Arbitrary file existence disclosure in Action Pack

Description

There is an information leak vulnerability in Action Pack. Specially crafted requests can be used to determine whether a file exists on the filesystem that is outside the Rails application's root directory. The files will not be served, but attackers can determine whether or not the file exists.

This only impacts Rails applications that enable static file serving at runtime. For example, the application's production configuration will say:

  config.serve_static_assets = true

This vulnerability applies to the following versions:

Versions Affected:  >= 3.0.0 
Not affected:       < 3.0.0, 4.2.0.beta4 
Fixed Versions:     3.2.21, 4.0.12, 4.1.8

Remediation

To work around this issue, set config.serve_static_assets = false in an initializer. This work around will not be possible in all hosting environments and upgrading is advised. All users running an affected configuration should either upgrade or use one of the work arounds immediately.