Imagine you are a programmer who deliberately left an opening in a piece of code during the build phase that allows you an unknown entry into the application. Using this opening in the code, which of the following types of attack could hackers use to gain unauthorized access to the application ?

A backdoor attack involves a deliberate hidden access point in software that allows unauthorized entry, bypassing normal authentication. This could be intentionally left by developers or maliciously inserted. Birthday attacks involve hash collisions, man-in-the-middle intercepts communications, and spoofing forges identities - none match the deliberate code opening scenario.