Impact

If an application using Puma allows untrusted input in a response header, an attacker can use newline characters (i.e. CR, LF or/r, /n) to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting.

While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS).

This is related to CVE-2019-16254, which fixed this vulnerability for the WEBrick Ruby web server.

Patches

This has been fixed in 4.3.2 and 3.12.3 by checking all headers for line endings and rejecting headers with those characters.

Workarounds

Add a Rack middleware to your application which accomplishes the same header check:

class HTTPResponseSplittingMiddleware
  CRLF_REGEX = /[\r\n]/.freeze

  def initialize(app)
    @app = app
  end

  def call(env)
    status, headers, body = @app.call(env)
    headers.reject! { |_k, v| CRLF_REGEX =~ v.to_s }
    [status, headers, body]
  end
end

References

• OWASP entry on HTTP Response Splitting
• The same issue fixed in WEBrick

For more information

If you have any questions or comments about this advisory:

• Open an issue in puma
• Email us a project maintainer. Email addresses are listed in our Code of Conduct.