Electrum 4.8.3 Fixes Flaw Malicious Servers Exploit

Some servers used by the Electrum bitcoin wallet are actively exploiting a weakness that can trick users into paying an attacker, the wallet’s developers said on 24 October. They have released Electrum 4.8.3 to fix it and are urging users to upgrade.

The security advisory, rated high severity, covers all versions before 4.8.3. “Some servers are performing this attack in the wild,” it says. Electrum received two user reports, and one of those users “bumped the fee of tx2 and lost money”. The advisory does not say how much was lost, and no CVE number has been assigned. TSN logs cases like this on its crypto hacks and exploits hub.

In brief

Electrum’s developers say some servers are hiding newly sent transactions and showing users a fake that pays a lookalike address, hoping they will raise its fee.
Two users reported it, and one lost money after bumping the fee. The amount has not been disclosed, and no CVE has been assigned.
Version 4.8.3 fixes it. Anyone who has not upgraded should check every output address and amount before signing.

How the attack works

Electrum is a light wallet. Instead of downloading the whole blockchain, it relies on a server to tell it about unconfirmed transactions. The developers point out that this trust is a documented part of the design. The attack abuses it.

According to the advisory, when a user broadcasts a new transaction, a malicious server can “swallow” it rather than pass it on. It then updates the wallet’s history with a conflicting transaction that sends the funds to the attacker’s address. That fake has invalid signatures, “but this is not checked (due to light client design)”. The wallet displays the fake as unconfirmed, and the real transaction disappears from view.

The fake alone cannot move any money. The danger comes next. If the user fee-bumps the fake with replace-by-fee (RBF), which people often do when a payment seems stuck, the wallet “will sign it with a higher fee, and hence pay the attacker”.

The advisory suggests the attackers set this up deliberately. Both users said the attacker’s address had been “grinded to match the last four chars of the legit address”, so a quick glance at the end of the address would not catch it. In the second report, the server built the fake “with zero fee (presumably in hope of the user bumping its fee)”.

The first report, from a user identified as dannydeezy, came in privately on 11 October. It described a 2-of-3 multisig wallet signed with two Trezor hardware devices. That user checked the outputs on both devices and pressed broadcast. The history then showed the full amount going, unconfirmed, to an unknown address, and the real transaction never reached the mempool. On 18 October a second user, Raju-K12, reported losing money after bumping the fee. Version 4.8.3 was tagged on 23 October, and the binaries and the advisory followed on 24 October.

What the fix changes, and what it does not

According to the advisory and the fix commit, Electrum now checks the signatures on unconfirmed transactions that spend from the wallet’s own addresses before adding them to its history. A forged spend like this one should therefore no longer appear. The developers say the protection has limits. Incoming transactions are still not validated, because the client “lacks a full consensus engine”. Confirmed transactions were already checked by Electrum’s existing SPV verification.

The 4.8.3 release notes list this as the release’s high-severity security fix. They credit four people who reported security issues and include a long list of other changes.

What Electrum users should do

The advisory’s own guidance is short:

Upgrade to 4.8.3. Download it from electrum.org, where signature files are published alongside each download. The developers add that users should update regularly in any case.
If you cannot upgrade yet, check before you sign. When signing any transaction, “for bumping the fee or not”, check that the output addresses and amounts are what you expect. Check the whole address, not just its last few characters.
Consider a server you trust. “Clients that connect to servers they trust (e.g. run their own) are not affected,” the advisory says.

TSN’s beginner guide to bitcoin self-custody covers wallet set-up more generally.

The weak point is the fee bump, not the first payment

What stands out is where the money was lost. The first payment was signed correctly, and on the first user’s account it was even checked on hardware devices. The theft depends on a second signature, given to a transaction the user never created. Fee-bumping feels like routine housekeeping, so it gets less scrutiny than the original payment. That is the moment the attack targets. A hardware wallet helps only if the user reads the destination on its screen again at that point.

A documented trust assumption became a working exploit

Electrum’s developers had always said that servers are trusted with unconfirmed transactions. This case shows that a known design trade-off can sit quietly until someone turns it into a working attack. The fix narrows the gap for a wallet’s own spends but does not close it for incoming payments. That puts the advice to use a trusted server, or run one, back at the centre for anyone holding meaningful sums. It echoes recent wallet incidents on TSN, from the Fearless Wallet key leak to the NGRAVE fake-alert drain, where the defence was the user checking what they were asked to approve.

Disclosure moved within two weeks of the first report

The developers moved from the first private report to a public fix in under two weeks, and from the confirmed loss to release in six days. That is in line with the faster approach Bitcoin Core adopted this week for easily found bugs. What remains undisclosed is how many servers ran the attack and how much was taken. Until Electrum says more, the one number users can act on is the version they are running, and the safe answer is 4.8.3.

Electrum 4.8.3 Fixes Flaw Malicious Servers Are Exploiting, Devs Say

How the attack works

What the fix changes, and what it does not

What Electrum users should do

The weak point is the fee bump, not the first payment

A documented trust assumption became a working exploit

Disclosure moved within two weeks of the first report

Sources

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