Do you think your bitcoin in cold wallet is protected by law of mathematics and also the most powerful computer network in the world? Think again

The recent proposal by Pieter Wuille revealed that core devs can push in a soft fork change that pretty much changes every way bitcoin works, thus hackers can spend your money without your consent

How is that possible? Because bitcoin is essentially an agreement (protocol) among nodes, if majority of the nodes around you agree that your bitcoin is gone, then it is gone! It does not matter how strong ECDSA is, all it takes is a group of nodes around you changed their rules (or so called sybil attack)

Somebody might wonder: Aren't miners suppose to be the honest nodes and stop all this? Unfortunately, in this case, miners or so called most powerful computer network in the world can not do anything about it

Why? Because everything in bitcoin is decided by its agreement among nodes. If the nodes changed their way of calculate blocks, then all the miners will be dropped from the new network, and all those ASICs in large mining farms will just become paperweight

This becomes a real threat when mining has become too centralized, e.g. only a few large pools are doing mining. So, even they are running the original version of bitcoin, if large group of nodes have upgraded to a different version, these miners will just be ignored as minority (new version can easily change the way that miner works). Of course without hash power the new version will worth nothing later on, but I guess the thieves only need to sell their stolen coins before others realize the problem

The critical point that have real financial impact are exchanges and web wallet services. If one of these nodes together with a group of malicious nodes changed their protocol, then they could easily take others' coin, sell on exchange and profit. If you are really paranoid and assume that every exchange might be a potential malicious actor like MTGOX, then they have many ways to profit unethically through a protocol change