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.
Typical Severity
Medium
Relationships
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
Nature
Type
ID
Name
ChildOf
Detailed Attack Pattern - A detailed level attack pattern in CAPEC provides a low level of detail, typically leveraging a specific technique and targeting a specific technology, and expresses a complete execution flow. Detailed attack patterns are more specific than meta attack patterns and standard attack patterns and often require a specific protection mechanism to mitigate actual attacks. A detailed level attack pattern often will leverage a number of different standard level attack patterns chained together to accomplish a goal.
Detailed Attack Pattern - A detailed level attack pattern in CAPEC provides a low level of detail, typically leveraging a specific technique and targeting a specific technology, and expresses a complete execution flow. Detailed attack patterns are more specific than meta attack patterns and standard attack patterns and often require a specific protection mechanism to mitigate actual attacks. A detailed level attack pattern often will leverage a number of different standard level attack patterns chained together to accomplish a goal.
Detailed Attack Pattern - A detailed level attack pattern in CAPEC provides a low level of detail, typically leveraging a specific technique and targeting a specific technology, and expresses a complete execution flow. Detailed attack patterns are more specific than meta attack patterns and standard attack patterns and often require a specific protection mechanism to mitigate actual attacks. A detailed level attack pattern often will leverage a number of different standard level attack patterns chained together to accomplish a goal.
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.
Techniques
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.
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.
Techniques
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.
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.
Techniques
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.
Exploit
Get victim to click URL: In order for the attack to be successful, the victim needs to access the malicious URL.
Techniques
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.
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.
Resources Required
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.
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.
Related Weaknesses
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
Description
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
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