Ruby on Rails: Explicit, dynamic render path: Dir. Trav + RCE

Possible Information Leak Vulnerability in Action View

There is a possible directory traversal and information leak vulnerability in
Action View. This vulnerability has been assigned the CVE identifier
CVE-2016-0752.

Versions Affected: All.
Not affected: None.
Fixed Versions: 5.0.0.beta1.1, 4.2.5.1, 4.1.14.1, 3.2.22.1

Impact

Applications that pass unverified user input to the render method in a
controller may be vulnerable to an information leak vulnerability.

Impacted code will look something like this:

def index
  render params[:id]
end

Carefully crafted requests can cause the above code to render files from
unexpected places like outside the application’s view directory, and can
possibly escalate this to a remote code execution attack.

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

Releases

The FIXED releases are available at the normal locations.

Workarounds

A workaround to this issue is to not pass arbitrary user input to the render
method. Instead, verify that data before passing it to the render method.

For example, change this:

def index
  render params[:id]
end

To this:

def index
  render verify_template(params[:id])
end

private
def verify_template(name)
  # add verification logic particular to your application here
end

Patches

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

• 3-2-render_data_leak.patch - Patch for 3.2 series
• 4-1-render_data_leak.patch - Patch for 4.1 series
• 4-2-render_data_leak.patch - Patch for 4.2 series
• 5-0-render_data_leak.patch - Patch for 5.0 series

Please note that only the 4.1.x and 4.2.x series are supported at present. Users
of earlier unsupported releases are advised to upgrade as soon as possible as we
cannot guarantee the continued availability of security fixes for unsupported
releases.

Credits

Thanks John Poulin for reporting this!