What is a double spend? What is a double spend?

As someone who uses Bitcoin, what do I need to know about how the Bitcoin system prevents double spends? Are there still circumstances where they can occur?
A double spend is an attack where the given set of coins is spent in more than one transaction. There are a couple main ways to perform a double spend:


Send two conflicting transactions in rapid succession into the Bitcoin network. This is called a race attack.
Pre-mine one transaction into a block and spend the same coins before releasing the block to invalidate that transaction. This is called a Finney attack.
Own 51+% of the total computing power of the Bitcoin network to reverse any transaction you feel like, as well as have total control of which transactions appear in blocks. This is called a 51% attack.


To prevent damages from the first attack - wait for one confirmation to appear on a given transaction. To prevent damage from the second attack - wait for 6 confirmations to appear on a transaction, or less if the transaction is small (but still require at least 1). Damage from the third attack can cripple the entire Bitcoin network, so don't worry about it - your business most likely won't be the main target (it's unlikely to happen without really big money getting involved).

For more information about all those attacks, you can check out my master thesis on Bitcoin security.