A firmware flaw in Coldcard hardware wallets weakened the random number generator used to create seed phrases, leading to the theft of roughly 594 bitcoin from about 500 single-signature wallets on July 30–31, 2026. The vulnerability affected Mk3, Mk4, Mk5, and Q devices running specific firmware versions, replacing the hardware random number generator with a predictable software substitute. That cut entropy from 128 bits down to 72 bits — enough to make some seeds guessable.
How the flaw worked
The bug lived in the seed generation code. On Mk3 devices with firmware 4.0.1 through 5.0.3, and on Mk4 and Mk5 devices before version 5.6.0, as well as Q devices before 1.5.0Q, the hardware random number generator was swapped out. Instead of drawing on the chip's true random source, the firmware fell back to a software-based generator that was far less random. The result: seeds with only 72 bits of entropy instead of the intended 128. That's a difference of 2^56 possible seeds — a gap that made brute-force attacks feasible for well-resourced adversaries.
Who was at risk
Not every Coldcard user was vulnerable. The risk depended on the device version and how the seed was generated. Seeds created with a BIP-39 passphrase or with at least 50 dice rolls were not considered compromised. The main exposure hit single-signature wallets, where one seed controls all funds. Multisignature setups require additional keys, so they were less affected. The attackers swept funds from roughly 500 wallets, suggesting they targeted seeds that were generated on the vulnerable firmware without extra entropy sources.
Fixed firmware and lingering trust
Coldcard released patched firmware: version 5.6.0 for Mk4 and Mk5, and 1.5.0Q for Q devices. Mk3 users were advised to upgrade to a later firmware or replace the device. The incident is a reminder that hardware wallets reduce online risks but still require trust in the firmware, the supply chain, and the user's own practices. A single firmware bug can undo the security promise of cold storage.
The question now is how many users have applied the fix — and whether any remaining vulnerable wallets still hold funds that could be swept.
