New to CAPEC? Start Here
Home > CAPEC List > CAPEC-3: Using Leading 'Ghost' Character Sequences to Bypass Input Filters (Version 3.6)  

CAPEC-3: Using Leading 'Ghost' Character Sequences to Bypass Input Filters

Attack Pattern ID: 3
Abstraction: Detailed
Status: Draft
Presentation Filter:
+ Description
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.
+ Likelihood Of Attack


+ Typical Severity


+ Relationships
Section HelpThis table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.267Leverage Alternate Encoding
Section HelpThis table shows the views that this attack pattern belongs to and top level categories within that view.
+ Execution Flow
  1. Determine if the source code is available and if so, examine the filter logic.
  1. 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.
  2. 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.
  1. 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)
+ 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.
+ Skills Required
[Level: Medium]
The ability to make an API request, and knowledge of "ghost" characters that will not be filtered by any input validation. These "ghost" characters must be known to not affect the way in which the request will be interpreted.
+ Consequences
Section HelpThis table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Access Control
Gain Privileges
Modify Data
+ Mitigations
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).
+ Example Instances

Alternate Encoding with Ghost Characters in FTP and Web Servers

Some web and FTP servers fail to detect prohibited upward directory traversals if the user-supplied pathname contains extra characters such as an extra leading dot. For example, a program that will disallow access to the pathname "../test.txt" may erroneously allow access to that file if the pathname is specified as ".../test.txt". This attack succeeds because 1) the input validation logic fails to detect the triple-dot as a directory traversal attempt (since it isn't dot-dot), 2) some part of the input processing decided to strip off the "extra" dot, leaving the dot-dot behind.

Using the file system API as the target, the following strings are all equivalent to many programs:


As you can see, there are many ways to make a semantically equivalent request. All these strings ultimately result in a request for the file ../test.txt.

+ References
[REF-1] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02.
+ Content History
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modification DateModifierOrganization
2015-11-09CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases
2015-12-07CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases
2017-01-09CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases
2017-05-01CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases
2017-08-04CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases, Description Summary, Payload
2018-07-31CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases, Attacker_Skills_or_Knowledge_Required
2020-07-30CAPEC Content TeamThe MITRE Corporation
Updated Mitigations
2021-06-24CAPEC Content TeamThe MITRE Corporation
Updated Related_Weaknesses
More information is available — Please select a different filter.
Page Last Updated or Reviewed: October 21, 2021