Postfix, Null Terminate, and Backslash
Attack Pattern ID: 53 (Detailed Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ 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 midstring 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 midstring may be used.

Attack 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.

  2. 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

  3. 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]
+ Attack Prerequisites

Null terminators are not properly handled by the filter.

+ 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
  • Data Modification
  • Information Leakage
  • Privilege Escalation
+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
158Improper Neutralization of Null Byte or NUL CharacterTargeted
172Encoding ErrorTargeted
173Failure to Handle Alternate EncodingTargeted
171Cleansing, Canonicalization, and Comparison ErrorsTargeted
74Failure to Sanitize Data into a Different Plane ('Injection')Targeted
20Improper Input ValidationTargeted
697Insufficient ComparisonTargeted
707Improper Enforcement of Message or Data StructureTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern52Embedding NULL Bytes 
Mechanism of Attack1000
ChildOfAttack PatternAttack Pattern267Leverage Alternate Encoding 
Mechanism of Attack (primary)1000
+ Related Security Principles
  • Reluctance to Trust

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Low
+ Technical Context
Architectural Paradigms
+ References
G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
+ Content History
G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.Cigital, Inc2007-03-01
Eugene LebanidzeCigital, Inc2007-02-26Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software"
Sean BarnumCigital, Inc2007-03-05Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Related Attack Patterns
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback