India, Aug. 3 -- Hackers have found a software flaw in Coldcard, a popular hardware Bitcoin wallet that is designed to keep cryptocurrency offline in "cold storage," and are stealing millions of dollars in an ongoing attack.

Coinkite Inc., the Canada-based company behind Coldcard wallets, warned users late last week that a security flaw had compromised some wallets. By Monday, hackers had stolen about 1,367 Bitcoin worth nearly $86 million from more than 4,500 wallets, according to Galaxy Research.

Coldcard devices are hardware wallets that store Bitcoin in cold wallets, which are considered one of the safest ways to protect cryptocurrency because they remain disconnected from the internet, according to Bloomberg.

Despite being offline...