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Date: Thu, 19 Aug 2021 09:37:07 -0700
From: Aaron Patterson <>
Subject: [CVE-2021-22942] Possible Open Redirect in Host Authorization Middleware

# 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:,

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
configuration files that look like this:

config.hosts <<  ''

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.

The fixed releases are available at the normal locations.

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/

      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? ||

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-authorzation-open-redirect.patch - Patch for 6.0 series
* 6-1-host-authorzation-open-redirect.patch - Patch for 6.1 series

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

Thanks to [@mshtawy]( for reporting

Aaron Patterson

Content of type "text/html" skipped

Download attachment "6-1-host-authorzation-open-redirect.patch" of type "application/octet-stream" (6201 bytes)

Download attachment "6-0-host-authorzation-open-redirect.patch" of type "application/octet-stream" (6035 bytes)

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