Electrum patches flaw that let malicious servers redirect bitcoin during fee bumps

How the attack worked

Electrum is a lightweight Bitcoin wallet, which means it depends on external servers to fetch transaction history and broadcast payments rather than verifying everything through a full Bitcoin node. According to the developers, older versions trusted those servers to describe unconfirmed transactions without validating the signatures attached to them.

That created an opening for a rogue server to intercept a payment before it reached the Bitcoin network and swap it in the wallet history for a fake transaction. The substituted payment sent the user's coins to an address controlled by the attacker, but used invalid signatures that Electrum did not flag at the time.

The wallet would still show the fake transfer as pending. The critical step came if the user then tried to accelerate confirmation by increasing the fee. Electrum could take the fraudulent transaction and create a valid replacement, which would then send the bitcoin to the attacker's address.

Replace-by-Fee turned the fake transaction into a real one

The attack hinged on Bitcoin's Replace-by-Fee mechanism, which allows an unconfirmed transaction to be replaced by a new version carrying a higher fee. In normal use, that feature helps users push through a payment that appears stuck in the mempool.

In the reported exploit, attackers used that expectation against the victim. In one case described by the developers, the fake transaction carried no fee at all, making it look especially likely that a fee bump was needed. They also generated destination addresses whose final four characters matched the intended recipient, making a quick visual check less reliable.

Two October reports prompted the patch

The first report was submitted on Oct. 11 and involved a two-of-three multisignature wallet using two Trezor hardware devices. That user said the legitimate payment had been verified on both devices, but later a different outgoing transaction appeared in Electrum's history. The developers said this user did not report a loss.

A second user reported the issue on Oct. 18 after increasing the fee on a substituted transaction and losing bitcoin. Electrum has not disclosed how much was stolen and has not identified the malicious servers involved.

The project released Electrum 4.8.3 on Oct. 24, six days after the first reported theft, according to the disclosure.

Who was at risk and what changes now

The new release adds signature checks for unconfirmed transactions that spend coins from the user's own wallet. That change is intended to prevent the wallet from accepting the kind of forged pending transaction used in the reported attacks.

Developers said earlier versions remain vulnerable when they are connected to malicious servers. By contrast, users who connected only to servers they trust, including their own servers, were not exposed to this specific attack.

Electrum's immediate guidance is to upgrade to version 4.8.3 and to carefully verify receiving addresses and amounts before signing payments. The next confirmed step is the rollout of the patched version; the developers have not publicly named the attackers or said whether additional victims have been identified.