CAPEC-244 XSS Targeting URI Placeholders

CAPEC-244 Metadata

Likelihood of Attack

High

Typical Severity

High

Overview

Summary

An attack of this type exploits the ability of most browsers to interpret "data", "javascript" or other URI schemes as client-side executable content placeholders. This attack consists of passing a malicious URI in an anchor tag HREF attribute or any other similar attributes in other HTML tags. Such malicious URI contains, for example, a base64 encoded HTML content with an embedded cross-site scripting payload. The attack is executed when the browser interprets the malicious content i.e., for example, when the victim clicks on the malicious link.

Prerequisites

Target client software must allow scripting such as JavaScript and allows executable content delivered using a data URI scheme.

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. 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.
2	Experiment	[Probe identified potential entry points for reflected XSS vulnerability] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various payloads formatted as data URI schemes using base to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited.	Use a list of XSS probe strings using different URI schemes to inject in parameters of known URLs. If possible, the probe strings contain a unique identifier to trace the injected string back to the entry point. Use a proxy tool to record results of manual input of XSS probes in known URLs.
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 payload formatted as a URI scheme, or use the URL returned when the URI scheme was given as input to the web application. 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

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.

Related Weaknesses (CWE)

CWE ID	Description
CWE-83	Improper Neutralization of Script in Attributes in a Web Page

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.