Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Phase: Implementation

Strategy: Output Encoding

Perform proper output validation and escaping to neutralize all code syntax from data written to code files.

"HTML injection" (see XSS) could be thought of as an example of this, but it is executed on the client side, not the server side. Server-Side Includes (SSI) are an example of direct static code injection.

This issue is most frequently found in PHP applications that allow users to set configuration variables that are stored within executable php files. Technically, this could also be performed in some compiled code (e.g. by byte-patching an executable), although it is highly unlikely.