URGENT: Coldcard Wallet Flaw Exposes Bitcoin Seeds, $70M Vanishes as AI Suspected in Breach

A significant security vulnerability in Coldcard Bitcoin hardware wallets, manufactured by Coinkite, has led to a hack resulting in over $70 million (more than a thousand bitcoins) in stolen funds. The exploit, which began to surface on social media on July 30th, involves a critical flaw in how the devices generated secure private keys, particularly for wallets created without robust user-generated entropy.

The root cause is a firmware bug, specifically identified in Coldcard MK3 devices with firmware versions 4.0.1 (March 2021) through 4.1.9. This bug caused the seed generation process to fall back to a weak software Pseudorandom Number Generator (PRNG) instead of utilizing the intended hardware true random number generator (TRNG). This resulted in seeds being generated with only approximately 40 bits of entropy, significantly less than the intended 128 bits, making them predictable enough for attackers to brute-force.

Initially, the Coldcard Mk3 model was confirmed to be affected, but later, Coinkite advised users of its Mk4, Mk5, and Q devices to also take precautions. Wallets most at risk are 12- or 24-word seeds generated by these devices that did not include user-generated dice rolls (at least 50 rolls) or a BIP 39 extra passphrase. Single-signature wallets are particularly vulnerable, with 594.5 Bitcoins, valued at over $35.7 million, moved from such addresses on Thursday alone. Blockchain analysts have since reported that total stolen funds have exceeded $70 million, with unusual patterns suggesting a single attacker who utilized a paid account at a well-known blockchain-services provider to query source addresses during the sweeps.

Coinkite, Bitcoin Magazine, and other industry experts have issued urgent advisories for affected users. The primary recommendation is to immediately move funds from any vulnerable Coldcard wallet to a new, secure word seed or a seed generated by a different device. If no other hardware wallet is available, users can generate a temporary passphrase using six BIP 39 words on their existing Coldcard to enhance security before creating a completely new, secure seed.

Coinkite has released fixed firmware versions: Mk4 and Mk5 users must update to version 5.6.0 or later, Q users to 1.5.0Q or later, and Mk3 users to 4.2.0 or later. However, it is crucial to understand that updating the firmware does not secure existing vulnerable seeds. Users must generate a brand-new wallet with a new seed *after* updating the firmware and then transfer their funds to these new addresses. Any keys generated by the vulnerable firmware remain compromised.

Cybersecurity engineer Peter Todd also addressed specific edge cases for multi-signature wallets, warning that a 2-of-3 setup with two compromised Coldcards could still be vulnerable if the multisig script is revealed. He suggested using MARA mining pool's private mempool mining service, Slipstream, to keep transactions secret until they are in a block, dramatically reducing an attacker's window.

Beyond the immediate crisis, NVK, a co-founder of Coldcard, emphasized that this event signals a broader tech shift. He highlighted the emerging