How to Prevent Cross-Site Scripting (XSS) Attacks

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Cross-site scripting attacks (XSS) are used to steal data and hijack browsing sessions so attackers can take action on a victim’s behalf. Attackers may use this opportunity to alter web pages, post on social accounts, initiate bank transfers or make fraudulent purchases.

This is accomplished by tricking applications and websites into sending malicious scripts through the web browser. The most common method for taking over user sessions is to inject code using form fields or other data input fields. Developer best practices such as sanitizing and encoding user input are critical for preventing XSS vulnerabilities.

Risks of XSS attacks

XSS attacks pose vital security risks to both users and businesses alike.

Risk to users

Attackers that hijack a user’s session can take a number of malicious actions that affect them, such as making fraudulent charges using their bank cards or directly transfer money to other accounts.

XSS attacks can also be used to extract sensitive information from cookies and databases. This information can be used for identity theft.

Risk to businesses

If an attacker gets a hold of stolen user credentials, they can wreak havoc for the business as well. If the user has admin rights, an XSS attack can extend to the server-side.

Businesses can also suffer from a loss of credibility and brand trust if an XSS attack becomes public knowledge. Users and customers are likely not to return to a site if they know their information could be at risk.

Types of XSS attacks

Reflected XSS attacks

In reflected XSS attacks, malicious scripts are injected directly into an HTTP request. The script is reflected from the server in an HTTP response and then executed in a user’s browser. This is the simplest type of XSS attack.

DOM-based XSS attacks

Document-object model (DOM) based attacks require no interaction with the server. The vulnerability is the browser-side script. Web applications read the malicious script directly from a query string. They are similar in this way to reflected XSS attacks.

Persistent/Stored XSS attacks

Persistent, also known as stored, XSS attacks are the most dangerous type of attack because they have the potential to affect every user who visits the site. In this case, scripts are injected into a database through form fields.

The script is then stored indefinitely in the website’s database. Every user who then enters the site is vulnerable to having their session hijacked.

How to prevent XSS attacks

There are a number of precautions you can take to prevent XSS attacks.

Keep software up-to-date

Software should always be kept up-to-date for many reasons, including fixing bugs, improving performance, installing new features and patching security vulnerabilities. Regularly updating software will greatly reduce the vulnerabilities that leave a site or application open to XSS vulnerabilities.

You should also audit all of your applications to determine which you need and which you rarely use. Get rid of all the apps you don’t use to further reduce the number of vulnerabilities.

Scan for vulnerabilities

You should regularly scan all web-facing infrastructure for vulnerabilities. Many vulnerability scanning tools can identify applications and web sites that are vulnerable to XSS attacks. Here are three options that can help identify those vulnerabilities.

1 Intruder

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Encode and sanitize user input

Input fields are the most common point of entry for XSS attack scripts. Therefore, you should always screen and validate any information input into data fields. This is particularly important if the data will be included as HTML output to protect against reflected XSS attacks.

Validation should occur on both the client-side and server-side as an added precaution. validating the data before it’s sent to servers will also protect against persistent XSS scripts. This can be accomplished using JavaScript.

Use escaping/encoding techniques for input uses where possible, such as HTML escape or JavaScript escape.

User input that must contain HTML can be cleaned via a trusted library for your development language.

Also, set the HttpOnly flag for cookies so they won’t be accessible via client-side JavaScript.

Use a web application firewall

A web application firewall (WAF) can be a powerful tool for protecting against XSS attacks. WAFs can filter bots and other malicious activity that may indicate an attack. Attacks can then be blocked before any script is executed and before it compromises the network.

Implement a content security policy

A content security policy (CSP) is an http response header that can define the functions a website is allowed to perform. They can be used to prevent a website from accepting any in-line scripts. This may be the strongest method at your disposal as it can completely block XSS attacks or at least greatly reduce the possibility of them.

Further reading:

Kyle Guercio
Kyle Guercio
Kyle Guercio has worked in content creation for six years contributing blog posts, featured news articles, press releases, white papers, and more for a wide variety of subjects in the technology space. He covers topics relating to servers and cybersecurity and has contributed to ServerWatch and

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