Electrum wallet security flaw is fixed after a user loses money in fee attack

What it is

Electrum 4.8.3, released 24 October 2026, fixes a flaw in which a malicious Electrum server can get a user to sign a payment to an attacker. Electrum's advisory says some servers are already running the attack. All versions before 4.8.3 are affected. Electrum calls it a critical security issue and urges all users to upgrade.

How it works

Electrum relies on servers for transaction history and for broadcasting payments, and its documentation treats the main server as trusted for unconfirmed transactions. The attack exploits that trust.

1. The server hides the user's original unconfirmed payment.
2. It shows a conflicting transaction in its place, paying an attacker-controlled address. This transaction has invalid input signatures.
3. Older clients display it as unconfirmed without checking those signatures.
4. If the user raises the fee with replace-by-fee (RBF), the wallet signs the altered payment and the funds go to the attacker.

In both user reports, the attacker's address matched the last four characters of the address the user meant to pay. A check that looks only at the end of the address would not catch this.

Preconditions / Affected

• Electrum earlier than 4.8.3.
• A connection to a malicious server. The advisory says clients connected to trusted servers, including the user's own, are not affected.
• The user manually increases the fee on an unconfirmed transaction.

Developers received two reports. One user lost money after raising a transaction's fee.

BULL wallet is not affected unless the user manually added a compromised server. By default it connects to two Bull Bitcoin servers and one Blockstream server, according to Bull Bitcoin CEO Francis Pouliot.

What to do

• Upgrade to Electrum 4.8.3. It checks signatures on inputs that spend the wallet's own coins before accepting an unconfirmed transaction into local history.
• Download only from electrum.org and verify the GPG signatures, as the download page instructs.
• If you cannot upgrade yet, check the destination address and amount manually before signing anything, including a fee increase. Compare the full address, not just the end.
• Connecting to your own server, or another server you trust, avoids this attack.
• BULL wallet users should avoid adding custom servers. Pouliot said Bull Bitcoin is adding the same client-side validation as Electrum, with no release date given.

Caveats

The page does not name the malicious servers, say how many exist, or give the total amount lost. It also gives no CVE identifier.