CAPEC-588 DOM-Based XSS – Attack Pattern Analysis

CAPEC-588 Metadata

Likelihood of Attack

High

Typical Severity

Very High

Overview

Summary

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.

Prerequisites

An application that leverages a client-side web browser with scripting enabled. An application that manipulates the DOM via client-side scripting. An application that failS to adequately sanitize or encode untrusted input.

Execution Flow

Step	Phase	Description	Techniques
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 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.
2	Experiment	[Probe identified potential entry points for DOM-based XSS vulnerability] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads and special characters to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited. Specific to DOM-based XSS, the adversary is looking for areas where input is being used to directly change the DOM.	Use a list of XSS probe strings 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 HTML special characters to inject into parameters of known URLs and check if they were properly encoded, replaced, or filtered out.
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. In DOM-based XSS, the malicious script might not even be sent to the server, since the victim's browser will manipulate the DOM itself. This can help avoid serve-side detection mechanisms.	Change a URL parameter to include a malicious script tag. Add a URL fragment to alter the value of the expected Document object URL. Send information gathered from the malicious script to a remote endpoint.
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.

Potential Solutions / Mitigations

Use browser technologies that do not allow client-side scripting. Utilize proper character encoding for all output produced within client-site scripts manipulating the DOM. Ensure that all user-supplied input is validated before use.

Related Weaknesses (CWE)

CWE ID	Description
CWE-20	Improper Input Validation
CWE-79	Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-83	Improper Neutralization of Script in Attributes in a Web Page

Related CAPECs

CAPEC ID	Description
CAPEC-63	An adversary embeds malicious scripts in content that will be served to web browsers. The goal of the attack is for the target software, the client-side browser, to execute the script with the users' privilege level. An attack of this type exploits a programs' vulnerabilities that are brought on by allowing remote hosts to execute code and scripts. Web browsers, for example, have some simple security controls in place, but if a remote attacker is allowed to execute scripts (through injecting them in to user-generated content like bulletin boards) then these controls may be bypassed. Further, these attacks are very difficult for an end user to detect.

CAPEC ID

Description

CAPEC-63

An adversary embeds malicious scripts in content that will be served to web browsers. The goal of the attack is for the target software, the client-side browser, to execute the script with the users' privilege level. An attack of this type exploits a programs' vulnerabilities that are brought on by allowing remote hosts to execute code and scripts. Web browsers, for example, have some simple security controls in place, but if a remote attacker is allowed to execute scripts (through injecting them in to user-generated content like bulletin boards) then these controls may be bypassed. Further, these attacks are very difficult for an end user to detect.

Taxonomy Mappings

Taxonomy: OWASP Attacks

Entry ID	Entry Name
Link	Reflected DOM Injection