Here is the OFFICIAL patch for System Shock 2. This will upgrade it from the original install of 1.15 to 2.3.
According to Irrational Games website
"This patch implements cooperative multiplayer for System Shock 2. Up to four players are supported over internet or LAN. In addition, several bug-fixes and additional features are implemented including the ability for the user to modify weapon degradation and monster re-spawning rates. This patch is only intended for the original release of System Shock 2. If you have purchased a re- released "budget" edition, you should not need this patch."