Possible Open Redirect in Host Authorization Middleware

There is a possible open redirect vulnerability in the Host Authorization
middleware in Action Pack. This vulnerability has been assigned the CVE
identifier CVE-2021-22942.

Versions Affected: >= 6.0.0.
Not affected: < 6.0.0
Fixed Versions: 6.1.4.1, 6.0.4.1

Impact

Specially crafted “X-Forwarded-Host” headers in combination with certain
“allowed host” formats can cause the Host Authorization middleware in
Action Pack to redirect users to a malicious website.

Impacted applications will have allowed hosts with a leading dot.
For example, configuration files that look like this:

config.hosts <<  '.EXAMPLE.com'

When an allowed host contains a leading dot, a specially crafted
Host header can be used to redirect to a malicious website.

This vulnerability is similar to CVE-2021-22881, but CVE-2021-22881 did not
take in to account domain name case sensitivity.

Releases

The fixed releases are available at the normal locations.

Workarounds

In the case a patch can’t be applied, the following monkey patch can be
used in an initializer:

module ActionDispatch
  class HostAuthorization
    HOSTNAME = /[a-z0-9.-]+|\[[a-f0-9]*:[a-f0-9.:]+\]/i
    VALID_ORIGIN_HOST = /\A(#{HOSTNAME})(?::\d+)?\z/
    VALID_FORWARDED_HOST = /(?:\A|,[ ]?)(#{HOSTNAME})(?::\d+)?\z/

    private
      def authorized?(request)
        origin_host =
          request.get_header("HTTP_HOST")&.slice(VALID_ORIGIN_HOST, 1) || ""
        forwarded_host =
          request.x_forwarded_host&.slice(VALID_FORWARDED_HOST, 1) || ""
        @permissions.allows?(origin_host) &&
          (forwarded_host.blank? || @permissions.allows?(forwarded_host))
      end
  end
end