Percent-encoded cookies can be used to overwrite existing prefixed cookie names

It is possible to forge a secure or host-only cookie prefix in Rack using
an arbitrary cookie write by using URL encoding (percent-encoding) on the
name of the cookie. This could result in an application that is dependent on
this prefix to determine if a cookie is safe to process being manipulated
into processing an insecure or cross-origin request.
This vulnerability has been assigned the CVE identifier CVE-2020-8184.

Versions Affected: rack < 2.2.3, rack < 2.1.4
Not affected: Applications which do not rely on __Host- and __Secure- prefixes to determine if a cookie is safe to process
Fixed Versions: rack >= 2.2.3, rack >= 2.1.4

Impact

An attacker may be able to trick a vulnerable application into processing an
insecure (non-SSL) or cross-origin request if they can gain the ability to write
arbitrary cookies that are sent to the application.

Workarounds

If your application is impacted but you cannot upgrade to the released versions or apply
the provided patch, this issue can be temporarily addressed by adding the following workaround:

module Rack
  module Utils
    module_function def parse_cookies_header(header)
      return {} unless header
      header.split(/[;] */n).each_with_object({}) do |cookie, cookies|
        next if cookie.empty?
        key, value = cookie.split('=', 2)
        cookies[key] = (unescape(value) rescue value) unless cookies.key?(key)
      end
    end
  end
end