/usr/lib/ruby/vendor_ruby/action_dispatch/middleware/templates/rescues/unknown_action.html.erb is in ruby-actionpack 2:4.2.10-0ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | <header>
<h1>Unknown action</h1>
</header>
<div id="container">
<h2><%= h @exception.message %></h2>
</div>
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