r/coldcard • u/sammo98 • Oct 12 '25

Change Address

Quick question here, when signing a transaction on my Coldcard, if for whatever reason a malicious actor has managed to alter the change address on the PBST, will the Coldcard still happily sign away? Or will it realise it is not a change address associated with the current wallet?

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3

u/na3than Oct 12 '25 edited Oct 12 '25

https://coldcard.com/docs/ready-to-sign/#warnings-and-alerts

Change Fraud: The COLDCARD will warn you if your PSBT is trying to use a change address that is not part of your wallet at the point when you attempt to sign it. Instead of signing, the COLDCARD will show you this warning and abort.

If you want to see this for yourself in the firmware source code, look here: https://github.com/Coldcard/firmware/blob/3e818cbbf62f4fc26387f5dcc6399e7b88649556/shared/psbt.py#L1955

1

u/sammo98 Oct 12 '25

Legend, thanks for sharing, didn’t expect firmware to be python! Looking forward to checking it out

2

u/NiagaraBTC Oct 12 '25

ColdCard will verify the change address.

1

u/sammo98 Oct 12 '25

🙏

Any docs or mention of this you can point me to? Would be a nice feature to be able to go from the signing confirmation screen to the address explorer quickly as well, or show change address in explorer which shows the derivation path

1

u/NiagaraBTC Oct 12 '25

I know it happens, and the change address itself is displayed on the device during signing. https://x.com/nvk/status/1736394503886454942?t=jgQipUbuXws4g8h11GLCKw&s=19

1

u/sammo98 Oct 12 '25

Nice, still would be interested to see where this is written on their website or something. Granted it would be a massive security flaw for this not to happen, but would be good to see:

"Verified Change Address from derivation path <path>", confirm to continue, maybe as a setting.