Affected: https://github.com/rails/rails

A possible open redirect vulnerability in the Host Authorization middleware in Action Pack >= 6.0.0 that could allow attackers to redirect users to a malicious website.

Affected: RubyGems: actionpack

# Overview

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:

```ruby 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:

```ruby 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_…