COLDCARD Delta PIN mode let a private key be recovered from two signatures

On 29 September 2025 the security firm Karma-X posted an advisory announcing an undisclosed vulnerability in Coldcard hardware wallets that, it said, affected current firmware and could lead to loss of funds or exposure of keys. An updated post on 30 September 2025 identified the issue as a flaw in Delta Mode, the duress (trick) PIN feature intended to make a forced unlock look normal.

According to Karma-X, the flaw allowed full recovery of the Bitcoin private key from two transaction signatures, in a scenario where an attacker gets the victim to sign twice. Its full technical write-up requires an account, so only this summary is public. Coinkite’s disclosure history confirms the report: it credits Nathan Landon with showing that the Delta PIN implementation allowed private-key recovery from two transaction signatures and says Coinkite published a patch the same day. The public firmware commit titled “deltamode timing fix” changes how Delta Mode computes the hashes it signs.

Coinkite lists the issue as patched in firmware 5.4.4 (Mk4/Mk5) and 1.3.4Q (Q), both released on 30 September 2025; the public release notes for those versions do not mention it. Karma-X’s first post described all Coldcard devices on current firmware as affected, while Coinkite lists the Mk4/Mk5 and Q release families.