Improper Validation of Syntactic Correctness of Input

The product receives input that is expected to be well-formed - i.e., to comply with a certain syntax - but it does not validate or incorrectly validates that the input complies with the syntax.


Description

Often, complex inputs are expected to follow a particular syntax, which is either assumed by the input itself, or declared within metadata such as headers. The syntax could be for data exchange formats, markup languages, or even programming languages. When untrusted input is not properly validated for the expected syntax, attackers could cause parsing failures, trigger unexpected errors, or expose latent vulnerabilities that might not be directly exploitable if the input had conformed to the syntax.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code loads and parses an XML file.

// Read DOM
try {
  ...
  DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
  factory.setValidating( false );
  ....
  c_dom = factory.newDocumentBuilder().parse( xmlFile );
} catch(Exception ex) {
  ...
}

The XML file is loaded without validating it against a known XML Schema or DTD.

See Also

Comprehensive CWE Dictionary

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Weaknesses without Software Fault Patterns

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