Insecure Default Initialization of Resource

Summary

Impact: In its default configuration, a Rails application that displays image variants may allow an unauthenticated attacker to read arbitrary files from the server, including the process environment. That environment typically holds `secret_key_base` and often credentials for external systems, which may in turn allow escalation to remote code execution or lateral movement to those systems. Details: libvips reads and writes file formats through "loaders" and "savers" (or more generally "operations"), many of which are backed by third-party libraries. It marks some of these operations as "unfuzzed", meaning they are unsafe for untrusted content, and several handle formats unrelated to web images. Active Storage did not disable the unfuzzed operations, so an attacker who can upload a crafted file and cause a variant to be generated from it may be able to invoke one. We are aware of a mechanism by which an attacker, by uploading a crafted file, is able to cause disclosure of the contents of arbitrary files accessible on the filesystem of the targeted application. One specific attack chain has been reported to us (see "Disclosure" below), but we do not assume it is the only one that exists. This issue affects activestorage versions 6.x prior to 7.2.3.2, 8.x prior to 8.0.5.1 and 8.1.x prior to 8.1.3.1.