CAPEC-199: XSS Using Alternate Syntax

Overview

CAPEC-199 (XSS Using Alternate Syntax) is a detailed-level attack pattern catalogued by MITRE in the Common Attack Pattern Enumeration and Classification (CAPEC). It describes a recurring method attackers use to exploit software weaknesses.

How the attack works

The phases an attacker typically follows to carry out this attack.

Step 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. 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.
Step 2
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.
- 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.
Step 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.
- 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.
Step 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.

What the attacker needs

Prerequisites

Target client software must allow scripting such as JavaScript.

Skills required

Low skill: To inject the malicious payload in a web page
High skill: To bypass non trivial filters in the application

Resources required

Ability to send HTTP request to a web application.

Consequences

What a successful CAPEC-199 attack can achieve.

Modify Data

Affects: Integrity

Read Data

Affects: Confidentiality

Execute Unauthorized Commands

Affects: Authorization

Run Arbitrary Code

Gain Privileges

Affects: Accountability, Authentication, Authorization, Non-Repudiation

Bypass Protection Mechanism

Affects: Access Control, Authorization

How to mitigate it

Defenses that reduce the risk of CAPEC-199.

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.

Examples

In this example, the adversary tries to get alert(1) 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: alert(1) 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: alert(1) In this example, we assume that the application needs to write a particular string in a client-side JavaScript context (e.g., HERE ). 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.

The CWE weaknesses that CAPEC-199 exploits.

CWE-87: Improper Neutralization of Alternate XSS Syntax

How this attack relates to others in the CAPEC hierarchy.

Parent

Frequently asked questions

Common questions about CAPEC-199.

What is CAPEC-199?

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.

How does a XSS Using Alternate Syntax attack work?

It typically unfolds over 4 phases. It begins with: [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.

How do you prevent CAPEC-199?

Design: Use browser technologies that do not allow client side scripting.

What weaknesses does CAPEC-199 target?

CAPEC-199 exploits 1 CWE weakness, including CWE-87 (Improper Neutralization of Alternate XSS Syntax).

How severe is CAPEC-199?

MITRE rates CAPEC-199 as High severity with high likelihood of attack.

References

Attack-pattern data is sourced from the MITRE CAPEC catalog (v3.9). Weakness associations link to the corresponding CWE entries on RadicalNotion.AI.