[CVE-2019-5419] Denial of Service Vulnerability in Action View

[Aaron Patterson <tenderlove () ruby-lang org>]

There is a potential denial of service vulnerability in MODULE / COMPONENT.
This vulnerability has been assigned the CVE identifier CVE-2019-5419.

Versions Affected:  All.
Not affected:       None.
Fixed Versions:     6.0.0.beta3, 5.2.2.1, 5.1.6.2, 5.0.7.2, 4.2.11.1

Impact
------
Specially crafted accept headers can cause the Action View template location
code to consume 100% CPU, causing the server unable to process requests.  This
impacts all Rails applications that render views.

All users running an affected release should either upgrade or use one of the
workarounds immediately.

Releases
--------
The 6.0.0.beta3, 5.2.2.1, 5.1.6.2, 5.0.7.2, and 4.2.11.1 releases are
available at the normal locations.

Workarounds
-----------
This vulnerability can be mitigated by wrapping `render` calls with
`respond_to` blocks.  For example, the following example is vulnerable:

```
class UserController < ApplicationController
  def index
    render "index"
  end
end
```

But the following code is not vulnerable:

```
class UserController < ApplicationController
  def index
    respond_to |format|
      format.html { render "index" }
    end
  end
end
```

Implicit rendering is impacted, so this code is vulnerable:

```
class UserController < ApplicationController
  def index
  end
end
```

But can be changed this this:

```
class UserController < ApplicationController
  def index
    respond_to |format|
      format.html { render "index" }
    end
  end
end
```

Alternatively to specifying the format, the following monkey patch can be
applied in an initializer:

```
$ cat config/initializers/formats_filter.rb
# frozen_string_literal: true

ActionDispatch::Request.prepend(Module.new do
  def formats
    super().select do |format|
      format.symbol || format.ref == "*/*"
    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-action-view-dos.patch - Patch for 6.0 series
* 5-2-action-view-dos.patch - Patch for 5.2 series
* 5-1-action-view-dos.patch - Patch for 5.1 series
* 5-0-action-view-dos.patch - Patch for 5.0 series
* 4-2-action-view-dos.patch - Patch for 4.2 series

Please note that only the 5.2.x, 5.1.x, 5.0.x, and 4.2.x 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.

Also note that the patches for this vulnerability are the same as CVE-2019-5418.

Credits
-------
Thanks to John Hawthorn <john@hawthorn.email> of GitHub

-- 
Aaron Patterson
http://tenderlovemaking.com/

Attachment: 4-2-action-view-dos.patch
Description:

Attachment: 5-0-action-view-dos.patch
Description:

Attachment: 5-1-action-view-dos.patch
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Attachment: 5-2-action-view-dos.patch
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Attachment: 6-0-action-view-dos.patch
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