The Vulnerability
Published 7/31/2026, 6:06:50 AM
Coldcard Mk3 users should not panic, but they must take immediate action if their setup falls into a specific high-risk category. On July 30, 2026, an attacker drained approximately 594.5 BTC (~$38.3 million) from roughly 500 single-signature addresses in a coordinated 25-minute window [Source: https://atlas21.com/594-bitcoin-drained-15-minutes-theft/]. While the incident is severe, the vulnerability is mathematically limited to seeds generated using the device's internal random number generator (RNG) on specific firmware versions without additional security layers.
The Vulnerability
The drain targeted a flaw in the Coldcard Mk3 firmware where a build setting caused the device to skip its hardware RNG, falling back to a predictable software substitute [Source: https://theblock.co/post/410235/coinkite-coldcard-mk3-warning]. This resulted in "weak entropy," allowing attackers to pre-calculate and compromise private keys.
Risk Assessment by User Type
The risk is not universal across all Coldcard users. Your level of exposure depends on how you generated your seed and whether you use a passphrase.
| User Category | Risk Level | Reason |
|---|---|---|
| Mk3 + Single-Sig + No Passphrase | CRITICAL | Seed generation was likely predictable; funds are at immediate risk [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/]. |
| Mk3 + BIP-39 Passphrase | LOW | A strong passphrase adds external entropy that protects the wallet even if the base seed is weak [Source: https://theblock.co/post/410235/coinkite-coldcard-mk3-warning]. |
| Mk3 + Dice Roll Generation | LOW | Manual dice rolls (99+ rolls) bypass the flawed internal RNG entirely [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/]. |
| Mk3 + Multi-Sig | LOW | Requires compromising multiple keys from different sources; no multisig drains have been reported [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/]. |
| Mk4, Mk5, or Q Models | NONE | These models use a different architecture and are confirmed unaffected by Coinkite [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/]. |
Technical Details
- Affected Firmware: Versions 4.0.1 (released March 2021) through 5.0.3 (the final Mk3 release) [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].
- Total Impact: While the primary event involved 594.5 BTC, further analysis by Block's engineering team suggests the total affected amount could be as high as 1,082 BTC when including earlier linked transactions [Source: https://atlas21.com/594-bitcoin-drained-15-minutes-theft/].
Recommended Action Plan
If you are using an affected Mk3 device, Coinkite advises moving funds with "calm and care" rather than rushing, which can lead to manual errors [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].
- Immediate Mitigation: If you cannot migrate to a new device immediately, add a strong BIP-39 Passphrase to your Mk3. This creates a new set of addresses. Move your funds from the "no-passphrase" accounts to these new passphrase-protected accounts.
- Long-term Solution: Generate a fresh seed on an unaffected device (Mk4, Mk5, Q, or a different hardware manufacturer) and sweep all funds to the new seed.
- Entropy Best Practice: For any future seed generation, use the dice-roll method to ensure the randomness is verified by you and not dependent on hardware firmware.
Conclusion: Panic is unnecessary for users who utilized manual entropy (dice rolls), multisig, or strong passphrases. However, for single-sig Mk3 users without a passphrase, the risk is immediate and requires a controlled migration of funds to a new, secure seed [Source: https://theblock.co/post/410235/coinkite-coldcard-mk3-warning].