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CAPEC-53: Postfix, Null Terminate, and Backslash

Attack Pattern ID: 53
Abstraction: Detailed
Status: Draft
Presentation Filter:
+ Description
If a string is passed through a filter of some kind, then a terminal NULL may not be valid. Using alternate representation of NULL allows an adversary to embed the NULL mid-string while postfixing the proper data so that the filter is avoided. One example is a filter that looks for a trailing slash character. If a string insertion is possible, but the slash must exist, an alternate encoding of NULL in mid-string may be used.
+ Likelihood Of Attack

High

+ Typical Severity

High

+ 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.
NatureTypeIDName
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
Explore
  1. Survey the application for user-controllable inputs: Using a browser, an automated tool or by inspecting the application, an adversary records all entry points to the application.

    Techniques
    Use a spidering tool to follow and record all links and analyze the web pages to find entry points. Make special note of any links that include parameters in the URL.
    Use a proxy tool to record all user input entry points visited during a manual traversal of the web application.
    Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.
    Manually inspect the application to find entry points.
Experiment
  1. Probe entry points to locate vulnerabilities: The adversary uses the entry points gathered in the "Explore" phase as a target list and injects postfix null byte(s) followed by a backslash to observe how the application handles them as input. The adversary is looking for areas where user input is placed in the middle of a string, and the null byte causes the application to stop processing the string at the end of the user input.

    Techniques
    Try different encodings for null such as \0 or %00 followed by an encoding for the backslash character.
Exploit
  1. Remove data after null byte(s): After determined entry points that are vulnerable, the adversary places a null byte(s) followed by a backslash such that they bypass an input filter and remove data after the null byte(s) in a way that is beneficial to them.

    Techniques
    If the input is a directory as part of a longer file path, add a null byte(s) followed by a backslash at the end of the input to try to traverse to the given directory.
+ Prerequisites
Null terminators are not properly handled by the filter.
+ Skills Required
[Level: Medium]
An adversary needs to understand alternate encodings, what the filter looks for and the data format acceptable to the target API
+ Indicators
Null characters are observed by the filter. The filter needs to be able to understand various encodings of the Null character, or only canonical data should be passed to it.
+ 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.
ScopeImpactLikelihood
Integrity
Modify Data
Confidentiality
Read Data
Confidentiality
Access Control
Authorization
Gain Privileges
+ Mitigations
Properly handle Null characters. Make sure canonicalization is properly applied. Do not pass Null characters to the underlying APIs.
Assume all input is malicious. Create an allowlist that defines all valid input to the software system based on the requirements specifications. Input that does not match against the allowlist should not be permitted to enter into the system.
+ Example Instances

A rather simple injection is possible in a URL:

http://getAccessHostname/sekbin/
helpwin.gas.bat?mode=&draw=x&file=x&module=&locale=[insert relative path here]
[%00][%5C]&chapter=

This attack has appeared with regularity in the wild. There are many variations of this kind of attack. Spending a short amount of time injecting against Web applications will usually result in a new exploit being discovered.

+ References
[REF-1] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02.
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2017-01-09CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns
2020-07-30CAPEC Content TeamThe MITRE Corporation
Updated Mitigations
2021-06-24CAPEC Content TeamThe MITRE Corporation
Updated Related_Weaknesses
2022-02-22CAPEC Content TeamThe MITRE Corporation
Updated Description, Execution_Flow, Skills_Required
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Page Last Updated or Reviewed: October 21, 2021