Prefix code
The prefix code is a five-digit code unique to each Federation starship. It is designed to allow authorized personnel to remotely control a ship's systems.
Potential security concerns
If prefix codes still use a five-digit format in the TNG era and beyond, a brute force attack may allow an attacker to lower a Federation starship's shields, depending on computer security measures. However, the prefix code can be disabled by bridge crew with a manual override or altered when the code is suspected of being compromised.
Worf, in the TNG episode "The Wounded", claimed that the prefix code would allow Cardassian warships to disable the shields of the USS Phoenix. This was not the case, as the Phoenix sustained negligible damage and subsequently returned fire and destroyed a Cardassian vessel. Therefore, either the Phoenix's crew altered the prefix code; the Phoenix's crew overrode the Cardassian remote command; or the Cardassians failed to use the prefix code correctly. Third option, and most likely when events of episode are taken into accaunt (as per Data's words, Phoenix has sustained direct hit, retreated and then returned, destroying Cardassian warship) is that Cardassian weapon (which seemed to be either phaser or disruptor, originating from red rectangle at forward part of ship, which was originally to be navigational defelector) is that Phoenix has taken direct hit on hull after its shields have been lowered but survived, retreating afterwards outside Cardassian weapons range and then returned, destroying warship.
Usage examples
The prefix code was used by Captain Kirk in The Wrath of Khan as a desperate attempt to gain a temporary tactical advantage against Khan. Khan, lacking formal training in starship command, was unfamiliar with the prefix code. The code allowed the Enterprise to remotely lower Reliant's shields and disable the Reliant's weapons with phaser fire.
In TNG "The Wounded", Picard reveals the prefix code of USS Phoenix to Cardassians to help them prevent an attack on a Cardassian cargo ship, but the code does not noticeably affect the outcome of the battle.