Possible Input Validation Circumvention in Active Model

Description

There is a possible input validation circumvention vulnerability in Active
Model. This vulnerability has been assigned the CVE identifier CVE-2016-0753.

Versions Affected: 4.1.0 and newer
Not affected: 4.0.13 and older
Fixed Versions: 5.0.0.beta1.1, 4.2.5.1, 4.1.14.1

Impact
------
Code that uses Active Model based models (including Active Record models) and
does not validate user input before passing it to the model can be subject to
an attack where specially crafted input will cause the model to skip
validations.

Vulnerable code will look something like this:

```ruby
SomeModel.new(unverified_user_input)
```

Rails users using Strong Parameters are generally not impacted by this issue
as they are encouraged to whitelist parameters and must specifically opt-out
of input verification using the `permit!` method to allow mass assignment.

For example, a vulnerable Rails application will have code that looks like
this:

```ruby
def create
params.permit! # allow all parameters
@user = User.new params[:users]
end
```

Active Model and Active Record objects are not equipped to handle arbitrary
user input. It is up to the application to verify input before passing it to
Active Model models. Rails users already have Strong Parameters in place to
handle white listing, but applications using Active Model and Active Record
outside of a Rails environment may be impacted.

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

Releases
--------
The FIXED releases are available at the normal locations.

Workarounds
-----------
There are several workarounds depending on the application. Inside a Rails
application, stop using `permit!`. Outside a Rails application, either use
Hash#slice to select the parameters you need, or integrate Strong Parameters
with your application.

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.

* 4-1-validation_skip.patch - Patch for 4.1 series
* 4-2-validation_skip.patch - Patch for 4.2 series
* 5-0-validation_skip.patch - Patch for 5.0 series

Please note that only the 4.1.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.

Credits
-------
Thanks to:

[John Backus](https://github.com/backus) from BlockScore for reporting this!

Am I vulnerable?

The constraints below list the versions that this vulnerability is patched in, and versions that are unaffected. If a patch is ready but unrealeased, then it is pending.

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Affected package information

Package Patched in Unaffected in
activemodel >= 5.0.0.beta1.1,~> 4.2.5, >= 4.2.5.1,~> 4.1.14, >= 4.1.14.1 <= 4.0.13