Bitcoin developer Peter Todd has cautioned holders about the concentrated risk of single-signature Bitcoin custody in the wake of a reported $38 million loss tied to Coldcard hardware wallets, reigniting debate over how large balances are secured with a single key.

Single-Signature Custody Under Renewed Scrutiny

A single-signature, or single-sig, wallet protects funds using one private key derived from a single seed phrase. When that key or the seed generation process is compromised, no secondary factor stands between an attacker and the full balance.

The warning follows a documented weakness in how certain Coldcard firmware versions generated seeds. Coinkite, the manufacturer behind Coldcard, has published a seed generation warning covering affected Coldcard Mk3 devices, identifying conditions under which entropy could be undermined. The Mk3 is an earlier generation of the Coldcard product line; Coinkite's current shipping model is the Mk4, which uses a different secure element architecture.

Coldcard devices occupy a notable position in the Bitcoin self-custody ecosystem, valued for their air-gapped design that uses SD card or NFC communication rather than direct USB connections. This incident highlights that even wallets designed to resist network-based attacks can carry fundamental weaknesses in how they generate the cryptographic material underpinning user funds.

The magnitude of the loss underscores the vulnerability. A theft at this scale demonstrates that when a single key is exposed, an entire holding can be transferred at once — a stark contrast to the incremental losses typical of other failure modes.

Cold Storage Weak Points Revealed

Independent analysis has centered on the seed generation flaw rather than user error. Engineers at Block described a predictable RNG fallback and 32-bit reseed in Coldcard firmware, a condition that can render private keys guessable if the random number generator degrades to a weak state.

This distinction is critical: device security and overall wallet security are separate concerns. A hardware wallet may be physically tamper-resistant while still producing cryptographically weak keys, shifting the risk from physical theft of the device to mathematical prediction of the key itself. Secure random number generation is foundational to all cryptographic systems, and predictable RNG failures have historically been among the most consequential classes of vulnerabilities across the technology landscape.

Reporting on the incident described funds being drained in approximately 15 minutes, consistent with an attacker who already possessed the ability to reconstruct the key rather than someone probing defenses in real time. The precise mechanics remain unconfirmed.

Holders Reassessing Custody Models

Single-sig setups remain appealing for their simplicity: one seed to back up, one signature required to spend, and fewer components to manage. However, that same convenience concentrates all risk into a single point of failure.

The incident is steering larger holders toward multi-layer custody strategies. Multi-signature arrangements require several independent keys to authorize a transaction, reducing the probability that one compromised seed can move an entire balance. This approach introduces operational complexity but does not eliminate risk entirely.

Custody considerations have intensified as institutional Bitcoin exposure grows. The core lesson from this drain is specific: the appropriate custody model balances convenience, control, and the value at stake — and single-sig arrangements may no longer meet that threshold for the largest holdings.