CVE-2023-28120 - Untrusted User Input Exploit Found in ActiveSupport 'bytesplice' Method of 'SafeBuffer' Class

A new vulnerability, CVE-2023-28120, has been discovered in the ActiveSupport library's SafeBuffer class, specifically when utilizing the bytesplice method with untrusted user input. This vulnerability allows an attacker to manipulate the contents of the buffer and potentially execute arbitrary code. In this long read, we dive into the specifics of CVE-2023-28120, including detailed exploit information and necessary mitigation steps, to ensure you are aware and well-prepared to address this issue in your applications.

The following snippet demonstrates the vulnerable code

# ActiveSupport SafeBuffer class
class SafeBuffer < String

  # Vulnerable bytesplice method
  def byteslice(*args)
    super(*args).tap do |substr|
      substr.instance_variable_set(:@html_safe, @html_safe)
    end
  end

end

# Attacker-controlled input
untrusted_input = gets.chomp

# SafeBuffer instance with untrusted user input
buf = SafeBuffer.new(untrusted_input)

# Calling bytesplice on SafeBuffer with user input
result = buf.byteslice(, 10)

For more information on this vulnerability, refer to the following official references

1. National Vulnerability Database (NVD)
2. Ruby on Rails Security Advisory

Exploit Details

The vulnerability exists in ActiveSupport's SafeBuffer class, which is intended to provide a safe method for handling potentially unsafe (i.e., user-provided) strings. By design, the Buffer should reject operations that attempt to mix trusted and untrusted strings. However, the bytesplice method does not properly ensure that untrusted input is combined with trusted data.

The exploit works by calling the bytesplice method on a SafeBuffer instance containing attacker-controlled untrusted input; byteslice does not adequately sanitize the untrusted input, allowing an attacker to modify the SafeBuffer's contents.

A potential attack scenario might involve a web application using SafeBuffer to store and manipulate critical data in string format, such as user session identifiers. An attacker could exploit CVE-2023-28120 to incrementally poison the SafeBuffer, and potentially hijack user sessions.

Mitigation

In order to mitigate this vulnerability, developers must ensure that untrusted user input is never passed to the bytesplice method of a SafeBuffer instance. Instead, the input should be sanitized or escaped before being used in any operations involving SafeBuffer.

To stay informed about the latest security updates and patch availability, developers should subscribe to the Ruby on Rails Security Mailing List.

Conclusion

The impact of CVE-2023-28120 largely depends on how developers rely on SafeBuffer to manage untrusted input in their applications. Nevertheless, it is important to understand and address this potential exploit immediately to ensure a secure and robust application environment.

By following the mitigation steps described above and keeping abreast of security updates, developers can feel confident that their applications are well-protected against this vulnerability.

Timeline

Published on: 01/09/2025 01:15:07 UTC
Last modified on: 01/09/2025 22:15:26 UTC