A Widespread Security Crisis
Coinkite, the Canadian manufacturer of Coldcard hardware wallets, has issued urgent security warnings following a massive, coordinated sweep of Bitcoin wallets that has resulted in the loss of over $70 million in assets. Security researchers and blockchain analysts have linked the coordinated theft to a fundamental flaw in how certain Coldcard devices generate private keys.
According to reports from Galaxy Research and engineers at Block, approximately 1,082 BTC have been drained from user wallets. While initial attention focused on the Coldcard Mk3, subsequent advisories from Coinkite expanded the scope of concern to include the Mk4, Mk5, and Q models, particularly those where the seed phrase was generated without sufficient entropy—or randomness—using the recommended 50+ dice rolls.
The Technical Root Cause
The vulnerability appears to stem from a firmware bug in Coldcard Mk3 devices, specifically those running firmware version 4.0.1 (released in March 2021) through version 5.0.3. Experts, including Kevin Loaec, CEO of Wizardsardine, hypothesize that a low-entropy random-number generator within the device’s software library caused the wallet to produce predictable seeds. Instead of the industry-standard 128 bits of entropy, affected devices were generating seeds with only ~40 bits, rendering them susceptible to brute-force attacks.
“Every single mnemonic generated [via a Coldcard] since 2021 will be public in the next few days,” Loaec warned, underscoring the severity of the threat. The vulnerability effectively allows attackers to derive private keys and drain funds from single-signature addresses without requiring physical access to the hardware device.
Immediate Mitigation Steps
Coinkite has released a firmware patch to address the issue for new seed generation. However, for existing users, the company and security experts emphasize that patching the device is insufficient if the underlying seed phrase remains compromised. Users are advised to take the following steps:
- Move Funds Immediately: Transfer all Bitcoin assets from potentially affected Coldcard wallets to a new, securely generated seed on a different device or a verified software wallet (e.g., Nunchuck, Blockstream Green, or Bluewallet).
- Verify Backup Entropy: If you are unsure of how your seed was generated, assume it is compromised. Only wallets created using the manual dice roll method (with at least 50 rolls) are considered safe from this specific entropy flaw.
- Use Passphrases: For users who cannot immediately migrate funds, adding a strong, independently selected BIP-39 passphrase can provide a temporary layer of defense, though this is not a permanent solution and does not replace the need for a new, secure seed.
The investigation into the origin of the $70 million sweep is ongoing. While no definitive link has been publicly proven to bridge the specific Mk3 bug to every single stolen wallet, the concentration of thefts in native SegWit addresses with flawed entropy patterns points to a systemic failure in the generation process.