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.
Medium

1
Explore
[Survey the application for user-controllable inputs] Using a browser or an automated tool, an adversary 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.
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 links 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.

2
Experiment
[Probe identified potential entry points for XSS using double characters] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads modified to use double characters and doubled special characters to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited.
Use a list of XSS probe strings using double characters to inject script in parameters of known URLs. If possible, the probe strings contain a unique identifier.
Use a proxy tool to record results of manual input of XSS probes in known URLs.
Use a list of doubled HTML special characters to inject into parameters of known URLs and check if they were properly encoded, replaced, or filtered out.

3
Experiment
[Craft malicious XSS URL] Once the adversary has determined which parameters are vulnerable to XSS, they will craft a malicious URL containing the XSS exploit. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from the victim.
Execute a script using an expression embedded in an HTML attribute, which avoids needing to inject a script tag.
Send information gathered from the malicious script to a remote endpoint.

4
Exploit
[Get victim to click URL] In order for the attack to be successful, the victim needs to access the malicious URL.
Send a phishing email to the victim containing the malicious URL. This can be hidden in a hyperlink as to not show the full URL, which might draw suspicion.
Put the malicious URL on a public forum, where many victims might accidentally click the link.

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 adversary 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.

CAPEC Content Team
The MITRE Corporation
2014-06-23

CAPEC Content Team
The MITRE Corporation
2017-05-01
Updated Related_Attack_Patterns, Related_Weaknesses

CAPEC Content Team
The MITRE Corporation
2022-02-22
Updated Description, Execution_Flow, Resources_Required

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