CAPEC Details
Name Using Leading 'Ghost' Character Sequences to Bypass Input Filters
Likelyhood of attack Typical severity
Medium Medium
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.
Execution Flow
Step Phase Description Techniques
1 Explore Determine if the source code is available and if so, examine the filter logic.
2 Experiment If the source code is not available, write a small program that loops through various possible inputs to given API call and tries a variety of alternate (but equivalent) encodings of strings with leading ghost characters. Knowledge of frameworks and libraries used and what filters they apply will help to make this search more structured.
3 Experiment Observe the effects. See if the probes are getting past the filters. Identify a string that is semantically equivalent to that which an adversary wants to pass to the targeted API, but syntactically structured in a way as to get past the input filter. That encoding will contain certain ghost characters that will help it get past the filters. These ghost characters will be ignored by the targeted API.
4 Exploit Once the "winning" alternate encoding using (typically leading) ghost characters is identified, an adversary can launch the attacks against the targeted API (e.g. directory traversal attack, arbitrary shell command execution, corruption of files)
Solutions Use an allowlist rather than a denylist 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-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
Related CAPECS
CAPEC ID Description
CAPEC-267 An adversary leverages the possibility to encode potentially harmful input or content used by applications such that the applications are ineffective at validating this encoding standard.