1,082 BTC stolen from Coldcard wallets via weak seed generation
In brief
- 1,082.65 BTC ($70M) swept from 1,196 Coldcard wallets in 41 minutes on July 30
- Coldcard firmware vulnerability exploited weak seed generation; devices never physically accessed
- Firmware failed to use hardware randomness, fell back to serial number and clock registers
- Mk2, Mk4, Q, and Mk5 models affected; Mk3 owners warned by Coinkite
The seed generation flaw
Coldcard's firmware was designed to draw the seed from a dedicated hardware randomness generator. An internal build setting caused the device to skip that generator and fall back to basic software substitution, seeded only from the chip's serial number and its clock registers.
This downgrade had catastrophic consequences. The range of possible keys the device could produce shrank from unimaginably vast to roughly four billion possibilities on newer Coldcard models. For a computer, that's trivial.
How the attack worked
The attacker generated candidate seeds on their own hardware, derived the addresses each would produce, and checked those addresses against the public blockchain. Every step ran on the attacker's machine. The victim's device was never involved and could have been powered off in a safe on another continent.
Of the drained wallets, 1,183 used the modern native segwit address format, seven used an older standard and six used an older one still. The attacker used a paid account at a well-known blockchain data provider to query source addresses during the sweeps.
What's affected and what comes next
Coinkite, Coldcard's maker, has warned Mk3 owners and says its newer devices are unaffected. Block's research places the Mk2, Mk4, Q and Mk5 models in scope.
There is no test an owner can run against their own wallet to determine whether their seed sits inside the reproducible range. This means affected users cannot verify their exposure. Galaxy warned further waves are likely if owners do not move their funds.
