What happened
On 24 October 2026, Electrum disclosed a high-severity wallet vulnerability affecting every version before 4.8.3. A malicious Electrum server could replace a payment that had just been broadcast with an invalid transaction paying the attacker. If the user then chose to raise the fee, the wallet could sign that substituted transaction and make the attacker's payment valid.
Electrum says servers were performing the attack in the wild. Two users reported the behaviour privately, and one said funds were stolen. The project has not published the loss amount, the servers involved or a total victim count.
Version 4.8.3 adds a signature check before an unconfirmed outgoing transaction enters the wallet's local history. Electrum tagged the fix on 23 October, published binaries on 24 October and advised users to upgrade.
What it changes
Electrum is a light wallet. Instead of downloading and validating the whole Bitcoin blockchain, it asks a server for transaction history. That server should be treated as untrusted, but the affected wallet accepted a server-supplied transaction as one of its own without first checking the signatures on inputs belonging to the wallet.
The advisory describes the sequence. The server withheld the real payment, inserted a conflicting transaction with the attacker's output, and showed it as unconfirmed. The fake transaction was not valid because the wallet had not signed it. The dangerous step came later: selecting replace-by-fee, or RBF, used the displayed transaction as the template. After the user approved the fee bump, the wallet supplied valid signatures.
The fix verifies signatures for wallet-owned inputs in unconfirmed transactions before displaying them in local history. A server can still withhold data or lie about what it has seen, but it can no longer use this exact invalid transaction to obtain a valid signature through the fee-bump screen.
For users, the immediate action is to update to 4.8.3 or later from Electrum's official distribution. The project's workaround for anyone who cannot update immediately is to verify every output address and amount again before approving a transaction or fee bump.
What it does not change
This was not a private-key extraction. The server did not learn the seed or sign by itself. It relied on presenting a changed payment and persuading the user to authorize it through an ordinary wallet action.
RBF was not the flaw either. RBF lets a sender replace an unconfirmed transaction with one paying a higher fee. The vulnerability was Electrum's failure to authenticate the transaction shown before the replacement was signed.
Users connected only to a trusted or self-hosted Electrum server were not exposed to this technique, according to the advisory. That does not make every public server malicious, nor does it remove the privacy tradeoff of sharing wallet queries with one.
The new check is deliberately narrow. It validates signatures for inputs Electrum recognizes as its own. Incoming unconfirmed transactions are not fully validated, and Electrum does not become a Bitcoin full node. Users still need to verify payment details on the signing screen.
Context
The disclosure followed two private reports. Electrum received the first on 11 October. A second report on 18 October described a loss and noted that the attacker's address shared the same last four characters as the intended address, a detail that could defeat a cursory visual check.
Three independent newsrooms examined the disclosure. CoinDesk focused on the malicious-server path and the reported theft. crypto.news traced the fee-bump sequence and upgrade guidance. CriptoNoticias checked the affected versions and the limits of the patch.
The reports agree on the mechanism, the 4.8.3 fix and at least one reported loss. None independently establishes how many servers attacked users or how much bitcoin was stolen. Those facts remain undisclosed.