Bitcoin holders reassess self-custody after critical Coldcard entropy flaw

A critical entropy bug in Coldcard hardware wallets has triggered a reassessment of trust assumptions among Bitcoin users managing their own private keys. The devices were designed to use STM32 "true random number generators" based on physical processes to create secure seed phrases, but a serious vulnerability was introduced after a firmware rewrite.

Beginning with firmware version 4.0.1 released in March 2021, Coldcard switched to MicroPython's Yasmarang PRNG instead of properly utilising the hardware RNG. Security analysts characterised this as an obviously insecure fallback mechanism. Coinkite, the device maker, has disputed characterisations of deliberate backdoor placement, though Bitcoin journalist Hodlnaut has speculated the flaw resulted from careless development practices and error suppression through random changes.

The entropy shortfall was severe across models: Mk2 and Mk3 devices generated seeds with only 40 bits of entropy, while Mk4, Mk5 and Q achieved around 70 bits—all far below the 128 bits required for secure 12-word seed phrases. Since the vulnerability became public, attackers have successfully brute-forced private keys, stealing over $100 million in Bitcoin. Wallets with added dice entropy, BIP-39 passphrases, or non-standard derivation paths proved more resilient. Developer James O'Beirne created a honeypot monitoring site called cktripwire to track which wallet types attackers are effectively targeting. The incident underscores the community principle: don't trust, verify.