CAPEC Details
Name Leverage Alternate Encoding
Likelyhood of attack Typical severity
High High
Summary 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.
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.
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 attacker 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 attacker 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.
  • Try to use different encodings of content in order to bypass validation routines.
Solutions 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.
Related Weaknesses
CWE ID Description
CWE-20 Improper Input Validation
CWE-73 External Control of File Name or Path
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-172 Encoding Error
CWE-173 Improper Handling of Alternate Encoding
CWE-180 Incorrect Behavior Order: Validate Before Canonicalize
CWE-181 Incorrect Behavior Order: Validate Before Filter
CWE-692 Incomplete Denylist to Cross-Site Scripting
CWE-697 Incorrect Comparison
Related CAPECS
CAPEC ID Description
CAPEC-153 An attacker exploits a weakness in input validation by controlling the format, structure, and composition of data to an input-processing interface. By supplying input of a non-standard or unexpected form an attacker can adversely impact the security of the target. For example, using a different character encoding might cause dangerous text to be treated as safe text. Alternatively, the attacker may use certain flags, such as file extensions, to make a target application believe that provided data should be handled using a certain interpreter when the data is not actually of the appropriate type. This can lead to bypassing protection mechanisms, forcing the target to use specific components for input processing, or otherwise causing the user's data to be handled differently than might otherwise be expected. This attack differs from Variable Manipulation in that Variable Manipulation attempts to subvert the target's processing through the value of the input while Input Data Manipulation seeks to control how the input is processed.