The particular attack you describe is infeasible in Bitcoin for sure, and almost certainly for any other cryptocurrency. The problem is that the miners certify

The particular attack you describe is infeasible in Bitcoin for sure, and almost certainly for any other cryptocurrency. The problem is that the miners certify that a given chain is valid, but nodes and other miners will apply block validity criteria on top of that. Block validity includes things like "transactions are properly signed" and "miner rewards are correctly calculated". This effectively means that creating a "fake" transaction that would empty someone's account and credit it to the thieves would require a valid signature from the original account. Otherwise nobody would accept the new block. Changing the block validity criteria is possible, but requires cooperation on a much more grand scale, especially for Bitcoin where there are many implementations of the protocol that would have to change to match the new block validity criteria (which, suspiciously, will contain a new criteria saying "oh, and the signature check criteria don't apply to transactions from this address"). Without some form of consensus, this would amount to "theftcoin" simply being a hard fork of Bitcoin, with the main chain continuing, potentially with less hash power.