Resolve relative paths in send_file:

If someone were to send a file to send_file() that includes '../',
then we would allow them to reach outside the directory we choose.
This is a possible security issue. (One can argue the user should
sanitize their input, but I think we simply shouldn't allow it.)

The problem is that Path::Tiny does the right thing and allows us to
reach there. To prevent that, we're resolving paths using
Path::Tiny's realpath() method and then subsumes() to see that the
file is within the original directory.

Otherwise, we send a 403 forbidden. There is also a test that verifies
this is done correctly.

lib/Dancer2/Core/App.pm

@@ -1044,8 +1044,14 @@ sub send_file {
             $self->with_return->( $self->response );
         };
 
-        $file_path = Path::Tiny::path( $dir, $path );
+        # resolve relative paths (with '../') as much as possible
+        $file_path = Path::Tiny::path( $dir, $path )->realpath;
 
+        # We need to check whether they are trying to access
+        # a directory outside their scope
+        $err_response->(403) if !Path::Tiny::path($dir)->realpath->subsumes($file_path);
+
+        # other error checks
         $err_response->(403) if !$file_path->exists;
         $err_response->(404) if !$file_path->is_file;
         $err_response->(403) if !-r $file_path;

t/dsl/send_file.t

@@ -23,6 +23,10 @@ use Ref::Util qw<is_coderef>;
         send_file 'index.html';
     };
 
+    get '/illegal' => sub {
+        send_file '../index.html';
+    };
+
     prefix '/some' => sub {
         get '/image' => sub {
             send_file '1x1.png';
@@ -144,6 +148,17 @@ test_psgi $app, sub {
         ok($r->is_success, 'send_file returns success');
         is($r->header('Content-Disposition'), 'inline; filename="1x1.png"', 'send_file returns correct inline Content-Disposition');
     };
+
+    subtest "Illegal path" => sub {
+        my $r = $cb->( GET '/illegal' );
+        is( $r->code, 403, 'Illegal path returns 403' );
+        is(
+            $r->content,
+            'Forbidden',
+            'Text content contains UTF-8 characters',
+        );
+    };
+
 };
 
 done_testing;