The Outpost Attack is a 20 minute long chaotic event in which an endless amount of infected will constantly spawn outside of the Outpost and attempt to break down the main doors of the Outpost (Even though the infected are low level, they will give extra experience points during the OA). If players fail to defend the front doors of the Outpost, then the Outpost Attack will be lost, no buffs will be awarded and the outpost mission givers will be disabled for an hour. However depending on how well you do in the Outpost Attack, and upon a successful defense, you will be awarded with either a Outpost Defender buff or Outpost Guardian buff depending on how many infected kills you have gotten during the Outpost Attack.There will be a long red health bar at the top of the screen whilst you are at the outpost attack location, representing the health of the outpost doors. The health is globally spread across all multiplayer servers and there is a limit for how much damage each server can take to make it fair for other servers.
AdminPwn has stated that he may add NPCs that help you defend the outpost for players using Single player in the future.
An Outpost Attack will happen at all 3 outposts twice a day, so there will be a total of 6 outpost attacks a day, the order is random however. Each attack will happen every 4 hours. A timer for how long is left before the next Outpost Attack can be seen at the bottom of the World Map. Furthermore, if you have your music turned on, then you can hear the outpost's alarm/siren in the distance if there is currently an outpost attack happening (can be heard from anywhere in the game).
Also the Outpost that is under attack will be surrounded by orange tiles on the World Map.
Like all infected, it is recommended to use a weapon with cheap ammo types for the outpost attack, or a melee weapon like the Machete is highly recommended. Melee is strongly recommended as the infected during outpost attacks are typically low level and therefor weak, however you will gain bonus exp during the Outpost Attack. By using a weapon such as the Machete which has the fastest attack speed out of all melee weapons, then you don't need much damage to start instantly killing swarms of infected near the door, making it much easier and costing you far less money than using ammo for a gun.
If the outpost attack is defended successfully then you will be awarded with a 2 hour long buff, either Outpost Guardian or Outpost Defender. The outpost will also continue to be safe and use-able like normal.The Outpost Defender Buff will give +20% Exp Gain, +20% Find Elite Item Chance and +20% Find Ammo Quantity. Which requires 100 infected kills during the Outpost Attack. The Outpost Guardian Buff will give +40% Exp Gain, +40% Find Elite Item Chance and +40% Find Ammo Quantity. Which requires 200 infected kills during the Outpost Attack (logging out or changing areas won't reset the kill count for the current Outpost Attack).
It is also recommended to buy some medical items, food and water before the outpost attack, as the combat will last for 30 minutes and you will be needing food and water by the end of it. Prices typically spike upwards for the food and water after an outpost attack ends also, which is why it's best to buy it before that happens.
During Outpost Attacks it's a very safe strategy to shoot the legs of the infected to incapacitate them, so that it takes longer for them to reach the door (also prevents more from spawning in and overwhelming you with their speed).