Coldcard hardware wallet flaw drains 594 BTC in 25-minute sweep

Jul 31, 2026

An attacker exploited a critical firmware vulnerability in Coldcard hardware wallets to steal 594 BTC (worth ~$38 million) from about 500 wallets in under 30 minutes. The flaw, introduced in March 2021, caused devices to use predictable, software-based key generation instead of a secure hardware randomizer. Coinkite, the wallet's manufacturer, and Block have warned users of affected models, particularly the Mk3, to move their funds immediately.

Coldcard wallet attack

▪After the initial sweep, 562 BTC of the stolen funds were consolidated into a single address.
▪An attacker stole approximately 594 bitcoin, worth about $38 million, from around 500 single-signature wallets.
▪Block's security team began investigating after receiving reports of Bitcoin being remotely stolen from non-Bitkey wallets.
▪The theft of 594 BTC occurred between 01:31 and 01:56 UTC on July 31, 2026, affecting the wallets in under 30 minutes.
▪Security researchers identified an additional 695 transactions with the same on-chain fingerprint, potentially raising the total stolen amount to 1,082.59 BTC.

Firmware vulnerability details

▪The vulnerability caused affected Coldcard devices to skip their hardware randomness generator and use predictable, software-based key generation.
▪Importing a compromised seed into a new wallet does not eliminate the threat, as the seed itself remains vulnerable.
▪The flawed software-based key generation was seeded with non-secret data, including the chip's serial number and clock registers.
▪A build setting instructed the device to skip the hardware randomness generator, and a library check only verified the setting's existence, not its state.

Affected device models

▪A wallet's vulnerability depends on the firmware version running when its seed was created, not when the hardware was purchased.
▪Coinkite, the manufacturer, issued a warning specifically for users of its Coldcard Mk3 signing device.
▪Coinkite's early analysis stated that Mk4, Q, and Mk5 devices were not affected by the vulnerability that led to the theft.
▪Block engineer Max Guise recommended that anyone affected by the vulnerability move their funds as soon as possible.
▪Block's investigation identified security flaws in multiple generations of Coldcard wallets, including the Mk2, Mk3, Mk4, Q, and Mk5.

Vulnerable firmware versions

▪The vulnerability in the Coldcard Mk3 model extends through firmware version 5.0.3, the final version to support the device.
▪Coinkite warned that seeds created on an Mk3 device running firmware version 4.0.1 or any later Mk3 version may be at risk.
▪The vulnerability was introduced in Coldcard firmware version 4.0.0, which was shipped in March 2021.

Compromised cryptographic functions

▪For Coldcard Mk2 and Mk3 firmware, a coding error caused wallet generation to rely on predictable values instead of hardware-generated randomness.
▪In addition to wallet seeds, the flawed generator also produced private keys for paper wallets, seed-splitting masks, and device cloning keys.
▪Wallets protected with weak 25th-word passphrases and some multisignature setups could also be at risk from the vulnerability.