Name Using Leading 'Ghost' Character Sequences to Bypass Input Filters
Summary Some APIs will strip certain leading characters from a string of parameters. An adversary can intentionally introduce leading "ghost" characters (extra characters that don't affect the validity of the request at the API layer) that enable the input to pass the filters and therefore process the adversary's input. This occurs when the targeted API will accept input data in several syntactic forms and interpret it in the equivalent semantic way, while the filter does not take into account the full spectrum of the syntactic forms acceptable to the targeted API.
Prerequisites The targeted API must ignore the leading ghost characters that are used to get past the filters for the semantics to be the same.
Solutions Perform white list rather than black list input validation. Canonicalize all data prior to validation. Take an iterative approach to input validation (defense in depth).
Related Weaknesses
CWE ID Description
CWE-20 Improper Input Validation
CWE-41 Improper Resolution of Path Equivalence
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-171 DEPRECATED: Cleansing, Canonicalization, and Comparison Errors
CWE-172 Encoding Error
CWE-173 Improper Handling of Alternate Encoding
CWE-179 Incorrect Behavior Order: Early Validation
CWE-180 Incorrect Behavior Order: Validate Before Canonicalize
CWE-181 Incorrect Behavior Order: Validate Before Filter
CWE-183 Permissive List of Allowed Inputs
CWE-184 Incomplete List of Disallowed Inputs
CWE-697 Incorrect Comparison
CWE-707 Improper Neutralization
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