File binder

File binders are utility software that allow a user to "bind" multiple files together, resulting in a single executable. They are commonly used by hackers to insert other programs such as Trojan [horse (computing)|Trojan horses] into otherwise harmless files, making them more difficult to detect. Malware builders often include a binder by default.
A polymorphic packer is a file binder with a polymorphic engine. It thus has the ability to make its payload mutate over time, so it is more difficult to detect and remove.