Beyond the Basics: Advanced Insights into XSS Vulnerabilities

By their very nature, web applications are constantly exposed to a wide range of attacks by cybercriminals, which is why they are involved in 26% of all breaches. Attackers continuously scan these exposed applications for known vulnerabilities, gathering intelligence to stage an attack. Once they have a significant collection of possible targets for attack, they rapidly attempt to exploit as many of the detected vulnerabilities as possible. Without proactively detecting and managing these vulnerabilities, organizations can quickly become compromised.

In this article, we will explore Cross-Site Scripting (XSS) attacks, one of the most common types of web attacks, investigating how they happen and what can be done to prevent them. 

What is an XSS attack? 

One of the most common types of web attacks that organizations face is an XSS attack. These attacks inject malicious scripts into web pages so a user can execute the script when accessing the page. Due to their severe impact, these attacks are so prevalent that they are number three in the current OWASP top 10 list.

These vulnerabilities are a critical security threat due to their potential to compromise user privacy and inflict significant organizational harm. XSS vulnerabilities can be exploited to hijack user sessions by stealing cookies or session tokens, granting attackers unauthorized access to sensitive personal and financial data. Beyond data theft, XSS attacks can also deface websites, undermining an organization’s reputation and eroding user trust. 

They can also redirect users to malicious sites, often for phishing purposes, and allow attackers to manipulate user interactions with a web application without their knowledge. This manipulation can range from submitting false information to initiating unauthorized financial transactions.

How do XSS attacks work?

XSS attacks are not all identical, further complicating the problem. There are three major varieties: stored, reflected, and DOM-based attacks. Each has a different mechanism for attack and varying levels of potential impact. 

Stored (or Persistent) XSS

These attacks require that malicious scripts be directly stored on the target web servers, such as in a database, forum post, visitor log, or comment field. The intent is that when a browser pulls the data with the offending script on it, the script will auto-execute in the user’s browser. This variety of scripts is particularly dangerous as they continue to be a threat until the malicious content is removed from the server, allowing it to continue to target large numbers of users. 

An example of this attack would be an attacker posting a comment on a blog that includes a script. Once the post is submitted, it is stored on the server until someone else opens that post, having the script automatically execute in their browser. 

Reflected (or Non-Persistent) XSS

Unlike a persistent XSS attack, the reflected version requires no storage of dangerous scripts. Instead, these attacks rely on embedding scripts into a URL or web request. These attacks rely on the user’s actions, such as clicking on a malicious link, submitting a form, or navigating to a malicious site. Once a user interacts with the script, a message is sent, which reflects off a web server and executes in the victim’s browser, appearing to have originated from the server. Attackers often have to trick the user into clicking on the malicious link, which may be done via phishing techniques, malicious ads, or shortened URLs that hide the whole URL string, such as Bit.ly. 

An example of a reflected attack would be an attacker sending a user’s link with a script embedded in it. When the user clicks on the link, the script is sent to the server and reflected back in the user’s browser, where it’s executed. Rather than persisting and affecting multiple users, this attack only executes once per user interaction. 

DOM-based XSS

A DOM-based XSS attack differs entirely from the other two XSS attacks, as it involves a web page’s Document Object Model (DOM). In these attacks, the payload is executed by modifying the DOM environment in a victim’s browser, often through client-side scripts, rather than sending a payload to the server. 

An example of this would come from a web application that uses data from a URL without proper validation. The lack of validation allows data inputs that can be used to modify the DOM, which can be altered to enable malicious scripts to be executed. These attacks are tricky to detect as the XSS executes on the client side, so the malicious code may not be present in the server’s response. 

How to stop XSS attacks?

A modern approach to preventing XSS attacks starts by embedding security directly into the development lifecycle. A comprehensive developer-first security platform uses multiple testing methods, including Static Application Security Testing (SAST) and Dynamic Application Security Testing (DAST), to provide a complete view of vulnerabilities.

AI-powered SAST tools analyze source code for flaws, while DAST tools test the running application for vulnerabilities that only appear during execution. By integrating these testing tools into the CI/CD pipeline, organizations empower developers to find and fix vulnerabilities early, long before they can be exploited in production.

When a possible vulnerability is detected, leveraging a strategy of distrusting all user input can help prevent these XSS attacks. This prevention strategy includes validating input to ensure it aligns with expected formats and sanitizing input to remove or neutralize harmful content effectively. As a rule of thumb, anytime there is input, there is the possibility of an XSS vulnerability, and input should be sanitized. 

The output should be neutralized through encoding to create a holistic defense. This process converts special characters to their HTML or URL-encoded equivalents, rendering any embedded scripts functionally useless. So even if malicious user-supplied data made it onto storage, it is unusable for future attacks.

Prevent XSS attacks with Snyk API & Web 

Secure application development requires a platform that not only finds vulnerabilities but also helps developers fix them fast. 

Snyk API & Web identifies over 3000 vulnerabilities, including critical ones like XSS and SQL injections. Snyk API & Web, as part of the Snyk AI Trust Platform, integrates directly into CI/CD pipelines. It empowers developers by delivering prioritized, actionable remediation advice within their existing workflows. This automated and developer-first approach ensures that applications are secured throughout their lifecycle without sacrificing speed.

Learn more about how Snyk API & Web can help you secure your applications by booking a demo.

FAQs

Can XSS attacks affect any website?

Only static websites that do not handle or display user input are immune to XSS attacks.

Are modern web frameworks immune to XSS attacks?

While modern frameworks can reduce the risk, they are not entirely immune, especially if not used correctly.

Can regular security updates prevent XSS attacks?

Regular updates can mitigate vulnerabilities, but proactive security measures and coding practices are also crucial.

Is it sufficient to just validate input to prevent XSS attacks?

Input validation is essential, but should be combined with output encoding and other security practices for effective prevention.