CAPEC-71 Using Unicode Encoding to Bypass Validation Logic

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.

1. [Explore] [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. They record all the links, the forms, the resources accessed and all other potential entry-points for the web application. 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 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.

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

• [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.

• Confidentiality, Access Control, Authorization: Bypass Protection Mechanism • Confidentiality, Integrity, Availability: Execute Unauthorized Commands • Integrity: Modify Data • Availability: Unreliable Execution

• 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 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.