The Mechanism of the Drain
Published 7/31/2026, 10:07:45 PM
The drain of 594.48 BTC (approximately $38.3 million) from Coldcard hardware wallets occurred between 01:31 and 01:56 UTC on July 30, 2026. The incident was not a physical breach of the devices but a brute-force attack targeting a catastrophic firmware flaw that resulted in predictable private keys.
The Mechanism of the Drain
The attacker executed a highly coordinated sweep, moving 1,324 UTXOs from roughly 500 single-signature wallets within a three-block window [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/]. The funds were consolidated into a single attacker-controlled address: bc1qq85v2c926eg6pgxhwp6q7lf6cnsz80qs3fcu9r [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].
The attack specifically targeted wallets with balances greater than 0.15 BTC that had been dormant since 2021, which aligns with the introduction of the vulnerability [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].
Technical Root Cause: Predictable Entropy
The vulnerability stemmed from a firmware bug introduced in March 2021 (Firmware 4.0.0). A build setting error caused the device to bypass its hardware True Random Number Generator (TRNG) and fall back to a software-based substitute [Source: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware].
| Feature | Intended Security | Actual Security (Mk3) |
|---|---|---|
| Entropy Source | Hardware TRNG | Software PRNG (Predictable) |
| Seeding Data | High-entropy noise | Serial Number + Clock Registers |
| Security Level | 128 bits | ~40 bits |
Because the software fallback was seeded with non-secret data—specifically the chip's serial number and internal clock registers—the resulting "random" seeds were reduced to approximately 40 bits of entropy [Source: https://blog.coinkite.com/entropy-technical-backgrounder/]. This allowed a modern computer to brute-force the ~4 billion possible combinations and pre-calculate the private keys for thousands of potential wallets [Source: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware].
Affected Devices and Risk
The risk is determined by the firmware version used at the time the seed was generated, not the current firmware version.
- Coldcard Mk3: Firmware versions 4.0.1 through 5.0.3 are confirmed to have the ~40-bit entropy flaw [Source: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware].
- Mk4, Mk5, and Q Models: These models exhibited reduced entropy (~72 bits) prior to recent hotfixes (Mk4/Mk5 v5.6.0; Q v1.5.0Q), though they were not the primary targets of this specific 594 BTC drain [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].
- Unaffected Users: Those who generated seeds using Dice Rolls, utilized BIP-39 Passphrases, or used the device within a Multisig setup were protected from this specific vulnerability [Source: https://blog.coinkite.com/entropy-technical-backgrounder/].
Current Status
While Coinkite has released firmware updates to restore proper entropy, these updates cannot fix a seed that was already generated with low entropy. Users who generated seeds on affected Mk3 firmware between March 2021 and July 2026 are advised to generate a completely new seed and migrate funds immediately [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/]. Some reports from Galaxy Research suggest total losses across all related vulnerabilities could be as high as 1,082.65 BTC, though this higher figure remains not independently confirmed [Note: not independently confirmed].