COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-267

Leverage Alternate Encoding

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.

Abstraction

Standard

Typical severity

High

Likelihood of attack

High

Understand the attack

What is this attack pattern?

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.

Execution flow

How does the attack proceed, step by step?

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.

Techniques used

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

Probe entry points to locate vulnerabilities

The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various payloads using a variety of different types of encodings to determine if an entry point actually represents a vulnerability with insufficient validation logic and to characterize the extent to which the vulnerability can be exploited.

Techniques used

Try to use different encodings of content in order to bypass validation routines.

Requirements

What does the attack require?

Prerequisites

The application's decoder accepts and interprets encoded characters. Data canonicalization, input filtering and validating is not done properly leaving the door open to harmful characters for the target host.

Skills required

Low: An adversary can inject different representation of a filtered character in a different encoding.
Medium: An adversary may craft subtle encoding of input data by using the knowledge that they have gathered about the target host.

Resources required

Not stated in the MITRE record.

Impact

What does a successful attack lead to?

Modify Data

Affected scopes: Integrity

Read Data

Affected scopes: Confidentiality

Execute Unauthorized Commands

Affected scopes: Authorization

Gain Privileges

Affected scopes: Accountability, Authentication, Authorization, Non-Repudiation

Bypass Protection Mechanism

Affected scopes: Access Control, Authorization

Unreliable ExecutionResource Consumption

Affected scopes: Availability

Defence

How is it prevented and mitigated?

Assume all input might use an improper representation. Use canonicalized data inside the application; all data must be converted into the representation used inside the application (UTF-8, UTF-16, etc.)

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. Test your decoding process against malicious input.

Real world

MITRE example instances

Example instance 1Detail

Microsoft Internet Explorer 5.01 SP4, 6, 6 SP1, and 7 does not properly handle unspecified "encoding strings," which allows remote adversaries to bypass the Same Origin Policy and obtain sensitive information via a crafted web site, aka "Post Encoding Information Disclosure Vulnerability." Related Vulnerabilities CVE-2010-0488

Example instance 2Detail

Adversaries may attempt to make an executable or file difficult to discover or analyze by encrypting, encoding, or otherwise obfuscating its contents on the system or in transit. This is common behavior that can be used across different platforms and the network to evade defenses.