A developer uses a 'whitelist' approach for input validation: only accept values matching an expected pattern. A security reviewer prefers this over a 'blacklist' approach. Why is allowlisting (whitelisting) fundamentally stronger for security?

• A.An allowlist defines a complete set of acceptable inputs; anything not matching is rejected. A blocklist attempts to enumerate all bad inputs - which is inherently incomplete since attackers can use encoding variants, new attack patterns, and context-specific bypasses not yet in the blocklist. Allowlists have no 'unknown unknowns' because the default is reject
• B.Allowlists are required by PCI-DSS regulations; blind SQL injection cannot exfiltrate data and only confirms whether a database is present; input validation alone fixes XSS, because data that enters clean can never become dangerous in a different output context; second-order injection is theoretical, as data written through a parameterized insert is marked tainted and re-escaped on read; XSS payloads cannot survive URL encoding, and frameworks that encode query parameters are immune by construction