Electrum’s fee bump became a theft tool

One of Bitcoin’s oldest software wallets has disclosed a bug that turned a routine attempt to speed up a payment into a way for attackers to steal it.

Electrum said on Oct. 24 that version 4.8.3 fixes a vulnerability affecting how the wallet handled unconfirmed transactions. CoinDesk reported that developers had received two reports in October, including one case in which a user raised the fee on what appeared to be a stuck payment and lost money.

The useful detail is not that a wallet had “a security issue.” Plenty do. It is where the trust sat. Electrum, unlike a full Bitcoin node, depends on outside servers to fetch transaction history and broadcast payments. Before the fix, it accepted those servers’ reports about pending transactions without verifying signatures in this specific case.

That left room for a malicious server to swap a legitimate outgoing payment in the wallet’s history with a fake one pointing to an attacker-controlled address. The substituted transaction contained invalid signatures, but Electrum did not catch that before showing it as pending.

The trap was set when the user tried to use Bitcoin’s replace-by-fee feature, which lets an unconfirmed payment be resent with a higher fee. Instead of merely paying miners more, the wallet could take the attacker’s substituted transaction and ask the user to sign a valid replacement. In plain English: the victim thought they were rescuing a delayed payment and instead authorized a new one to the thief.

Developers told users that one malicious transaction carried no fee at all, which made it look stuck and therefore likely to invite a fee bump. They also said attackers generated destination addresses whose final four characters matched the intended recipient’s address. That is not cryptographic wizardry so much as a bet that people inspect addresses the way most of us inspect plumbing invoices: briefly, and with too much faith.

One report, dated Oct. 11, involved a two-of-three multisignature wallet using two Trezor hardware devices. That user did not report losing funds. A second report, dated Oct. 18, came from a user who said they increased the fee on a substituted transaction and lost money. Electrum has not disclosed the amount stolen or identified the servers involved.

The limit of the claim matters. This was not a break of Bitcoin itself, and Electrum said users who connected only to servers they trusted, including their own, were not exposed to the reported attack. Earlier versions, however, remain vulnerable when used with malicious servers, which is an awkward reminder that in crypto, “self-custody” often still rents someone else’s plumbing.