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Cross-site scripting

Cross-site scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. It allows an attacker to circumvent the same-origin policy, which is designed to segregate different websites from each other. Cross-site scripting vulnerabilities normally allow an attacker to masquerade as a victim user, to carry out any actions that the user is able to perform and to access any of the user's data. If the victim user has privileged access within the application, then the attacker might be able to gain full control over all of the application's functionality and data.

How does XSS work?

Cross-site scripting works by manipulating a vulnerable web site so that it returns malicious JavaScript to users. When the malicious code executes inside a victim's browser, the attacker can fully compromise their interaction with the application.
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What are the types of XSS attacks?

There are three main types of XSS attacks. These are:
  • Reflected XSS, where the malicious script comes from the current HTTP request.
  • Stored XSS, where the malicious script comes from the website's database.
  • DOM-based XSS, where the vulnerability exists in client-side code rather than server-side code.

Reflected XSS

Reflected cross-site scripting (or XSS) arises when an application receives data in an HTTP request and includes that data within the immediate response in an unsafe way.
Suppose a website has a search function which receives the user-supplied search term in a URL parameter:
The application echoes the supplied search term in the response to this URL:
<p>You searched for: gift</p>
Assuming the application doesn't perform any other processing of the data, an attacker can construct an attack like this:
This URL results in the following response:
<p>You searched for: <script>/* Bad stuff here... */</script></p>
If another user of the application requests the attacker's URL, then the script supplied by the attacker will execute in the victim user's browser, in the context of their session with the application.

Stored XSS

Stored cross-site scripting (also known as second-order or persistent XSS) arises when an application receives data from an untrusted source and includes that data within its later HTTP responses in an unsafe way.
Suppose a website allows users to submit comments on blog posts, which are displayed to other users. Users submit comments using an HTTP request like the following:
POST /post/comment HTTP/1.1
Host: vulnerable-website.com
Content-Length: 100
After this comment has been submitted, any user who visits the blog post will receive the following within the application's response:
<p>This post was extremely helpful.</p>
Assuming the application doesn't perform any other processing of the data, an attacker can submit a malicious comment like this:
<script>/* Bad stuff here... */</script>
Within the attacker's request, this comment would be URL-encoded as:
Any user who visits the blog post will now receive the following within the application's response:
<p><script>/* Bad stuff here... */</script></p>
The script supplied by the attacker will then execute in the victim user's browser, in the context of their session with the application.

DOM-based XSS

DOM-based XSS vulnerabilities usually arise when JavaScript takes data from an attacker-controllable source, such as the URL, and passes it to a sink that supports dynamic code execution, such as eval() or innerHTML. This enables attackers to execute malicious JavaScript, which typically allows them to hijack other users' accounts.
To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript.
The most common source for DOM XSS is the URL, which is typically accessed with the window.location object. An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path.
In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. Additionally, the website's scripts might perform validation or other processing of data that must be accommodated when attempting to exploit a vulnerability. There are a variety of sinks that are relevant to DOM-based vulnerabilities. Please refer to the list below for details.
The document.write sink works with script elements, so you can use a simple payload, such as the one below:
document.write('... <script>alert(document.domain)</script> ...');
Lab examples:
"><svg onload=alert(1)>
^ Breakout
^ Mechanism abuse
<img src=1 onerror=alert(1)>
^ Image - error creation

DOM XSS combined with reflected and stored data

Some pure DOM-based vulnerabilities are self-contained within a single page. If a script reads some data from the URL and writes it to a dangerous sink, then the vulnerability is entirely client-side.
However, sources aren't limited to data that is directly exposed by browsers - they can also originate from the website. For example, websites often reflect URL parameters in the HTML response from the server. This is commonly associated with normal XSS, but it can also lead to so-called reflected+DOM vulnerabilities.
In a reflected+DOM vulnerability, the server processes data from the request and echoes the data into the response. The reflected data might be placed into a JavaScript string literal, or a data item within the DOM, such as a form field. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink.
eval('var data = "reflected string"');