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A critical security vulnerability, known as CVE-2021-33621, has been discovered in Ruby’s Common Gateway Interface (CGI) that could potentially put millions of users at risk. In this article, we’ll explore what CVE-2021-33621 is, what it affects, its CVSS score, and how you can protect yourself from it.

What is CVE-2021-33621?

CVE-2021-33621 is a security vulnerability in Ruby’s CGI that allows HTTP header injection and response splitting. This vulnerability could potentially be exploited by attackers to perform cross-site scripting (XSS) attacks, steal sensitive data, or execute arbitrary code on a user’s system.

What does CVE-2021-33621 affect?

According to the Ruby vendor’s website, the vulnerability affects applications that use the CGI module and are running the following versions:

  • cgi gem 0.3.3 or earlier
  • cgi gem 0.2.1 or earlier
  • cgi gem 0.1.1,, or 0.1.0

CVSS Score: The CVSS score for CVE-2021-33621 is 9.8, indicating that it is a critical vulnerability that requires immediate attention.

References: You can find more information about CVE-2021-33621 on the MITRE website ( and the software vendor’s website.

How can you protect yourself from CVE-2021-33621?

To protect yourself from this vulnerability, it is recommended that you review your code to ensure that untrusted input is not being passed to any CGI functions. It is also recommended that you upgrade to a patched version of Ruby as soon as possible. You can find more information about the vulnerability and the patches on the MITRE website and the Ruby vendor’s website.