CAPEC-245 - Cross-Site Scripting Using Doubled Characters, e.g. %3C%3Cscript

The attacker 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 attacker 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.






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.

The attacker must trick the victim into following a crafted link to a vulnerable server or view a web post where the dangerous commands are executed.

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.