From Halopedia, the Halo wiki
The spoofer is a small device used by the UNSC forces to force open doors and override certain pieces of equipment by sending in an electrical current that stabilizes the device that may be locked, damaged, or otherwise unable to function.
They are standard equipment to Marine fireteams when deployed into ground engagements. They appear to be very advanced, as they are capable of even overriding the Forerunner locks on doors in Installation 04.
In 2552, Private Kappus used one to open the door to the Flood containment facility on Installation 04. He was ordered by Captain Jacob Keyes to open the door that happened to be the only thing holding back the Flood. Unwillingly and unknowingly, Kappus unleashed the Flood on Installation 04, dooming him and the rest of his squad, with the exception of Sgt. Johnson and the "paranoid Marine". When the Master Chief later arrives, he opens the same door by pushing a button on the spoofer.
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