Home > CAPEC List > CAPEC-53: Postfix, Null Terminate, and Backslash (Version 2.11)  

CAPEC-53: Postfix, Null Terminate, and Backslash

Postfix, Null Terminate, and Backslash
Definition in a New Window Definition in a New Window
Attack Pattern ID: 53
Abstraction: Detailed
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

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.

+ Attack Steps
  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.

+ Attack Prerequisites
  • Null terminators are not properly handled by the filter.

+ Typical Severity


+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
+ Examples-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.

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Medium

An attacker needs to understand alternate encodings, what the filter looks for and the data format acceptable to the target API

+ Probing Techniques

Test the program with various inputs and observe the behavior of the filter. Overtime it should be possible to understand what the filter is expecting.

+ Indicators-Warnings of Attack

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.

+ Solutions and 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 a white list that defines all valid input to the software system based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Modify application data
Read memory
Gain privileges / assume identity
+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Low
+ Technical Context
Architectural Paradigms
+ References
[R.53.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
+ Content History
CAPEC Content TeamThe MITRE Corporation2014-06-23Internal_CAPEC_Team
CAPEC Content TeamThe MITRE Corporation2017-01-09Updated Related_Attack_PatternsInternal

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Page Last Updated or Reviewed: July 31, 2017