Coinkite released Coldcard firmware 5.6.1 on August 20, directly addressing a seed-generation vulnerability that enabled attackers to steal 1,778.84 BTC from over 8,600 addresses—confirmed at $112.7 million by Galaxy Research.
The flaw traces back to 2021 firmware. Mk2 and Mk3 devices generated seeds with only 40 bits of effective entropy; Mk4, Mk5, and Q devices fell to roughly 72 bits. Both fall catastrophically short of the required 128 bits. Attackers exploited this by reconstructing candidate seeds offline and cross-referencing derived Bitcoin addresses against the blockchain—no hardware access or recovery phrases needed.
Galaxy identified at least 15 distinct attackers and found 1,531 BTC from confirmed thefts still sitting unmoved. About 246 BTC moved post-exploit, with 65 percent flowing through CoinJoin to obscure traceability. The medium-confidence loss estimate, including suspected fourth-wave activity, reaches 2,417.35 BTC ($153 million).
The August 20 release covers Mk4 and Mk5 as well as version 1.5.1Q for the Q model, following a three-week security review after a July 31 hotfix. Every new seed now demands user-generated randomness: at least 65 keypresses with irregular timing, or 50 dice rolls, or 128 coin flips. This input combines with fresh entropy from the STM32 hardware RNG and both secure elements.
Coinkite replaced its Yasmarang pseudo-random generator with SHA-256 Hash_DRBG, a cryptographically robust standard. Boot-time checks now halt the device if the hardware RNG path fails, preventing startup compromises.
Coldcard also hardened transaction signing. The device now re-checks partially signed Bitcoin transactions immediately before signing and stops with a "Transaction modified" warning if a compromised USB-connected computer altered the payload after user review. Updates also tighten USB controls, improve firmware validation, isolate Delta Mode, secure backup mechanisms, and refine RNG fault handling.
Users who generated seeds on affected firmware between March 2021 and July 2026 must create an entirely new seed and migrate holdings to the new wallet. The firmware patch alone does not salvage existing recovery phrases.
