Coldcard Firmware Flaw Linked to Theft of 1,082 Bitcoin in 41 Minutes

A defect in the firmware of COLDCARD hardware wallets degraded randomness when generating seed phrases and has been linked to coordinated sweeps that drained more than 1,000 Bitcoin in under an hour.

On 30 July 2026 an automated attacker emptied 1,082.65 BTC from 1,196 addresses in only 41 minutes. The operation followed a predictable pattern of rapid, offline seed enumeration made possible by insufficient entropy in seeds created on affected COLDCARD devices from Coinkite.

The vulnerability originated in an integration error introduced in March 2021. Instead of using the hardware RNG of the STM32 microcontroller, certain firmware versions routed seed creation to a deterministic software PRNG seeded solely with the chip’s unique identifier and boot-time registers. No additional entropy was mixed after startup, reducing effective entropy to approximately 40 bits on Mk3 units and 72 bits on Mk4, Mk5 and Q models—far below the 128 bits expected from a standard 12-word BIP39 seed.

Seeds remain vulnerable based on the firmware version present at the moment of creation, not the version running today. Affected ranges include Mk2 and Mk3 devices on versions 4.0.0–4.1.9, Mk4 and Mk5 devices before 5.6.0, and Q devices before 1.5.0Q. Corrected builds were also released in the Edge branches for earlier Mk4/Mk5 and Q hardware.

Coinkite published an emergency firmware release on 31 July 2026 that improves entropy for new seeds. Updating the device does not strengthen an already-generated weak seed; funds must be moved to a freshly created seed after the update.

Users who added manual entropy with dice are considered unaffected only if they performed at least 50 fair, independent, and private rolls. A strong, unique BIP39 passphrase creates a separate wallet and adds a defensive layer but does not replace the need to replace the underlying seed. Multisig setups are safe only when the required quorum does not rely exclusively on keys generated by vulnerable firmware.

Owners of potentially affected addresses are advised to monitor on-chain activity and move funds at the first sign of consolidation or unauthorized sweeps. Once an attacker locates a valid seed, automated draining can occur within minutes.