OOB XSS Vulnerability

Anandhu Krishnan
Published on
14 May 2024
1 min read


Out-of-Band (OOB) XSS occurs when an attacker exploits a Cross-Site Scripting vulnerability to send malicious data to a server that is different from the one hosting the vulnerable web application. Unlike traditional XSS, which typically exfiltrates data directly to the attacker’s controlled server via the same connection, OOB XSS involves the attacker’s server communicating with a different, often less secure, server to collect the data. This technique can be used to bypass security measures like Content Security Policy (CSP) and firewall rules, making it particularly dangerous and difficult to detect. The attacker can exploit this vulnerability to steal sensitive information, execute commands, or manipulate the target system in ways that are harder to trace and mitigate.


  • Sanitize and Validate Inputs**: Ensure all user inputs are properly sanitized and validated to prevent injection of malicious scripts.
  • Implement a strong CSP to restrict the sources of executable scripts and limit the ability of malicious scripts to communicate with external servers.
  • Perform regular security testing, including static and dynamic code analysis, to identify and fix potential XSS vulnerabilities.
  • Avoid including sensitive data in HTTP response headers that can be exploited by malicious scripts.
  • Train developers on secure coding practices and the risks associated with XSS, including OOB XSS.
  • Use WAFs to detect and block malicious traffic that attempts to exploit XSS vulnerabilities.
  • Utilize frameworks and libraries that provide built-in protections against XSS and OOB XSS.
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Written by
Anandhu Krishnan
Anandhu Krishnan
Lead Engineer
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