***# In bitcoin's Necronomicon of possible attacks and weaknesses, one reigns supreme – the 51% attack.
If there is a fear that has played on people's minds as the end-of-days scenario for bitcoin, it is this. Attackers who hold more than 50% of hashing power could stop transactions from confirming and even reverse some transactions. They could undermine the whole project.
Bitcoin's design and its system of economic incentives has been set up specifically to combat the destructive potential of a 51% attack. And it has worked. The 51% attack has remained a hypothetical bogeyman. Until now.
By all indication, a coordinated 51% attack will begin on, or around, Nov. 16. That's when a consortium of miners representing substantially more than 50% of the network's hashing power and an allied group of blockchain startups will seek to increase the block size.
This will require a hard fork, which while controversial, is a legitimate desire. In itself, this is not an attack.*# **