CAPEC-53 Postfix, Null Terminate, and Backslash

CAPEC ID: 53

CAPEC-53 Metadata

Likelihood of Attack

High

Typical Severity

High

Overview

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 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.

Prerequisites

Null terminators are not properly handled by the filter.

Execution Flow

Step Phase Description Techniques
1 Explore [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.
  • 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.
2 Experiment [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.
  • Try different encodings for null such as \0 or %00 followed by an encoding for the backslash character.
3 Exploit [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.
  • 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.

Potential Solutions / 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.

Related Weaknesses (CWE)

CWE ID Description
CWE-20 Improper Input Validation
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-158 Improper Neutralization of Null Byte or NUL Character
CWE-172 Encoding Error
CWE-173 Improper Handling of Alternate Encoding
CWE-697 Incorrect Comparison
CWE-707 Improper Neutralization

Related CAPECs

CAPEC ID Description
CAPEC-267 An adversary leverages the possibility to encode potentially harmful input or content used by applications such that the applications are ineffective at validating this encoding standard.

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