Electrum, one of Bitcoin's oldest software wallets, relies on outside servers to fetch transaction histories and broadcast payments. Its developers disclosed a vulnerability on Saturday that some of those servers were already exploiting.

Before the fix, the wallet did not check signatures on pending transactions reported by servers. A malicious server could swap a user's payment for a fake one sending coins to an attacker, and when the user tried to speed it up with Replace-by-Fee, Electrum signed a valid replacement paying the attacker. Spoofed addresses matched the last four characters of the real destination.

Developers received two reports in October, and at least one user confirmed losing funds after raising the fee on a substituted transaction.

Version 4.8.3, released on Oct. 24, adds signature checks for unconfirmed transactions spending the user's own coins. Earlier versions remain vulnerable when connected to malicious servers, while users running only trusted servers, including their own, were not exposed.