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Coldcard firmware bug collapsed Bitcoin seed randomness for some devices

Coinkite says affected users must generate a completely new seed and move funds, since old phrases could be reconstructed by attackers exploiting the flawed randomness path.

Reported · Written up by our desk · Market Structure

CryptoSlate reports that Block’s Bitcoin Engineering and Security team, along with independent Bitcoin Core developers, traced recent Coinkite Coldcard wallet losses to a firmware defect that weakened the devices’ Bitcoin self custody seed generation before any user action.

According to CryptoSlate, Mk2 and Mk3 units running firmware 4.0.1 through 4.1.9 produced seeds where cryptographic randomness collapsed into a small, searchable set. The problem stemmed from a firmware change that diverted random number generation from the STM32 hardware source to MicroPython’s deterministic Yasmarang fallback, with Mk4, Q, and Mk5 models producing seeds with about 72 bits of entropy, still below the design’s 128-bit target.

CryptoSlate also reports that the issue could affect owners even if they stored recovery words offline for years, because an attacker could reconstruct the key space by searching the narrow output of the fallback generator. Coinkite’s advisory, as described by CryptoSlate, says firmware updates only protect seeds generated after installation, so seeds created through the flawed path require full replacement.

CryptoSlate adds that a seed weakness can persist when recovery words are imported elsewhere, since the origin property travels with the recovery phrase. Coinkite’s technical notes further say the vulnerable firmware path shipped starting in 2021 until the July disclosure, a five-year window during which the source code was public and the flaw was not detected.

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