COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record
CAPEC-199
XSS Using Alternate Syntax
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.
Abstraction
Detailed
Typical severity
High
Likelihood of attack
High
Understand the attack
What is this attack pattern?
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.
Execution flow
How does the attack proceed, step by step?
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.
Techniques used
- 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
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 used
- 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.
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 used
- 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
Get victim to click URL
In order for the attack to be successful, the victim needs to access the malicious URL.
Techniques used
- 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.
Requirements
What does the attack require?
Prerequisites
- Target client software must allow scripting such as JavaScript.
Skills required
- Low: To inject the malicious payload in a web page
- High: To bypass non trivial filters in the application
Resources required
- Ability to send HTTP request to a web application.
Impact
What does a successful attack lead to?
Modify Data
Affected scopes: Integrity
Read Data
Affected scopes: Confidentiality
Execute Unauthorized Commands
Affected scopes: Authorization
Gain Privileges
Affected scopes: Accountability, Authentication, Authorization, Non-Repudiation
Bypass Protection Mechanism
Affected scopes: Access Control, Authorization
Defence
How is it prevented and mitigated?
1
Design: Use browser technologies that do not allow client side scripting.
2
Design: Utilize strict type, character, and encoding enforcement
3
Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification.
4
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.
5
Implementation: Perform input validation for all remote content, including remote and user-generated content
6
Implementation: Perform output validation for all remote content.
7
Implementation: Disable scripting languages such as JavaScript in browser
8
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.
Real world
MITRE example instances
Example instance 1Detail
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.