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CAPEC-199: XSS Using Alternate Syntax

Attack Pattern ID: 199
Abstraction: Detailed
Presentation Filter:
+ Description
An adversary uses alternate forms of keywords or commands that result in the same action as the primary form but which may not be caught by filters. For example, many keywords are processed in a case insensitive manner. If the site's web filtering algorithm does not convert all tags into a consistent case before the comparison with forbidden keywords it is possible to bypass filters (e.g., incomplete black lists) by using an alternate case structure. For example, the "script" tag using the alternate forms of "Script" or "ScRiPt" may bypass filters where "script" is the only form tested. Other variants using different syntax representations are also possible as well as using pollution meta-characters or entities that are eventually ignored by the rendering engine. The attack can result in the execution of otherwise prohibited functionality.
+ Likelihood Of Attack

High

+ Typical Severity

High

+ Relationships
Section HelpThis 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.
NatureTypeIDName
ChildOfDetailed Attack PatternDetailed 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.588DOM-Based XSS
ChildOfDetailed Attack PatternDetailed 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.591Reflected XSS
ChildOfDetailed Attack PatternDetailed 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.592Stored XSS
Section HelpThis table shows the views that this attack pattern belongs to and top level categories within that view.
+ Execution Flow
Explore
  1. 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. 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. Make special note of any links that include parameters in the URL. Manual traversal of this type is frequently necessary to identify forms that are GET method forms rather than POST forms.
    Use a browser to manually explore the website and analyze how it is constructed. Many browser's plugins are available to facilitate the analysis or automate the URL discovery.
Experiment
  1. Probe identified potential entry points for XSS vulnerability: Possibly using an automated tool, an adversary requests variations on the inputs they surveyed before using alternate syntax. These inputs are designed to bypass incomplete filtering (e.g., incomplete HTML encoding etc.) and try many variations of characters injection that would enable the XSS payload. They record all the responses from the server that include unmodified versions of their script.

    Techniques
    Use a list of XSS probe strings to inject in parameters of known URLs. If possible, the probe strings contain a unique identifier. Attempt numerous variations based on form, format, syntax & encoding.
    Use a proxy tool to record results of manual input of XSS probes in known URLs.
  2. 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
    Change a URL parameter to include a malicious script tag created using alternate syntax to bypass filters.
    Send information gathered from the malicious script to a remote endpoint.
Exploit
  1. 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
Target client software must allow scripting such as JavaScript.
+ Skills Required
[Level: Low]
To inject the malicious payload in a web page
[Level: High]
To bypass non trivial filters in the application
+ Resources Required
Ability to send HTTP request to a web application.
+ Consequences
Section HelpThis table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Integrity
Modify Data
Confidentiality
Read Data
Authorization
Execute Unauthorized Commands
Accountability
Authentication
Authorization
Non-Repudiation
Gain Privileges
Access Control
Authorization
Bypass Protection Mechanism
+ Mitigations
Design: Use browser technologies that do not allow client side scripting.
Design: Utilize strict type, character, and encoding enforcement
Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification.
Implementation: Ensure all content coming from the client is using the same encoding; if not, the server-side application must canonicalize the data before applying any filtering.
Implementation: Perform input validation for all remote content, including remote and user-generated content
Implementation: Perform output validation for all remote content.
Implementation: Disable scripting languages such as JavaScript in browser
Implementation: Patching software. There are many attack vectors for XSS on the client side and the server side. Many vulnerabilities are fixed in service packs for browser, web servers, and plug in technologies, staying current on patch release that deal with XSS countermeasures mitigates this.
+ Example Instances

In this example, the adversary tries to get <script>alert(1)</script> executed by the victim's browser. The target application employs regular expressions to make sure no script is being passed through the application to the web page; such a regular expression could be ((?i)script), and the application would replace all matches by this regex by the empty string. An adversary will then create a special payload to bypass this filter:

<scriscriptpt>alert(1)</scscriptript>

when the applications gets this input string, it will replace all "script" (case insensitive) by the empty string and the resulting input will be the desired vector by the adversary:

<script>alert(1)</script>

In this example, we assume that the application needs to write a particular string in a client-side JavaScript context (e.g., <script>HERE</script>). For the adversary to execute the same payload as in the previous example, they would need to send alert(1) if there was no filtering. The application makes use of the following regular expression as filter

((\w+)\s*\(.*\)|alert|eval|function|document)

and replaces all matches by the empty string. For example each occurrence of alert(), eval(), foo() or even the string "alert" would be stripped. An adversary will then create a special payload to bypass this filter:

this['al' + 'ert'](1)

when the applications gets this input string, it won't replace anything and this piece of JavaScript has exactly the same runtime meaning as alert(1). The adversary could also have used non-alphanumeric XSS vectors to bypass the filter; for example,

($=[$=[]][(__=!$+$)[_=-~-~-~$]+({}+$)[_/_]+($$=($_=!''+$)[_/_]+$_[+$])])()[__[_/_]+__[_+~$]+$_[_]+$$](_/_)

would be executed by the JavaScript engine like alert(1) is.

+ References
[REF-69] "OWASP Cheatsheets". XSS Filter Evasion Cheat Sheet. The Open Web Application Security Project (OWASP). <https://www.owasp.org/www-community/xss-filter-evasion-cheatsheet>.
[REF-70] "OWASP Testing Guide". Testing for Cross site scripting. v2. The Open Web Application Security Project (OWASP). <http://www.owasp.org/index.php/Testing_for_Cross_site_scripting>.
[REF-71] "Non-alphanumeric XSS cheat sheet". <http://sla.ckers.org/forum/read.php?24,28687>.
[REF-72] "WASC Threat Classification 2.0". WASC-08 - Cross Site Scripting. The Web Application Security Consortium (WASC). 2010. <http://projects.webappsec.org/Cross-Site+Scripting>.
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23
(Version 2.6)
CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2017-05-01
(Version 2.10)
CAPEC Content TeamThe MITRE Corporation
Updated Description Summary, Related_Attack_Patterns, Related_Weaknesses
2019-04-04
(Version 3.1)
CAPEC Content TeamThe MITRE Corporation
Updated Consequences
2020-07-30
(Version 3.3)
CAPEC Content TeamThe MITRE Corporation
Updated Example_Instances, Execution_Flow
2022-02-22
(Version 3.7)
CAPEC Content TeamThe MITRE Corporation
Updated Example_Instances, Execution_Flow
2022-09-29
(Version 3.8)
CAPEC Content TeamThe MITRE Corporation
Updated Example_Instances
Previous Entry Names
Change DatePrevious Entry Name
2017-05-01
(Version 2.10)
Cross-Site Scripting Using Alternate Syntax
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Page Last Updated or Reviewed: October 21, 2021