Coldcard Firmware 5.6.1 Forces Manual Entropy After 594.5 BTC Theft

Coinkite has shipped firmware 5.6.1 for its Coldcard Mk4 and Mk5 devices, and 1.5.1Q for the Q model, that refuses to generate a new wallet seed unless the owner first supplies manual randomness: at least 50 dice rolls, 128 coin flips, or 65 timed key presses. The mandate arrives three weeks after a random-number generator flaw let attackers sweep 594.5 BTC from 500 Coldcard addresses on July 30, a theft valued at between roughly $100 million and $130 million depending on the bitcoin price used.

What went wrong with the RNG

Coldcard was designed to pull seed entropy exclusively from its onboard hardware generator, the STM32 TRNG plus two secure elements. Coinkite traced the failure to a build and link error that left a software fallback active when it should have been disabled.

That error routed the random-number call to MicroPython’s Yasmarang pseudo-random generator, which had sat unnoticed in the seed-generation path since March 2021. Seeds produced under the bug carried roughly 72 bits of entropy instead of the 128 bits Coldcard was built to deliver, a gap large enough for attackers to brute-force affected wallets.

Two fixes stacked on top of each other

Firmware 5.6.1 adds a boot-time check that confirms the random-number call actually reaches the hardware path before the device will proceed. If that check fails, the device halts rather than generating a weak seed.

Coinkite also replaced Yasmarang with a SHA-256 Hash_DRBG built to the NIST SP 800-90A specification. The new generator is seeded at startup with a full 256-bit digest pulled from both secure elements; earlier firmware had truncated that seed value down to just 32 bits.

On top of both hardware-level fixes, every new seed now demands manual entropy from the owner. The design mirrors an approach pushed by Bitcoin developer Peter Todd, who has long argued that user-supplied randomness should back up device generators, not replace trust in them entirely.

Why holders should care

For existing Coldcard owners, the update does not retroactively fix seeds generated before the patch. Anyone who set up a wallet on affected firmware, particularly one created after March 2021, should treat that seed as compromised and move funds to a wallet generated under 5.6.1 or 1.5.1Q.

The episode is a reminder that hardware wallets are only as trustworthy as the code running inside them. A single build error sat undetected for more than four years before it was exploited at scale against 500 addresses.

Read more: Coldcard Forces User-Supplied Entropy After Bitcoin Theft Pegged at $114M-$130M

Sources

• cryptopotato.com
• decrypt.co