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December 18, 2023

What is Blind XSS?

Blind cross-site scripting fires somewhere you cannot see, often in an admin panel hours later. How to test for it when there is no visible response.

Brackish Security4 min read

You may have heard of Reflected Cross Site Scripting (XSS) or Stored XSS, but what is Blind XSS? Unlike traditional XSS attacks, where the immediate impact is visible, Blind XSS vulnerabilities are typically triggered when the malicious input is viewed by a different user, often an administrator or a support person, at a later time (which makes it also a Stored XSS).

The Mechanics of Blind XSS

Blind XSS attacks occur when an attacker injects a malicious script into areas of a web application that are not immediately rendered or visible. This script lies dormant until it is triggered by a specific user action or viewed by a specific audience. Unlike standard XSS, where the attacker gets immediate feedback, Blind XSS is "blind" to its immediate execution.

Common Targets

Areas commonly targeted in Blind XSS attacks include:

Basically just think about where your input may be going. Does a moderator have to review your comment in a special moderator panel, or even review your profile where you've placed a Blind XSS payload in your personal profile?

In fact, Brackish Security researchers recently found a Blind XSS in an open source application's log viewing page. Every request that was sent to this particular application logged the User Agent and rendered HTML or JS that was found in the user agent. This vulnerability would have allowed an attacker to compromise administrator accounts and perform remote code execution.

Detecting Blind XSS

The stealthy nature of Blind XSS makes it particularly challenging to detect. Traditional security measures like real-time input sanitization checks often miss these payloads because they don't manifest immediately. As a result, Blind XSS can remain unnoticed for long periods, increasing the potential damage. Fortunately there are several tools available to help you detect Blind XSS.

Prevention Strategies

Preventing Blind XSS requires a multi-layered approach, focusing on both immediate and deferred data handling practices.

Input Validation and Sanitization

Implement stringent input validation and sanitization on both client and server sides. This includes:

Content Security Policy (CSP)

Employing a robust Content Security Policy can significantly mitigate the impact of Blind XSS by restricting where scripts can be loaded from and executed.

Regular Audits and Code Reviews

Conducting thorough audits and code reviews can help identify and rectify potential Blind XSS vulnerabilities. This is particularly crucial for areas processing user-generated content. Contact us if you want a security professional to review your code.

Penetration Testing

We at Brackish Security always recommend White Box Web and Mobile Application Penetration Testing. It allows the testing team to get the greatest amount of coverage to ensure the application is secure. It enables the team to find vulnerabilities that black box testing struggles to find, such as Blind XSS.

Real World Blind XSS

Take a look at these bug bounty reports for some real world Blind XSS vulnerabilities.

https://hackerone.com/reports/880591

https://hackerone.com/reports/948929

Conclusion

Blind XSS poses a unique challenge to developers and pentesters alike. Its covert nature requires a proactive and multifaceted approach to prevention and detection. Regular audits, robust input handling practices, and thorough testing are crucial in safeguarding against these stealthy attacks. As cybersecurity experts, staying vigilant and constantly evolving our strategies is key to mitigating the risks posed by Blind X

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