Check for absolute paths in server URL before passing to find

Various double slashes and URL encodings can bypass current checks.
In the case of the file existing, the server will 500 instead of 403
which leaks the existence but not the contents of the file.

Props to @eadz for finding this.

lib/sprockets/server.rb

@@ -33,16 +33,16 @@ def call(env)
       # Extract the path from everything after the leading slash
       path = unescape(env['PATH_INFO'].to_s.sub(/^\//, ''))
 
-      # URLs containing a `".."` are rejected for security reasons.
-      if forbidden_request?(path)
-        return forbidden_response
-      end
-
       # Strip fingerprint
       if fingerprint = path_fingerprint(path)
         path = path.sub("-#{fingerprint}", '')
       end
 
+      # URLs containing a `".."` are rejected for security reasons.
+      if forbidden_request?(path)
+        return forbidden_response
+      end
+
       # Look up the asset.
       asset = find_asset(path, :bundle => !body_only?(env))
 
@@ -90,7 +90,7 @@ def forbidden_request?(path)
         #
         #     http://example.org/assets/../../../etc/passwd
         #
-        path.include?("..")
+        path.include?("..") || Pathname.new(path).absolute?
       end
 
       # Returns a 403 Forbidden response tuple
@@ -222,7 +222,7 @@ def headers(env, asset, length)
       #     # => "0aa2105d29558f3eb790d411d7d8fb66"
       #
       def path_fingerprint(path)
-        path[/-([0-9a-f]{7,40})\.[^.]+$/, 1]
+        path[/-([0-9a-f]{7,40})\.[^.]+\z/, 1]
       end
 
       # URI.unescape is deprecated on 1.9. We need to use URI::Parser

test/test_server.rb

@@ -182,10 +182,28 @@ def app
   end
 
   test "illegal require outside load path" do
-    get "/assets/../config/passwd"
+    get "/assets//etc/passwd"
     assert_equal 403, last_response.status
 
-    get "/assets/%2e%2e/config/passwd"
+    get "/assets/%2fetc/passwd"
+    assert_equal 403, last_response.status
+
+    get "/assets//%2fetc/passwd"
+    assert_equal 403, last_response.status
+
+    get "/assets/%2f/etc/passwd"
+    assert_equal 403, last_response.status
+
+    get "/assets/../etc/passwd"
+    assert_equal 403, last_response.status
+
+    get "/assets/%2e%2e/etc/passwd"
+    assert_equal 403, last_response.status
+
+    get "/assets/.-0000000./etc/passwd"
+    assert_equal 403, last_response.status
+
+    get "/assets/.-0000000./etc/passwd"
     assert_equal 403, last_response.status
   end