So having a miner in your pocket is certainly one way to execute a double spend. That's a Finney attack.[0] The other and less costly way is all about node con

So having a miner in your pocket is certainly one way to execute a double spend. That's a Finney attack.[0] The other and less costly way is all about node connectivity.[1] If you are going to execute a double spend all you 'need' to do is broadcast two txs spending the same output at the same time--one going to the merchant the other spending it back to yourself--and cross your fingers and hope the double spend gets put in a block. Now, you can increase the probability (e.g. shielding the merchant's node from your double spend) of success but it's not a sure bet. And the higher the node connectivity of the network (i.e. how many nodes are each node connected to) the harder this attack becomes. There has been some work in this area, especially in pursuit of the efficacy of double spend attacks in light of some minor changes to the protocol which (I think) were rolled into core with the most release 0.9.0.[2] It's about .09% probability of success. [0]https://en.bitcoin.it/wiki/Weaknesses#The_Finney_attack https://en.bitcoin.it/wiki/Weaknesses#The_Finney_attack [1]http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23fc0/P2P2013_041.pdf http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23... [2]http://www.tik.ee.ethz.ch/file/848064fa2e80f88a57aef43d7d5956c6/P2P2013_093.pdf http://www.tik.ee.ethz.ch/file/848064fa2e80f88a57aef43d7d595...