An Important Update on ColdCard
This article is to help people who currently have a ColdCard device be safe. We will update it with current best practices as more details are made available. For a full run down of the events and a deep explanation of how this happened see our blog: The ColdCard Hack.
Coinkite and Block published security advisories today about a flaw in Coldcard firmware. Seeds generated on a Coldcard Mk2 or Mk3 running firmware 4.0.0 or later (March 2021 onward) may be predictable, and attackers are reportedly already draining affected wallets. Mk4, Q, and Mk5 carry a smaller version of the same flaw. Dice-roll seeds, and Mk2/Mk3 on older firmware, are not affected.
If you don’t use a Coldcard, no action is needed.
Bitcoin Well is not affected. We’re non-custodial, we never hold your keys, and this doesn’t touch your account, our ATMs, or Infinite.
If you do use one, here’s how to protect yourself tonight. Work carefully; a rushed migration can cost you more than the bug.
1. Check where your seed was created. A Mk2 or Mk3 on firmware from March 2021 or later means treat that seed as compromised, even if you’ve since moved it to another wallet.
2. If you have a device from a different manufacturer, generate a fresh seed on it, verify the backup, confirm a receive address on the device screen, and send a small test transaction. Once it arrives, move the rest. Keep your old backup until everything is confirmed.
3. If the affected Coldcard is all you have tonight, use Coinkite’s interim fix: add a long, random, unique BIP-39 passphrase (not your PIN) directly on the device, back it up separately from your seed words, and move bitcoin to that new passphrase wallet. Test transaction first. A typo creates a different wallet, so follow Coinkite’s steps exactly: https://coldcard.com/docs/passphrase/ Never type the passphrase into a computer, phone, or website.
4. Already using a BIP-39 passphrase with that seed? Coinkite says your risk is minimal. Plan a proper migration soon anyway.
Going forward: when you migrate, consider a device that supports dice-roll entropy, so your seed never depends on a chip’s randomness alone. For larger holdings, ask your Executive Bitcoin Advisor about multisig across two or three devices from different makers, so no single device is a point of failure.
If you need a device to migrate to, the Blockstream Jade Plus is one option that supports dice-roll entropy, the exact safeguard this incident is about. It also happens to be the one hardware wallet we're currently able to offer a discount on: use code bitcoinwell at checkout for a discount, and JADEPLUSEXPRESS for free shipping. Here's our step-by-step setup guide:
https://help.bitcoinwell.com/en/article/how-to-set-up-and-use-blockstream-jade-plus-15d8mwg/ Whatever device you choose, the migration steps above still apply.
Official advisories:
Coinkite: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/
Block: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware
Expect scams to follow this news. Never enter your seed words into a website, app, or “checker” tool. Bitcoin Well will never ask for your seed.
Stuck or unsure? Send us an email to hello@bitcoinwell.com, or reach your Executive Bitcoin Advisor. We’ll help.
— Your Team at Bitcoin Well
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