Block Discloses Critical Coldcard Vulnerabilities Affecting Bitcoin Hardware Wallets
In brief
- Block disclosed critical vulnerabilities in Coldcard Mk2, Mk3, Mk4, Q, and Mk5 hardware wallets
- Firmware flaws reduced wallet randomness, enabling attackers to remotely steal Bitcoin within an hour
- Researchers identified 1,082.59 BTC potentially stolen across 695+ earlier transactions
How the Attack Worked
The attack initially targeted single-signature wallets and occurred over roughly an hour. Wallets protected with weak 25th-word passphrases and some multisignature setups could also be at risk. The first vulnerability in Mk2 and Mk3 firmware caused wallet generation to rely on predictable values instead of sufficient hardware-generated randomness. In Mk4, Q, and Mk5 devices, a firmware flaw reduced additional randomness to just 32 bits, making seed derivation far more predictable than intended.
Importing an affected seed into another wallet does not eliminate the threat.
Scope of Compromise
Block privately disclosed its findings to Coinkite before making them public. Researchers identified 695 earlier transactions matching the same on-chain fingerprint, accounting for an additional 488.11 BTC. The total amount potentially stolen would rise to 1,082.59 BTC according to Block's preliminary analysis.
Block's engineering and security teams started receiving reports of Bitcoin being remotely stolen from non-Bitkey wallets. Researchers warned the campaign is likely still active. Bitcoin self-custody users are being urged to move their funds after the disclosure, and Block engineer Max Guise stated in a company update that affected holders should act quickly.
"Personally I recommend that anyone affected move funds as soon as they can safely do so" — Max Guise, Block engineer
The vulnerability represents a significant breach of trust in hardware wallet security. Coldcard devices have long been marketed as a gold standard for self-custody, and the discovery of predictable randomness in wallet generation undermines that reputation. Users who relied on these devices now face the difficult choice of manually recovering funds and migrating to alternative wallets while the threat remains active.
