Electrum Flaw Let Rogue Servers Hijack Fee Bumps
Electrum 4.8.3 closes a hole where a malicious server could swap a pending payment and wait for the user to raise the fee.
Electrum versions before 4.8.3 let a malicious server replace a user's just-broadcast payment with a fake, invalidly signed transaction, then get the wallet to sign it at a higher fee when the user used Replace-by-Fee. Electrum released the fix on 24 October 2026; users of their own servers were not affected.
The Ledger Desk · 3 min read- Electrum's advisory rates the flaw High and lists every version before 4.8.3 as affected; 4.8.3 is the patched release.
- A malicious server could swallow a just-broadcast payment and show the wallet a conflicting transaction with invalid signatures that pays the attacker.
- The loss happened only when the user manually fee-bumped the substituted transaction; the wallet then signed it with valid signatures.
- Users connected to servers they run or trust were not exposed, according to the advisory.
- Before signing any transaction, including a fee bump, check the output addresses and amounts, because the attacker matched the last four characters of the real address.
Electrum, one of Bitcoin's oldest software wallets, shipped version 4.8.3 on 24 October 2026 to close a flaw that let a malicious server trick users into signing their own coins away when they tried to speed up a payment. The project's advisory rates it High and says some servers were exploiting it in the wild. CoinDesk reported on 26 October that at least one theft has occurred. Neither source gives a loss figure.
How the substitution worked
According to the advisory, Electrum trusts the server it connects to for unconfirmed transactions. When a user broadcast a payment, a malicious server could swallow it and update the wallet's history with a conflicting transaction paying the attacker. That substitute carried invalid signatures, which the light client did not check. The wallet displayed the substitute as unconfirmed and no longer showed the original payment, so the user saw a payment that appeared stuck.
Why the fee bump completed the theft
The advisory says the theft needed one more step from the victim: manually raising the fee. Electrum then signed a replacement for the fake transaction with valid signatures at a higher fee, and the replacement paid the attacker. In the second user's report, the substitute was built with zero fee, which made it look stuck and invited a fee bump. The attacker also generated an address whose last four characters matched the legitimate destination.
What the fix covers and what it leaves open
Per the advisory, Electrum 4.8.3 now verifies the signatures of inputs that belong to the user's own wallet before adding unconfirmed transactions to local history. The project says incoming transactions are still not validated, because a full check would mean recursively validating unconfirmed parents until reaching mined transactions. The control gap was therefore narrow and specific: the wallet accepted a server's claim about spending the user's own coins without checking the signature.
Timeline and exposure
The advisory dates the first private report to 11 October, when no fee-bump loss occurred, and a second report on 18 October from a user who lost money after bumping the fee. The fix was tagged on 23 October and released with public disclosure on 24 October. CoinDesk counts six days from the first reported theft to the fix. Developers have not named the malicious servers, and users of servers they trust or run were not exposed.
What wallet users and integrators should do
Anyone on Electrum below 4.8.3 should upgrade. The advisory's stated workaround is to verify output addresses and amounts when signing any transaction, bumping the fee or not, and to compare full addresses rather than their endings. A stuck payment that the wallet shows differently from what the user sent is a warning sign, not a reason to bump. Firms that build on light-client wallets should list which server responses the wallet signs against. Still unknown: the total stolen and which servers were involved.
- Which wallet was affected and which versions?
- Electrum, according to CoinDesk and the project's security advisory. The advisory lists versions below 4.8.3 as affected and 4.8.3 as the patched release.
- How did the attack work?
- A malicious server swallowed the user's broadcast transaction and put a conflicting, invalidly signed one in the wallet's history, paying the attacker. If the user then raised the fee, Electrum signed a valid replacement that sent the coins to the attacker.
- How much was stolen?
- CoinDesk reports that at least one theft occurred and that a second user reported losing money on 18 October. Neither the article nor the advisory gives an amount, and developers have not identified the malicious servers.
- What should Electrum users do?
- Upgrade to 4.8.3, and verify output addresses and amounts before signing any transaction, fee bump or not. The advisory says users connected only to servers they trust, such as their own, are not affected.
- Bitcoin wallet bug let rogue servers steal funds when users tried to speed up payments — CoinDesk
- Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing it (GHSA-6jp4-8v48-cc5v) — Electrum project (spesmilo/electrum security advisories)