Last updated June, 2018
Simply put, a 51% attack is when malicious actors take over more than 50% of the hashing power of a coin. This allows them to write and verify things to the blockchain that aren’t legitimate.
In order to increase the speed at which exchanges allow funds to move in and out, many have been slowly lowering the number of confirmations on the blockchain required before allowing funds to leave their exchange.
This allows the malicious people to spend their coins, withdraw something else, and at the same time spend the same coins on a different exchange.
On June 2nd, a 51% attack occurred on the ZenCash network. They were able to double-spend 23,000 ZenCash which is approximately $550,000 USD at the time.
The good thing about these attacks is that they can only double-spend coins they own – and it appears that exchanges are left holding the bag? I expect exchanges will increase the number of confirmations required before allowing funds to be transferred.
What I’m not sure about is how this will affect ZenCash itself…. will people lose faith in the coin? Was it the coins fault, or the exchange’s fault for allowing funds to leave with only 1 or 2 confirmations?