Vulnerability Instance

Possible Open Redirect Vulnerability in Action Pack
There is a possible Open Redirect Vulnerability in Action Pack. This
vulnerability has been assigned the CVE identifier CVE-2021-22903.

Versions Affected:  >= v6.1.0.rc2
Not affected:       < v6.1.0.rc2
Fixed Versions:     6.1.3.2

Impact
------
This is similar to CVE-2021-22881: 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.

Since rails/rails@9bc7ea5, strings in config.hosts that do not have a leading
dot are converted to regular expressions without proper escaping. This causes,
for example, config.hosts << "sub.example.com" to permit a request with a Host
header value of sub-example.com.

Workarounds
-----------
The following monkey patch put in an initializer can be used as a workaround:

```ruby
class ActionDispatch::HostAuthorization::Permissions
  def sanitize_string(host)
    if host.start_with?(".")
      /\A(.+\.)?#{Regexp.escape(host[1..-1])}\z/i
    else
      /\A#{Regexp.escape host}\z/i
    end
  end
end
```