What is Cross-Site Scripting (XSS)?

  • By Greg Brown
  • Published: Aug 21, 2023
  • Last Updated: Aug 29, 2023

What is Cross-Site Scripting

Microsoft engineers coined the phrase Cross-Site Scripting in early 2000, and the attacks have grown to nearly 30% of all web application assaults. More than 60% of web applications are estimated to be susceptible to XSS attacks.

Cross-site scripting occurs when attackers inject malicious code, in the form of client-side scripts, into a web page. Cross-site scripting was designed initially to breach data across websites but evolved into other injectable forms of aggression. XSS relies on known vulnerabilities in web-based applications, servers the applications run on, or the plug-in systems the applications rely on. The effects of the XSS attacks can range from annoyance to significant security risk. 

What Is Cross-Site Scripting (XSS)?

Cross-site scripting vulnerabilities allow attackers to bypass access controls like a same-origin policy. Across the web, security depends on a variety of mechanisms, including the concept of same-origin policy. This concept states that content from one site has permission to access resources on a web browser. Content from any URL with the same URI Scheme, such as hostname or port number, can share these permissions.

Attackers exploit the vulnerabilities and “fold the malicious content into the content being delivered from the compromised site.” The resulting combined content then arrives at the client-side web browser. It appears to the browser that all content has been delivered from a trusted source and operates with full permission. Attackers then inject malicious code into web pages or frames, with the full permission given to it by the original web page. Attackers have elevated access privileges to sensitive information, cookies, and other information the browser maintains.

What Are the Types of XSS Attacks?

No single standardized classification of XSS exists; however, most computer experts distinguish XSS into two primary types, persistent and non-persistent. Additional sources will further distinguish cross-site scripting into other attack types, such as traditional and DOM-based.

Persistent XSS Attacks (Stored Attacks)

Persistent XSS attacks occur when the malicious code or script is permanently attached or stored on the server or the vulnerable web application. When a desired action is carried out on the vulnerable application, the code is activated and carries out its programmed response. Persistent Cross Site Scripting attacks are similar to non-persistent attacks, with the only difference being that the code is stored permanently on the vulnerable web application.

Non-Persistent XSS Attacks

Non-persistent XSS Attacks occur when vulnerable applications are accessed, and the malicious code returns the programmed response vector. Vulnerable applications display malicious code, and it is not stored. Non-persistent attacks require an application that allows user input without adequately sanitizing the data. Malicious code or scripts are usually delivered by conventional means of attack, such as phishing emails, obscured URLs on a vulnerable web page, or malicious links or attachments. Malicious code inserted into the user’s web browser can silently direct users to an external site where current session cookie data is stored.

DOM-Based XSS Attacks

DOM-based attacks are when the hacker takes advantage of the user’s web page DOM elements. Similar in nature to the reflected cross-site scripting attack, the code is delivered via a URL containing the malicious script. Instead of loading the malicious code through an HTTP response, attacks are delivered by manipulating the DOM environment. Attackers control user input fields and append malicious code to an element within a web page’s HTML, and the attacker then forces their code to be executed. 

Self-XSS Attacks

Self-XSS Attacks are a product of the modern social engineering world. This cross-site scripting attack uses social engineering to trick users into running the malicious script themselves. Users can be tricked into copying and pasting harmful code and inserting the script into the web browser’s address bar. This type of cyber attack is considered by many to be a True Cross Site Scripting attack. It relies on social engineering to activate the code rather than inserting a malicious script into a vulnerability in a web application. If carried out properly, these attacks can cause as much or more damage than others.

How to Prevent XSS Attacks?

Preventing Cross-Site Scripting Attacks

According to IBM, their solution to prevent an XSS Attack is ”the application must validate all the input data, make sure that only the listed data is allowed, and ensure that all variable output in a page is encoded before it is returned to the user.”

Since 2000, there have been several other solutions developed by tech giants to counteract cross-site scripting attacks.

  • Never trust user input. Users introduce the risk of cross-site scripting with every input they carry out. Every input must be treated as untrusted and validated as soon as possible. An effective technique is to filter each piece of data and remove any harmful keywords. Configure a filter to identify scripting strings not expected from the standard user inputs. 
  • Use Encoding/Escaping is a technique that modifies standard characters in the data and ensures they are not considered active or dangerous data. The technique may involve combining HTML, CSS, JavaScript, and URL Encoding. Always use existing libraries to escape or encode the user’s data. 
  • Sanitize the HTML when the user’s data contains HTML, encoding, or escaping that may break valid tags. Use a security-focused library for parsing and cleaning the HTML. 
  • Content Security Policy or CSOP is a browser-based method allowing you to whitelist HTTP headers. Use the technique to approve and disapprove sources encountered by the browser. 

The Impact of Cross-Site Scripting (XSS) Attacks

The most viable option to mitigate XSS attacks is to scan the codebase regularly for the presence of any cross-site scripting vulnerabilities. Using any external code base could introduce vulnerabilities to an XSS attack. Importing a compromised library can also be a huge security risk. If the code base is extensive, it is advisable to scan regularly with a powerful and automatic scanner such as White Source Bolt. Regular scans give the user confidence in the data and ward off any potential threats, specifically XSS vulnerabilities. 

Tech giants like Microsoft and Cisco constantly develop solutions to prevalent code attacks such as the XSS and others.

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