Root Cause: Predictable Random Number Generation

The Coldcard exploit, which occurred on July 31, 2026, resulted in the theft of approximately 594 BTC (~$38 million) in a rapid 25-minute window, with later estimates rising to over 1,082 BTC (~$70 million) [Source: https://www.coindesk.com/tech/2026/07/31/major-bitcoin-wallet-flaw-drains-594-btc-in-25-minute-sweep]. The incident was not a physical hardware breach but a catastrophic firmware-level entropy failure that made private keys predictable [Source: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware].

Root Cause: Predictable Random Number Generation (PRNG)

The exploit stemmed from a firmware bug introduced in March 2021 (version 4.0.0/4.0.1). A build setting error caused the device to bypass its hardware True Random Number Generator (TRNG) and fall back to a weak software-based pseudo-random number generator (PRNG) [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].

• The Vulnerability: Instead of generating 128 bits of true entropy, the software fallback used non-secret chip data—specifically the chip's serial number and internal clock registers—to seed the wallet [Source: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware].
• Entropy Collapse: This reduced the security of the generated seeds from the intended 128 bits to approximately 40 bits. This level of entropy is low enough for attackers to brute-force and reconstruct private keys using modern computing power [Source: https://www.coindesk.com/tech/2026/07/31/major-bitcoin-wallet-flaw-drains-594-btc-in-25-minute-sweep].
• Duration: The vulnerability remained undetected in the firmware for over 1,900 days (5 years) before being exploited [Source: https://blog.coinkite.com/coldcard-mk3-seed-generation-warning/].

Attack Execution and Scope

The attacker identified vulnerable wallets on-chain and swept funds across approximately 500 to 1,000 single-signature wallets between 01:31 and 01:56 UTC on July 31, 2026 [Source: https://www.coindesk.com/tech/2026/07/31/major-bitcoin-wallet-flaw-drains-594-btc-in-25-minute-sweep].

[Source: https://www.tradingview.com/news/cointelegraph:43ee49822094b:0-coldcard-issues-mk3-warning-as-experts-examine-38m-bitcoin-wallet-drain/] [Source: https://www.techtimes.com/articles/322392/20260731/coldcard-hardware-wallet-hacked-via-firmware-bug-that-bypassed-rng-five-years.htm]

Critical Security Advisory

Coinkite and security researchers have emphasized that firmware updates do NOT fix existing compromised seeds. Because the seed itself was generated with weak entropy, the private key remains predictable regardless of subsequent firmware patches [Source: https://blog.coinkite.com/entropy-technical-backgrounder/].

Required Actions for Affected Users:

1. Generate a NEW seed on a device with updated firmware.
2. Use physical dice rolls (at least 50) to ensure true entropy, bypassing the device's internal RNG [Source: https://blog.coinkite.com/entropy-technical-backgrounder/].
3. Migrate all funds to the new seed immediately.
4. Do NOT simply restore an old Mk3 seed into a new device, as the seed itself is the point of failure.

The exploit highlights a critical risk in hardware wallets: even if the hardware is secure, a single line of code in the firmware can compromise the mathematical foundation of the wallet's security. While the Mk3 was the primary device affected, any user who generated a seed on vulnerable firmware between 2021 and 2026 is at risk [Source: https://www.techtimes.com/articles/322392/20260731/coldcard-hardware-wallet-hacked-via-firmware-bug-that-bypassed-rng-five-years.htm].