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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 attacker 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


+ 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. An attacker first probes to figure out what restrictions on input are placed by filter, such as a specific characters on the end of the URL.
  1. The attacker then injects a string of their choosing with a null terminator (using an alternate encoding such as %00), followed by a backslash (%5C), followed by some additional characters that are required to keep the filter happy
  2. The malicious string then passes through the filter and passed to the underlying API. Everything after the null terminator is ignored. This may give an attacker the opportunity to access file system resources to which they should not have access and do other things.

    Some popular forms in which this takes place:



    PATH[alternate encoding of the NULL][additional characters required to pass filter]
+ Prerequisites
Null terminators are not properly handled by the filter.
+ Skills Required
[Level: Medium]
An attacker 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.
Modify Data
Read Data
Access Control
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:

helpwin.gas.bat?mode=&draw=x&file=x&module=&locale=[insert relative path here]

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
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
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
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Page Last Updated or Reviewed: October 21, 2021