Active Storage allowed transformation methods that were potentially unsafe

# Active Storage allowed transformation methods potentially unsafe

Active Storage attempts to prevent the use of potentially unsafe image
transformation methods and parameters by default.

The default allowed list contains three methods allow for the circumvention
of the safe defaults which enables potential command injection
vulnerabilities in cases where arbitrary user supplied input is accepted as
valid transformation methods or parameters.

Impact
------
This vulnerability impacts applications that use Active Storage with the image_processing processing gem in addition to mini_magick as the image processor.

Vulnerable code will look something similar to this:
```
<%= image_tag blob.variant(params[:t] => params[:v]) %>
```

Where the transformation method or its arguments are untrusted arbitrary input.

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

Workarounds
-----------
Consuming user supplied input for image transformation methods or their parameters is unsupported behavior and should be considered dangerous.

Strict validation of user supplied methods and parameters should be performed
as well as having a strong [ImageMagick security
policy](https://imagemagick.org/script/security-policy.php) deployed.

Credits
-------

Thank you [lio346](https://hackerone.com/lio346) for reporting this!