An attacker supplies the target software with input data that contains sequences of special characters designed to bypass input validation logic. This exploit relies on the target making multiples passes over the input data and processing a "layer" of special characters with each pass. In this manner, the attacker can disguise input that would otherwise be rejected as invalid by concealing it with layers of special/escape characters that are stripped off by subsequent processing steps. The goal is to first discover cases where the input validation layer executes before one or more parsing layers. That is, user input may go through the following logic in an application: <parser1> --> <input validator> --> <parser2>. In such cases, the attacker will need to provide input that will pass through the input validator, but after passing through parser2, will be converted into something that the input validator was supposed to stop.

https://capec.mitre.org/data/definitions/43.html

Related CWE definitions

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
The product constructs all or part of a command, data structure, or record using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify how it is parsed or interpreted when it is sent to a downstream component.
The product constructs all or part of a command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended command when it is sent to a downstream component.
The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.
The product validates input before applying protection mechanisms that modify the input, which could allow an attacker to bypass the validation via dangerous inputs that only arise after the modification.
The product validates data before it has been filtered, which prevents the product from detecting data that becomes invalid after the filtering step.
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are explicitly allowed by policy because the inputs are assumed to be safe, but the list is too permissive - that is, it allows an input that is unsafe, leading to resultant weaknesses.
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete, leading to resultant weaknesses.
The product compares two entities in a security-relevant context, but the comparison is incorrect, which may lead to resultant weaknesses.
The product does not ensure or incorrectly ensures that structured messages or data are well-formed and that certain security properties are met before being read from an upstream component or sent to a downstream component.
Please note that CAPEC definitions are provided as a quick reference only. Visit http://capec.mitre.org/ for a complete list of CAPEC entries and more information.
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