USER BULLETIN · FSA-2026-001 · 27 August 2026
A malicious app could trick your devices into signing a hidden output
Frostsnap device firmware · Read the technical report
We know of no affected users and no loss of funds. Exploiting this required malicious software in place of the Frostsnap app; we routinely search for such apps and have found none that leverages this bug. All device firmware before v0.4.0 is affected: update your app and your devices.
What the device screen is for
Before a Frostsnap device signs, it shows you who is being paid, how much, and the fee, and you approve on the device itself. If the phone or laptop running the Frostsnap app were compromised and tried to pay someone else, the device screen would still show what was really about to be signed, and you could refuse.
What was wrong
A signing request from the app can mark an output as belonging to your own wallet. That is how change is marked. The device left such outputs off the screen, and its check of the request was faulty: it verified who owned the coins being spent, but not that an output marked as yours was yours. The mark names a key, and the output’s address is derived from whatever key it names. A request could name someone else’s key, and the device would treat the output as change and not show it. The fee on the screen did not give it away either, because the fee is what remains after accounting for every output, hidden ones included.
So a malicious app could request signatures on a transaction containing an output the device neither checked nor displayed.
What it required
A malicious version of the Frostsnap app talking to your device. The genuine app never builds such a request: every output it marks as yours carries your own wallet’s key. No such malicious app has been reported to us, nor found by our routine searches.
On its own this defect could not lose funds, and a wallet left alone is not at risk. But for these outputs, the safety of the funds depended on the Frostsnap app being honest, which is the one dependency the device screen exists to remove. That is why we treat it as a security flaw rather than an ordinary bug.
What to do
Update your Frostsnap app and your device firmware to v0.4.0, making sure the app comes from a genuine source. Three fixes are included: the device checks that an output claimed as yours is yours; it limits what a request may claim; and it shows transfers back to your own wallet instead of leaving them off the screen. The device still omits one output from the screen: the ordinary change of a payment to someone else. This keeps what you review short, and the change is omitted only after the device has verified it really comes back to your own wallet.
Exploiting the defect requires feeding a malicious transaction to a device, so upgrade each device before you next sign with it; the app will prompt you. Upgrade the devices within reach now. For devices you keep further away, in inheritance or emergency arrangements, upgrade them before they are next depended upon: a recovery could happen without you there to check the screen.
Where to ask
The fixes are #525, #540 and #546. The technical report has the detail.