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CAPEC-71: Using Unicode Encoding to Bypass Validation Logic

Using Unicode Encoding to Bypass Validation Logic
Definition in a New Window Definition in a New Window
Attack Pattern ID: 71
Abstraction: Detailed
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

An attacker may provide a Unicode string to a system component that is not Unicode aware and use that to circumvent the filter or cause the classifying mechanism to fail to properly understanding the request. That may allow the attacker to slip malicious data past the content filter and/or possibly cause the application to route the request incorrectly.

+ Attack Steps
  1. Survey the application for user-controllable inputs: Using a browser or an automated tool, an attacker follows all public links and actions on a web site. He records all the links, the forms, the resources accessed and all other potential entry-points for the web 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.

  1. Probe entry points to locate vulnerabilities: The attacker uses the entry points gathered in the "Explore" phase as a target list and injects various Unicode encoded payloads 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 Unicode encoding of content in Scripts in order to bypass validation routines.

    Try to use Unicode encoding of content in HTML in order to bypass validation routines.

    Try to use Unicode encoding of content in CSS in order to bypass validation routines.

+ Attack Prerequisites
  • Filtering is performed on data that has not be properly canonicalized.

+ Typical Severity


+ Typical Likelihood of Exploit

Likelihood: Medium

+ Methods of Attack
  • Modification of Resources
  • API Abuse
  • Injection
+ Examples-Instances


A very common technique for a Unicode attack involves traversing directories looking for interesting files. An example of this idea applied to the Web is


In this case, the attacker is attempting to traverse to a directory that is not supposed to be part of standard Web services. The trick is fairly obvious, so many Web servers and scripts prevent it. However, using alternate encoding tricks, an attacker may be able to get around badly implemented request filters.

In October 2000, an adversary publicly revealed that Microsoft's IIS server suffered from a variation of this problem. In the case of IIS, all the attacker had to do was provide alternate encodings for the dots and/or slashes found in a classic attack. The Unicode translations are

. yields C0 AE
/ yields C0 AF
\ yields C1 9C

Using this conversion, the previously displayed URL can be encoded as


Related Vulnerabilities

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Medium

An attacker needs to understand Unicode encodings and have an idea (or be able to find out) what system components may not be Unicode aware.

+ Indicators-Warnings of Attack

Unicode encoded data is passed to APIs where it is not expected

+ Solutions and Mitigations

Ensure that the system is Unicode aware and can properly process Unicode data. Do not make an assumption that data will be in ASCII.

Ensure that filtering or input validation is applied to canonical data.

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
Bypass protection mechanism
Execute unauthorized code or commands
Run Arbitrary Code
Modify application data
DoS: crash / exit / restart
+ Relevant Security Requirements

Canonicalize data prior to performing any validation or filtering on it. Be aware of alternate encodings.

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: MediumIntegrity Impact: HighAvailability Impact: Medium
+ Technical Context
Architectural Paradigms
+ References
[R.71.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
[R.71.2] [REF-3] "Common Weakness Enumeration (CWE)". CWE-20 - Input Validation. Draft. The MITRE Corporation. 2007. <http://cwe.mitre.org/data/definitions/20.html>.
+ 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