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

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