Prefix code
The Prefix Code is a five-digit code unique to each Federation starship. It is designed to allow authorised personnel to remotely override ship's systems.
Potential security concerns
If Prefix Codes still use the five-digit format in the Next Generation era and beyond, a brute force attack can 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 to be compromised.
Worf in the TNG episode The Wounded claimed the Prefix Code would render USS Phoenix defenceless. This was not the case, asthe ship subsequently fired and destroyed a Cardassian vessel at range. Therefore, Captain Maxwell either easily disabled the Prefix Code, or the Prefix Code only affects certain ship systems.
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, without formal training in starship command, was unfamiliar with the prefix code. The code allowed Enterprise to remotely lower Reliant's shields.
In The Wounded, Picard reveals the Prefix Code of USS Phoenix to Cardassians.