CAPEC-18 XSS Targeting Non-Script Elements

CAPEC-18 Metadata

Likelihood of Attack

High

Typical Severity

Very High

Overview

Summary

This attack is a form of Cross-Site Scripting (XSS) where malicious scripts are embedded in elements that are not expected to host scripts such as image tags (<img>), comments in XML documents (< !-CDATA->), etc. These tags may not be subject to the same input validation, output validation, and other content filtering and checking routines, so this can create an opportunity for an adversary to tunnel through the application's elements and launch a XSS attack through other elements. As with all remote attacks, it is important to differentiate the ability to launch an attack (such as probing an internal network for unpatched servers) and the ability of the remote adversary to collect and interpret the output of said attack.

Prerequisites

The target client software must allow the execution of scripts generated by remote hosts.

Execution Flow

Step	Phase	Description	Techniques
1	Explore	[Survey the application for user-controllable inputs] Using a browser or an automated tool, an adversary records all entry points for inputs that happen to be reflected in a client-side non-script element. These non-script elements can be located in the HTML content (head, body, comments), in an HTML tag, XML, CSS, etc.	Use a spidering tool to follow and record all non-static links that are likely to have input parameters (through forms, URL, fragments, etc.) actively used by the Web application. 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 vulnerability] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited.	Manually inject various script payloads into each identified entry point using a list of common script injection probes that typically work in a client-side non-script elements context and observe system behavior to determine if script was executed. Since these probes may have to be injected in many different types of non-script elements, they should cover a variety of possible contexts (CSS, HTML tag, XML, etc.). Use an automated injection attack tool to inject various script payloads into each identified entry point using a list of common script injection probes that typically work in a client-side non-script elements context and observe system behavior to determine if script was executed. Since these probes may have to be injected in many different types of non-script elements, they should cover a variety of possible contexts (CSS, HTML tag, XML, etc.). Use a proxy tool to record results of the created requests.
3	Experiment	[Create malicious XSS content] Once the adversary has determined which entry points are vulnerable to XSS, they will interact with the web application to store the malicious content. Because of the nature of this attack, it is mostly carried out through stored XSS, although it is possible to perform this attack using reflected XSS. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from a victim.	Store a malicious script on a page, disguised as a non-script element, that will execute when viewed by the victim. Use a tool such as BeEF to store a hook into the web application. This will alert the adversary when the victim has accessed the content and will give the adversary control over the victim's browser, allowing them access to cookies, user screenshot, user clipboard, and more complex XSS attacks.
4	Exploit	[Get victim to view stored content] In order for the attack to be successful, the victim needs to view the stored malicious content on the webpage.	Send a phishing email to the victim containing a URL that will direct them to the malicious stored content. Simply wait for a victim to view the content. This is viable in situations where content is posted to a popular public forum.

Potential Solutions / Mitigations

In addition to the traditional input fields, all other user controllable inputs, such as image tags within messages or the likes, must also be subjected to input validation. Such validation should ensure that content that can be potentially interpreted as script by the browser is appropriately filtered. All output displayed to clients must be properly escaped. Escaping ensures that the browser interprets special scripting characters literally and not as script to be executed.

Related Weaknesses (CWE)

CWE ID	Description
CWE-80	Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

Related CAPECs

CAPEC ID	Description
CAPEC-588	This type of attack is a form of Cross-Site Scripting (XSS) where a malicious script is inserted into the client-side HTML being parsed by a web browser. Content served by a vulnerable web application includes script code used to manipulate the Document Object Model (DOM). This script code either does not properly validate input, or does not perform proper output encoding, thus creating an opportunity for an adversary to inject a malicious script launch a XSS attack. A key distinction between other XSS attacks and DOM-based attacks is that in other XSS attacks, the malicious script runs when the vulnerable web page is initially loaded, while a DOM-based attack executes sometime after the page loads. Another distinction of DOM-based attacks is that in some cases, the malicious script is never sent to the vulnerable web server at all. An attack like this is guaranteed to bypass any server-side filtering attempts to protect users.
CAPEC-591	This type of attack is a form of Cross-Site Scripting (XSS) where a malicious script is "reflected" off a vulnerable web application and then executed by a victim's browser. The process starts with an adversary delivering a malicious script to a victim and convincing the victim to send the script to the vulnerable web application.
CAPEC-592	An adversary utilizes a form of Cross-site Scripting (XSS) where a malicious script is persistently "stored" within the data storage of a vulnerable web application as valid input.