CWE 159

Precise terminology for the underlying weaknesses does not exist. Therefore, these weaknesses use the terminology associated with the manipulation.

Developers should anticipate that special elements 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.

Phase: Architecture and Design

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 displayed 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.

The variety of manipulations that involve special elements is staggering. This is one reason why they are so frequently reported.

Customized languages and grammars, even those that are specific to a particular product, are potential sources of weaknesses that are related to special elements. However, most researchers concentrate on the most commonly used representations for data transmission, such as HTML and SQL. Any representation that is commonly used is likely to be a rich source of weaknesses; researchers are encouraged to investigate previously unexplored representations.

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