Security advisory

BitBox02 firmware advisory, August 2026

On 17 August 2026 BitBox released firmware 9.26.5 for the BitBox02, fixing three security issues. Tenshu shows a notice next to every connected BitBox02 running older firmware, and this page is what it links to.

Update the firmware. For almost everyone that is the whole job. None of these issues weakens the seed your BitBox already generated — unlike last month's Coldcard advisory, where the seeds themselves came out badly. So there is normally no key to rotate, no new Vault to build and no coins to move.

The exception is a device that was actually attacked, rather than merely vulnerable. Two of the three issues end in an attacker running their own code on the device, and code running on a BitBox can reach the seed it holds — which no later firmware update undoes. Both require something specific to have happened to you, covered below. BitBox reports no evidence that any of the three was ever exploited.

The authority on what was fixed is BitBox's own release post.

What was fixed

Bootloader

affected through 9.26.1 · fixed in 9.26.2

Malicious firmware could be installed on a genuine device, but only after a successful phishing attack — the user had to be tricked into running a counterfeit BitBoxApp and unlocking the device for it.

Memory corruption

affected through 9.26.4 · fixed in 9.26.5

Arbitrary code execution on a device that has not been set up yet, when it is connected to a malicious host. A device with a wallet already on it was never reachable this way.

Silent Payments

affected 9.21.0 – 9.26.4 · fixed in 9.26.5

An implementation flaw that could lock funds to an unintended payment address, which an attacker could then hold for ransom.

Firmware 9.26.5 also carries smaller hardening changes to data handling, cryptographic operations, input validation and device-state protection, and fixes an unrelated iOS/iPadOS swapping bug.

Two of the three have carve-outs that depend on which BitBox you own: the BitBox02 Nova was never affected by the bootloader issue, and the Bitcoin-only edition was never affected by the memory-corruption issue. Tenshu does not distinguish editions — every BitBox arrives over USB as a plain "BitBox02" — so it shows one notice below 9.26.5 rather than guessing which issues reach your particular device. Updating settles all three either way.

What this means for a Cube

None of this weakens the seed your device generated. Nothing here touches how your BitBox produced its recovery words, so on a device that was not itself attacked, the key it holds in your Vault is exactly as good as it was. That is the difference between this advisory and the Coldcard one: there, the seed itself came out badly on every affected device, and no firmware update could repair it. Here the update is the remedy — with the one qualifier that follows, which is doing real work rather than hedging.

The memory-corruption issue needs three things at once: a Multi-edition BitBox on firmware 9.26.4 or below, still uninitialised, connected to a malicious host. Miss any one of them and it never applied to you. The Bitcoin-only edition was never affected at all, and neither was any device that already had a wallet on it. What matters here is the edition, not the model — the Nova's exemption above covers the bootloader issue only, so a Nova running Multi-edition firmware is in scope like any other Multi. If you set your Multi up on a computer you trust, it was never reachable either. Only if you set one up on a machine you had reason to distrust, and did so before updating, should you treat that device's seed as suspect and replace the key — the procedure is the one written up in the Coldcard advisory.

The bootloader issue required you to be phished first — into installing a counterfeit BitBoxApp and unlocking the device for it. If you believe that happened to you, the same applies: replace the key on that device. Firmware from 9.26.2 onward closes the hole regardless, and if you have updated at any point since that release you already have the fix.

Silent Payments do not reach your Vault. Tenshu never builds a silent-payment send with a BitBox key. If the same device signs in other wallets that do support them, that is where the issue lives — and updating fixes it there too.

A Cube is a multisig wallet, so a single key cannot move funds on its own. That is worth knowing, but it is not the reason to take this calmly. The reason is that this incident does not reach your keys at all.

Nothing about how Tenshu connects to, signs with, or imports from a BitBox02 has changed. A flagged device stays fully usable.

Updating the device

Tenshu reads the firmware version each BitBox02 reports and flags anything below 9.26.5. The flag clears by itself once the device reports the new firmware — there is nothing to dismiss and nothing to come back to.

  1. 1

    Open the BitBoxApp you already have

    Use the update link inside the app, or type bitbox.swiss/download into your browser yourself. Do not follow a download link out of an email: this advisory describes an attack that began with a counterfeit BitBoxApp, and security announcements reliably attract phishing campaigns that imitate them.

  2. 2

    Plug in the BitBox02 and unlock it

    Nothing needs to be prepared first. Your wallet, your recovery words and your Vault are untouched by this update.

  3. 3

    Update from Manage device

    Run the firmware update and confirm it on the device itself. When it finishes, check that the device reports 9.26.5 or later.

  4. 4

    That is the whole job

    Next time you connect the device to Tenshu the advisory notice will be gone. There is no descriptor to re-register, no new Vault to build and no funds to move — this incident never reached your keys.

Get the app from the right place

The bootloader attack this release closes started with a counterfeit BitBoxApp, so how you get the update matters as much as getting it. Use the update link inside the app you already have, or type bitbox.swiss/download yourself — never a download link out of an email. You can verify the app signature before installing if you want to check for yourself.

BitBox will never ask for your recovery words, and neither will COINCUBE. No genuine firmware update needs them typed anywhere but on the device itself. Anyone asking is stealing from you.

References

Questions about your own setup: [email protected].