[CVE-2021-22881] Possible Open Redirect in Host Authorization Middleware

[Rafael França]

There is a possible open redirect vulnerability in the Host Authorization
middleware in Action Pack. This vulnerability has been assigned the CVE
identifier CVE-2021-22881.

Versions Affected:  >= 6.0.0
Not affected:       < 6.0.0
Fixed Versions:     6.1.2.1, 6.0.3.5

Impact
------
Specially crafted "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 <<  '.tkte.ch'
```

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

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:

```ruby
module ActionDispatch
  class HostAuthorization
    private
      def authorized?(request)
        valid_host = /
          \A
          (?<host>[a-z0-9.-]+|\[[a-f0-9]*:[a-f0-9\.:]+\])
          (:\d+)?
          \z
        /x

        origin_host = valid_host.match(
          request.get_header("HTTP_HOST").to_s.downcase)
        forwarded_host = valid_host.match(
          request.x_forwarded_host.to_s.split(/,\s?/).last)

        origin_host && @permissions.allows?(origin_host[:host]) && (
          forwarded_host.nil? || @permissions.allows?(forwarded_host[:host]))
      end
  end
end
```

Patches
-------
To aid users who aren't able to upgrade immediately we have provided patches for
the two supported release series. They are in git-am format and consist of a
single changeset.

* 6-0-host-authorization-open-redirect.patch
* 6-1-host-authorization-open-redirect.patch

Please note that only the 5.2, 6.0, and 6.1 series are supported at present. Users
of earlier unsupported releases are advised to upgrade as soon as possible as we
cannot guarantee the continued availability of security fixes for unsupported
releases.

Credits
-------

Thanks to @tktech (https://hackerone.com/tktech) for reporting this issue!