Prevent remote shell execution in #apply

If the operations are coming from user input, this could allow the user
to execute arbitrary shell commands via `Kernel#system` and
`Kernel#spawn`:

  ImageProcessing::Vips.apply({ system: "echo something" })

We prevent this by using `#public_send` instead of `#send`, which goes
to method missing instead of calling private methods, which include
`Kernel#system` and `Kernel#spawn`.

CHANGELOG.md

@@ -1,3 +1,7 @@
+## HEAD
+
+* Prevent remote shell execution when using `#apply` with operations coming from user input (@janko)
+
 ## 1.12.1 (2020-11-06)
 
 * Fix format fallback for files ending with a dot on Ruby 2.7+ (@coding-chimp)

lib/image_processing/chainable.rb

@@ -34,13 +34,13 @@ def instrumenter(&block)
     def apply(operations)
       operations.inject(self) do |builder, (name, argument)|
         if argument == true || argument == nil
-          builder.send(name)
+          builder.public_send(name)
         elsif argument.is_a?(Array)
-          builder.send(name, *argument)
+          builder.public_send(name, *argument)
         elsif argument.is_a?(Hash)
-          builder.send(name, **argument)
+          builder.public_send(name, **argument)
         else
-          builder.send(name, argument)
+          builder.public_send(name, argument)
         end
       end
     end

test/pipeline_test.rb

@@ -258,4 +258,19 @@
       ImageProcessing::Vips.valid?(@portrait)
     end
   end
+
+  it "doesn't allow making system calls" do
+    ImageProcessing::Vips.source(@portrait).apply(system: "touch foo.txt")
+    refute File.exist?("foo.txt")
+
+    assert_raises Vips::Error do
+      ImageProcessing::Vips.source(@portrait).spawn("touch foo.txt").call
+    end
+    refute File.exist?("foo.txt")
+
+    assert_raises MiniMagick::Error do
+      ImageProcessing::MiniMagick.source(@portrait).spawn("touch foo.txt").call
+    end
+    refute File.exist?("foo.txt")
+  end
 end