Developers should anticipate that substitution characters will be injected/removed/manipulated in the input vectors of their software system. Use an appropriate combination of black lists and white lists to ensure only valid, expected and appropriate input is processed by the system.

Assume all input is malicious. Use a standard input validation mechanism to validate all input for length, type, syntax, and business rules before accepting the data to be diplayed or stored. Use an "accept known good" validation strategy.

Use and specify a strong output encoding (such as ISO 8859-1 or UTF 8).

Do not rely exclusively on blacklist validation to detect malicious input or to encode output. There are too many variants to encode a character; you're likely to miss some variants.

Inputs should be decoded and canonicalized to the application's current internal representation before being validated. Make sure that your application does not decode the same input twice. Such errors could be used to bypass whitelist schemes by introducing dangerous inputs after they have been checked.