Electrum before 4.8.3: malicious server can swap a broadcast tx and trick users into RBF-signing a payment to the attacker

Summary

Electrum treats the server it connects to as trusted for unconfirmed transactions. A malicious server can drop a transaction the user just broadcast and insert a conflicting transaction with invalid signatures that sends the outputs to an attacker-controlled address. Older clients do not check those signatures, show the substitute as unconfirmed, and hide the original. If the user then fee-bumps it with RBF, the wallet signs a valid payment to the attacker.

Affected

• Electrum (Bitcoin wallet, desktop and Android) versions before 4.8.3
• Fixed in 4.8.3 (released 23-24 October 2026): the client now verifies signatures on wallet-owned inputs of mempool transactions before adding them to local history

Impact

Users connected to a malicious Electrum server can lose funds when they fee-bump a substituted transaction. Per the advisory, some servers were exploiting this in the wild; the developers received two user reports, one involving lost funds, and the attacker's output address was ground to match the last four characters of the legitimate destination. The advisory rates it High; Electrum's release announcement calls it a critical security issue. 0xCERT rates it Critical because in-the-wild exploitation is confirmed.

Recommended actions

• Upgrade to Electrum 4.8.3, downloaded only from electrum.org, and verify the GPG signatures
• Until upgraded, check every output address in full and every amount before signing any transaction, including RBF fee bumps; do not rely on the first or last characters of an address
• Wallets and libraries built on the Electrum protocol: validate signatures of wallet-owned inputs in unconfirmed transactions received from servers

Source & attribution

This 0xCERT advisory summarises the Electrum security advisory GHSA-6jp4-8v48-cc5v published by SomberNight (https://github.com/spesmilo/electrum/security/advisories/GHSA-6jp4-8v48-cc5v), credited to dannydeezy and Raju-K12, and the Electrum 4.8.3 release notes. Details may change; refer to the original source for the authoritative record.