirks, and contextual variations that attackers exploit. Consequently, adversaries can bypass these rudimentary filters by using alternative syntaxes or obfuscation methods that were not explicitly listed in the exclusion rules. To avoid this vulnerability, security engineers must abandon restrictive denylists in favor of robust allowlist strategies or context-aware output encoding. By strictly defining acceptable input formats and ensuring data is properly escaped based on its specific context within the HTML structure, developers can effectively neutralize XSS threats regardless of the attacker’s evasion techniques.