Security advisory
Coldcard firmware advisory, July 2026
On 30 July 2026 Coinkite published a security advisory: on affected Coldcard firmware, wallet seeds were generated with far less randomness than intended. Tenshu shows a notice next to every connected Coldcard, and this page is the procedure it links to.
The authority on what is affected and which firmware fixes it is Coinkite's advisory. The technical root cause is written up in Block's engineering analysis.
What this means for a Cube
A Cube is not a single wallet. It holds a Spark wallet for everyday spending and, when you set one up, a Vault for savings. A Coldcard only ever holds a key in your Vault — so what this advisory means for you depends on how that Vault is built.
If your Vault needs more than one signature to spend — the way we recommend building one — a single key cannot move funds on its own. Someone who could reproduce one of your seeds still could not spend, because your descriptor requires another signature. You have time to do this properly rather than in a panic.
If it does not, treat this as urgent. A Vault is not required to be multisig, and a spending path can be satisfied by a single key: if the affected Coldcard key is enough on its own — a single-key Vault, or a recovery path whose timelock has passed — then nothing stands between that seed and your coins. Move the funds before anything else.
Either way, a firmware update fixes how new seeds are generated but cannot change a seed that already exists. If a key in your Vault was generated on affected firmware, that key should be replaced — and because a Vault's descriptor is fixed once it is created, replacing a key means building a new Vault around the new key and moving the coins across.
Your Spark wallet is a single-key wallet, but this advisory does not reach it: its keys come from your Cube's own seed, generated by Tenshu when you created the Cube, never by a Coldcard. The one exception is a Cube you created by restoring seed words that a Coldcard produced on affected firmware — those words carry the same weakness, and a single-key wallet has no second signature to fall back on.
If the same Coldcard also holds wallets in other software — outside Tenshu — nothing in your Cube protects those. Deal with any that the Coldcard key alone can spend first.
Two exceptions come straight from the advisory. A seed created with 50 or more independent, private dice rolls is not at risk from this issue: your dice supplied the entropy, not the broken generator. It is also why we suggest rolling the replacement seed rather than letting the device produce it. And a strong, unique BIP-39 passphrase is an independent barrier that reduces exposure — though it does not repair the seed underneath it, and Coinkite still recommends migrating.
Nothing about how Tenshu connects to, signs with, or imports from a Coldcard has changed. A flagged device stays fully usable.
Which devices are affected
Tenshu reads the firmware version each Coldcard reports. Firmware inside an affected range — or a version it cannot read — gets update and rotate; firmware at or past the fix gets rotate recommended. The second is not an all-clear, and Tenshu never shows one: patched firmware says nothing about when the seed on the device was created.
| Line | Affected | First fixed |
|---|---|---|
| Mk2 / Mk3 | 4.0.1 – 4.1.9 inclusive | 4.2.0 |
| Mk4 / Mk5 | below 5.6.0 | 5.6.0 |
| Mk4 / Mk5 (Edge) | below 6.6.0X | 6.6.0X |
| Q | below 1.5.0Q | 1.5.0Q |
| Q (Edge) | below 6.6.0QX | 6.6.0QX |
Replacing the key: a new Vault
Tenshu calls this rotating a key, but a descriptor cannot be edited once its Vault exists. What you are really doing is building a second Vault with the new key and moving the coins across. Every step uses a flow Tenshu already has; nothing here is specific to this advisory.
- 1
Update the device firmware
Follow Coinkite's upgrade instructions for your model and confirm the fixed version above is installed. Updating firmware does not change the seed already on the device — it is the prerequisite for the next step, not the fix.
- 2
Generate a new seed from your own dice rolls
This advisory is about a device generating a seed with less randomness than it claimed, so do not take the device's word for it a second time. Use the dice-roll option in the Coldcard's new-seed flow and roll at least 50 times, in private: the entropy then comes from your dice rather than from the generator that failed — the same 50-roll threshold Coinkite names as unaffected. Write the words down. Do not restore the old seed onto the updated firmware; the point is a key whose randomness you can account for yourself. The device now has a new master fingerprint, which is what makes it a different key as far as any descriptor is concerned.
- 3
Create a new Cube and build its Vault
A Vault's descriptor is fixed the moment it is created — a key cannot be swapped out of a Vault that already exists, and a Cube holds one Vault. So the replacement is a new Vault in a new Cube: create the Cube, run the Vault Builder, and use the Coldcard's new xpub where the old key used to be. Keychain keys cannot be carried over — a key may take part in one Vault only, ever, so each keyholder needs a fresh key on their Keychain. Register the new descriptor on every signing device, and back up the new Cube's Recovery Kit.
- 4
Sweep the funds into the new Vault
Building a Vault does not move coins, and your old Vault keeps working until you empty it. Take a receive address from the new Vault, verify it on a device, then send the full balance to it from the old Vault, signing with the old keys — the affected Coldcard included, if a path you are using needs it. If the Vault holds a lot, do a small test spend first and confirm it arrives.
- 5
Verify, then retire the old Cube
The new Vault shows the swept balance confirmed, the old Vault is empty, every device has the new descriptor registered, and the new seed backup is stored the way you store the others. Keep the old Cube's descriptor and backups until all of that is true — until the sweep confirms, the funds still live under the old descriptor.
Tenshu keeps showing the advisory badge on the Coldcard afterwards. That is expected: the badge marks the device, not the key, and no firmware version can prove when a seed was created. You can collapse the notice from the device list once you have acted on it.
References
- Coinkite — COLDCARD security advisory — what is affected and which firmware fixes it.
- Coinkite — upgrading COLDCARD firmware — the update procedure per model.
- Block engineering — predictable RNG fallback and 32-bit reseed — the technical root cause.
Questions about your own setup: [email protected].