What is Cross-Site Scripting Prevention?
Cross-site scripting prevention is the process of detecting and remediating XSS vulnerabilities in your websites or web applications before they hit production. The detection of XSS vulnerabilities can be done automatically, using an automated vulnerability scanner, or manually by performing penetration tests. In this article you will learn the best practices for cross-site scripting prevention and how you can implement them immediately. At the end of the article, you will also find a link to a free, developer-focused vulnerability scanner so you can start detecting and remediating cross-site scripting vulnerabilities early and often, as part of your development pipelines
In this article, you will learn:
- What is Cross-Site Scripting Prevention?
- How does cross-site scripting work?
- What are the types of XSS attacks?
- How important is Cross-site scripting prevention?
- Cross site scripting protection
- XSS Prevention with code examples
- Cross-site scripting prevention in Python (Django)
- Cross-site scripting prevention in Ruby (Rails)
- Cross-site scripting prevention in Java (Java Server Pages)
- Cross-site scripting prevention in C# (ASP.NET)
- Cross-site scripting prevention in Node
- Cross-site scripting prevention in PHP
- Cross-site scripting prevention in AngularJS
- Cross-site scripting prevention in React
- How can automated tools help prevent cross-site scripting?
How does cross-site scripting work?
Cross-Site Scripting (XSS) attacks are a form of injection attack, where malicious scripts are injected into trusted web applications.
An attacker can use the web application to send malicious code, typically in the form of a browser side script, to a different end user, resulting in an XSS attack.
XSS vulnerabilities are very common, occurring where a web application uses input from a valid user contained within the generated output, but without the appropriate validation or encoding.
With the malicious script sent to the user, their browser is unable to categorically know that the script should not be trusted, and subsequently executes the script. This script can then access a multitude of data, including any cookies, session tokens, or indeed any other sensitive information that may be retained by the browser for that site.
What are the types of XSS attacks?
There are three main types of XSS attacks:
- Reflected XSS: malicious script comes from the current HTTP request
- Stored XSS: malicious script comes from the website’s database
- DOM-based XSS: where the vulnerability exists in client-side code rather than server-side code.
How important is Cross-site scripting prevention?
The damage from exploiting an XSS vulnerability depends on the sensitivity of the data your site handles. Here are some examples where hackers exploited XSS vulnerable apps:
- Spreading worms on social media: Facebook, Twitter and YouTube have all been successfully attacked in this way.
- Session hijacking: Malicious JavaScript may be able to send the session ID to a remote site under the hacker’s control, allowing the hacker to impersonate that user by hijacking a session in progress.
- Identity theft: If the user enters confidential information such as credit card numbers into a compromised website, these details can be stolen using malicious JavaScript.
- Denial of service attacks and website vandalism.
- Theft of sensitive data, like passwords.
- Financial fraud on banking sites.
Cross-site scripting protection
Escape dynamic content
Usually, when a web page is rendered, dynamic content is woven into HTML. If the dynamic content is improperly treated, an attacker can use that to construct a stored XSS attack. The attacker would abuse an editable field by inserting some malicious JavaScript code, which is evaluated in the browser when another user visits that page.
You may not want your users to author raw HTML unless your site is a content-management system. Escape all dynamic content coming from a data store, so the browser knows it is to be treated as the contents of HTML tags, as opposed to raw HTML.
Escaping dynamic content generally consists of replacing significant characters with the HTML entity encoding:
” "
# #
& &
‘ '
( (
) )
/ /
; ;
< <
> >
As you will see in the code samples below, most modern frameworks will escape dynamic content by default.
By escaping editable content in this way, the content will never be treated as executable code by the browser. This will prevent most XSS attacks.
Whitelist values
If a dynamic data item can only take a handful of valid values, restrict the values in the data store. Also, make sure your rendering logic only permits known, proper values.
An example where you may want to use this approach is by asking a user to select their country from a dropdown list,, instead of having them typing it in manually.
Implement a content-security policy
Modern browsers support Content-Security Policies. Content-Security Policies allow the author of a web-page to control where JavaScript and other resources can be loaded and executed from.
In order for an XSS attack to be possible, the attacker has to be able to run malicious scripts on a user’s web page – either by injecting inline
