[CVE-2018-3760] Path Traversal in Sprockets

[Rafael Mendonça França]

There is an information leak vulnerability in Sprockets. This vulnerability
has been assigned the CVE identifier CVE-2018-3760.

Versions Affected: 4.0.0.beta7 and lower, 3.7.1 and lower, 2.12.4 and lower.
Not affected: NONE
Fixed Versions: 4.0.0.beta8, 3.7.2, 2.12.5

Impact
------
Specially crafted requests can be used to access files that exists on
the filesystem that is outside an application's root directory, when the Sprockets server is
used in production.

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

Releases
--------
The 4.0.0.beta8, 3.7.2 and 2.12.5 releases are available at the normal locations.

Workarounds
-----------
In Rails applications, work around this issue, set `config.assets.compile = false` and
`config.public_file_server.enabled = true` in an initializer and precompile the assets.

This work around will not be possible in all hosting environments and upgrading is advised.

Patches
-------
To aid users who aren't able to upgrade immediately we have provided patches for the three supported release series.
They are in git-am format and consist of a single changeset.

* 4-0-fix-path-traversal.patch - Patch for the 4.0.x release series
* 3-7-fix-path-traversal.patch - Patch for the 3.7.x release series
* 2-12-fix-path-traversal.patch - Patch for the 2.12.x release series

Credits
-------

Thanks to Orange Tsai from DEVCORE for reporting this issue.

Rafael França