Nearly 1,200 Coldcard wallets emptied and $70 million in Bitcoin stolen in just 41 minutes. Yet, the perpetrator never gained access to the devices, recovery phrases, or the victims' computers. He simply managed to reconstruct their seed phrases... On July 30, an individual stole 1,082.65 BTC, or approximately $70 million, from 1,196 Coldcard wallets. The funds were transferred in several batches to four addresses. An attack carried out entirely offline. He didn't need physical access to the devices, as the problem stemmed from the way some seed phrases had been generated. A bug in certain versions of the Coldcard firmware resulted in seeds that were far too predictable. The firmware was supposed to use a hardware random number generator. Due to a misconfiguration, some devices used a software generator based, among other things, on the chip's serial number and data related to its internal clock. This information appeared random, but could actually be reproduced or limited to a much smaller number of possibilities. The attacker could generate millions of candidate seeds on their own machines and then calculate the associated Bitcoin addresses. When a match was found, they obtained the same private key as the owner and could sign a valid transaction. The emptied wallets used several Bitcoin address formats, primarily native SegWit. Several generations of Coldcard could be affected, including the Mk2, Mk3, Mk4, Mk5, and Q, depending on the firmware used when the seed was created. The attacker reportedly left a trace by using a paid account with a blockchain data provider to verify the addresses during the attack. The recorded queries correspond precisely to the number, order, and timing of the emptied wallets. This information was reportedly passed on to the authorities. Coldcard confirms the vulnerability and recommends immediate migration of funds to another wallet.