By their very nature, web applications are constantly exposed to a wide range of attacks by cybercriminals, which is why web applications are a part of 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.

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 accessed. Due to their severe impact, these attacks are so prevalent that they are number 3 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 enable 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 of these has a different mechanism for attack and varying levels of potential impact. 

Stored (or Persistent) XSS

These attacks require that malicious scripts are 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 of 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 a user’s link with a script embedded in it being sent by an attacker. 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 in a way that will 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?

Identifying XSS vulnerabilities is a crucial initial step in preventing such attacks. Here, code analysis tools play an essential role. Static Application Security Testing (SAST) tools are particularly effective, as they methodically evaluate code to pinpoint potential vulnerabilities. These tools work by scrutinizing the source code before the application is running. Alternatively, Dynamic Application Security Testing (DAST) tools assess the operating code in a running state, testing it for possible security breaches. DAST tools detect vulnerabilities manifest only during an application’s execution. Building testing with these tools into the lifecycle of an application as part of development and operations helps ensure that vulnerabilities can be caught and remediated before attackers can use them. 

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 effectively remove or neutralize harmful content. 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 With Probely 

Secure application development requires the right tools to detect vulnerabilities accurately. Probely identifies over 3000 vulnerabilities, including critical ones like XSS and SQL injections. Probely helps organizations proactively identify and resolve potential vulnerabilities by integrating directly into development workflows, enhancing efficiency through automated and continuous security checks. With Probely, developers secure their applications throughout their lifecycle, preparing them to handle your organization’s most sensitive data. 

Learn more about how Probely can help you secure your applications by doing a 14-day fully-featured free trial.

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?

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?

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