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 deployed.

Credits

Thank you lio346 for reporting this!