CAPEC-245

Detailed Abstraction Level
Meta — Very abstract, high-level category
Standard — Specific enough to understand
Detailed — Tied to specific technique Draft MITRE CAPEC Status
Stable — Fully reviewed and complete
Draft — Under development
Incomplete — Partially defined
Deprecated — No longer recommended
Obsolete — Replaced by another CAPEC Severity: Medium
XSS Using Doubled Characters

Description

The adversary bypasses input validation by using doubled characters in order to perform a cross-site scripting attack. Some filters fail to recognize dangerous sequences if they are preceded by repeated characters. For example, by doubling the < before a script command, (<<script or %3C%3script using URI encoding) the filters of some web applications may fail to recognize the presence of a script tag. If the targeted server is vulnerable to this type of bypass, the adversary can create a crafted URL or other trap to cause a victim to view a page on the targeted server where the malicious content is executed, as per a normal XSS attack.

Prerequisites

The targeted web application does not fully normalize input before checking for prohibited syntax. In particular, it must fail to recognize prohibited methods preceded by certain sequences of repeated characters.

Mitigations

Design: Use libraries and templates that minimize unfiltered input.

Implementation: Normalize, filter and sanitize all user supplied fields.

Implementation: The victim should configure the browser to minimize active content from untrusted sources.