Coldcard testnet mode could be used to sign a real bitcoin transaction
Who is affected
A Coldcard owner on firmware 3.1.9 or earlier persuaded to switch the device to testnet and sign an attacker-supplied transaction could lose real bitcoin. Coinkite considers the attack unrealistic because switching to testnet requires explicit action on the device.
What happened
On 24 November 2020 Shift Crypto researcher Marko Bencun disclosed that a Coldcard switched to Bitcoin testnet could be made to sign a mainnet transaction. The attack adapts a coin-isolation bypass first published against Ledger devices in August 2020; Coinkite had initially said Coldcard was unaffected because it supports no altcoins, but it does support testnet.
According to CriptoNoticias, a user who confirms what appears to be a testnet transaction sends the same amount of real bitcoin to the attacker, and the wallet shows it as a testnet transaction. The attack assumes a compromised computer and requires the user to be persuaded to switch the device to testnet, for example to test a transaction.
Coinkite states that Coldcards ship in mainnet mode, that there is no remote (USB) way to switch to testnet, and that the attacker also needs the victim’s xpub and UTXOs. In firmware 3.2.1 it added a warning and moved the testnet setting into its “Danger Zone” menu; it said it did not see the need to ask users to upgrade. Researchers disputed Coinkite’s view that the attack is unrealistic, arguing social engineering could make it practical.
Timeline: reported in early August 2020 (4 August according to Coinkite’s disclosure history; CriptoNoticias gives 1 August) under a 90-day embargo; a beta fix was released on 23 November and the issue was published on 24 November.
Cryptocurrency is a digital or virtual currency secured by cryptography, operating on decentralized blockchain networks to enable secure, peer-to-peer transactions.
Coldcard is an air-gapped hardware wallet for Bitcoin, made by Coinkite, that stores private keys offline and signs transactions without internet exposure.
Bitcoin (BTC) is the first decentralized cryptocurrency, launched in 2009. It uses blockchain technology for secure, peer-to-peer digital transactions without intermediaries.
Read full definitionA testnet is a blockchain network used for testing and development, where developers can experiment without using real cryptocurrency. Examples include Goerli and Sepolia.
Read full definitionMainnet is the primary blockchain network where actual transactions occur, as opposed to testnets. It represents the live, functioning version of a blockchain.
Read full definitionAn altcoin is any cryptocurrency other than Bitcoin.
XPUB is an extended public key (xpub) in hierarchical deterministic (HD) wallets. It derives child public keys and addresses from a master public key without exposing private keys.
Read full definitionUTXO (Unspent Transaction Output) is a unit of cryptocurrency from a previous transaction that remains unspent and serves as input for new transactions in blockchains like Bitcoin.
Read full definitionFirmware is the software that runs on a hardware wallet itself: it generates keys, shows transaction details on the screen and signs.
Read full definitionA phishing attack tricks crypto users into revealing private keys, seed phrases, or login details via fake emails, websites, or messages mimicking legitimate wallets or exchanges.
Read full definitionSources
Independent reporting and research this entry was checked against.
- Coinkite — Testnet Considered Useful (24 Nov 2020)
- CriptoNoticias — Vulnerabilidad en el monedero Coldcard permitiría robar bitcoins con ingeniería social (27 Nov 2020)
- Journal du Coin — Le cauchemar des bitcoiners : une faille corrigée pour éviter de perdre bêtement tous vos BTC (26 Nov 2020)
- Coinkite — COLDCARD security disclosure history
Learn more
Guides on protecting yourself against this type of incident (Firmware vulnerability) and the glossary terms that describe it.
Can Hardware Wallets Be Hacked?
Hardware wallets are extremely secure, but not invincible. Understand the real risks and attack vectors.
Read GuideIs a Hardware Wallet Really Safe?
Hardware wallets are the safest option for most people — but you can still make mistakes. Here's what to know.
Read GuideFirmware
Firmware is the software that runs on a hardware wallet itself: it generates keys, shows transaction details on the screen and signs.
Bootloader
A bootloader is a small program that initializes hardware and loads the main operating system or firmware on a device, often used in blockchain hardware wallets for secure startup.
Firmware Update
Firmware Update installs new low-level software on a hardware wallet to fix bugs, enhance security, or add features in cryptocurrency devices.
Cite this entry
WalletInsights. “Coldcard testnet mode could be used to sign a real bitcoin transaction”. Hardware wallet security incident tracker. Last verified November 3, 2026. https://walletinsights.io/en/security-incidents/coldcard-testnet-isolation-bypass-2020/
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