We just released paperclip v4.2.2. This contains only a security patch over v4.2.1, and everyone is encouraged to upgrade. The commit message explains the problem and fix:
There is an issue where if an HTML file is uploaded with a .html extension, but the content type is listed as being
image/jpeg, this will bypass a validation checking for images. But it will also pass the spoof check, because a file named .html and containing actual HTML passes the spoof check.
This change makes it so that we also check the supplied content type. So even if the file contains HTML and ends with .html, it doesn’t match the content type of
image/jpegand so it fails.
This vulnerability was assigned JVN #83881261.
paperclip is a Rubygem that that lets you attach files to ActiveRecord models.
Thanks to Jon Yurek and Mike Burns for their work on this, and special thanks to MORI Shingo of DeNA Co., Ltd. for reporting the vulnerability.