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:
- User feedback forms
- Support ticket systems
- Comment sections that undergo moderation
- Logs
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.
- interactsh is an online version of the tool that Nuclei uses for testing. This site is useful for manual testing for short durations.
- Burp Collaborator – Burp collaborator is another tool that is useful for short term testing. It is a tool that is commonly used to test for out of band vulnerabilities.
- BXSS Hunter – BXSS Hunter is one of numerous online tools that can be setup to catch Blind XSS vulnerabilities 24/7. You can even set it up to send you emails or Discord webhooks if one of your Blind XSS payloads is hit.
- Custom Server – If you have a server you own, you could simply set a canary file and have it message you when it is requested. Or you could even install the interactsh locally.
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:
- Whitelisting allowable HTML tags
- Encoding user inputs
- Regularly updating libraries and frameworks to patch known vulnerabilities
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 XSS and similar threats.
