A malicious server could change the recipient shown in a wallet. Raising the pending transaction’s fee then produced a fresh signature and paid the attacker. Versions before 4.8.3 are affected.
Bitcoin wallet Electrum has fixed a flaw that lets an attacker steal funds by tricking a user into signing a forged transaction. Its October 24 security advisory says some servers are actively carrying out the attack. The high-severity issue affects versions before 4.8.3; one of the two users who contacted developers reported losing money.
In a separate statement on October 25, Electrum explained how its downloadable software can be verified: five people independently build and sign the program files hosted on its website. Users are advised to obtain updates from the official site and check the GPG signatures that attest to the binaries matching the published source code.
Raising the fee can authorise a different recipient
Electrum is a lightweight wallet that relies on servers instead of downloading and validating the entire Bitcoin blockchain as a full node would. Its official FAQ says it connects to roughly 10 servers for block headers and fee estimates. One main server supplies the transaction history for the user’s addresses and broadcasts new payments. For transactions that have not yet entered a block, the wallet trusts the information provided by that server.
A malicious main server can withhold the genuine transaction instead of passing it to the Bitcoin network. It then sends the wallet a conflicting transaction that spends the same funds but substitutes the attacker’s recipient address. The replacement has invalid signatures, which older versions do not check. The user sees the forgery awaiting confirmation in the wallet history, while the original payment is hidden.
The loss occurs when the user manually raises the pending payment’s fee. Replace-by-fee, or RBF, allows an unconfirmed transaction to be replaced with a higher-fee transaction. When the user raises the forged transaction’s fee, signs the replacement and broadcasts it, the payment goes to the attacker. Merely opening the wallet or connecting to a malicious server is not enough to send the money in the reported attack: the user must sign again.
Checking only the last four address characters is insufficient. Both reports described attacker addresses ending in the same four characters as the intended recipient. For users who have not updated, developers advise checking the complete output addresses and amounts whenever they sign a transaction, including when raising its fee.
Two reports, with no loss amount disclosed
| Date | Development |
|---|---|
| October 11, 2026 | dannydeezy privately reported seeing a different transaction after sending a payment. The fee-bump theft mechanism had not yet been identified. |
| October 18, 2026 | Raju-K12 privately reported losing money after increasing a transaction’s fee. |
| October 23, 2026 | Version 4.8.3 was tagged in the code repository with the fix. |
| October 24, 2026 | Installable program files were released and the vulnerability was publicly disclosed. |
The first incident report describes a genuine payment checked and signed on two Trezor hardware wallets. The wallet required signatures from two of its three keys. The payment had 31 inputs, meaning previously created transaction outputs being spent, and a single destination output. In the replacement later shown by the server, almost all the money went to a lookalike attacker address and a small portion to a previously used address belonging to the wallet. The record first appeared as unconfirmed, then local; the genuine payment never reached the Bitcoin network.
According to the report, all 62 signatures across the 31 inputs were valid for the original transaction and invalid for the forgery. The user had checked the destination on both devices for the original payment; the server later displayed a different payment in the history. The advisory does not report a compromise of the Trezor devices’ private keys.
The forged transaction in the second report had a zero fee. Developers suggest the attacker may have intended to encourage a fee increase, but that motive has not been established. The disclosure gives no total victim count, amount of Bitcoin stolen or information about reimbursement.
What version 4.8.3 checks
The patch documented in the 4.8.3 release notes checks signatures on inputs that spend from addresses owned by the wallet. It does so before adding an unconfirmed transaction to local history. The advisory also says users connected to a server they trust, such as one they operate themselves, are not affected by the reported attack.
Electrum still does not validate every transaction as a full node would. Incoming payments that have not been confirmed, for example, do not receive the same input-signature checks. Developers explain that the client lacks a full Bitcoin consensus engine and an interpreter for transaction scripts. Fully checking inputs belonging to others would also require following earlier unconfirmed transactions back until reaching a mined transaction.
Payments already included in a block are checked using simplified payment verification, or SPV. It uses proof of inclusion in a block, rather than independently enforcing every network rule as a full node does.
“Users should upgrade”
Verifying the update’s signature
As of October 25, the official Electrum download page lists 4.8.3 as its latest release. A valid GPG signature confirms that the file was signed with the corresponding signing key. Users also need to compare the key’s identifying fingerprint with independent sources. Checking only a file hash shown on the same download page offers insufficient protection: a compromised website could replace both the file and its hash.
Electrum’s October 25 statement on independent builds and GPG signatures
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The official FAQ recommends backing up the wallet’s recovery words on paper before updating. The program installation is separate from wallet files, although an older release may not read a file format changed by a newer version.
