Thursday, July 30, 2026

Coldcard

Funds lost

$130.0M

Attack type

Other

Category

Other

Summary

Starting July 30, 2026, Bitcoin hardware wallet manufacturer Coldcard (by Coinkite) disclosed a critical firmware entropy defect that enabled attackers to systematically brute-force and drain user funds offline, resulting in cumulative losses estimated between $85 million and $130 million+ (~1,900 to 2,055+ BTC) across multiple organized attack waves.

Details of the Exploit

The vulnerability originated from a firmware code refactoring introduced in version 4.0.1 (March 2021), where seed phrase generation calls were migrated from ckcc.rng_bytes() to ngu.random.bytes(). This migration unintentionally caused the underlying rng_get() function to resolve to MicroPython's software Pseudo-Random Number Generator (PRNG) fallback instead of Coldcard's dedicated True Hardware Random Number Generator (TRNG). Consequently, generated wallet seeds possessed severely degraded entropy, approximately 40 bits on Mk3 devices and ~72 bits on newer models, down from the intended 128/256 bits.

Attackers leveraged this reduced search space to pre-compute and offline brute-force the predictable private keys for affected single-signature addresses, executing sweeping transactions across four distinct waves without requiring physical access to the hardware devices. Coinkite issued security advisories across affected models (Mk3, Mk4, Q, Mk5) while security researchers worked with victims to perform Replace-By-Fee (RBF) cancellations for pending mempool sweeps.

Source: De.Fi REKT Database. Data as of November 5, 2026.