Malicious Electrum Servers Steal Bitcoin by Baiting Users Into Fee Bumps

Introduction

A critical vulnerability affecting Electrum wallets prior to version 4.8.3 allows a malicious public server to trick users into signing a forged transaction when they attempt to bump a low‑fee payment. The flaw was disclosed on Oct. 24, 2026, after at least one victim lost Bitcoin. Upgrading to the patched release or manually checking transaction outputs mitigates the risk.

How the attack works

Electrum relies on its server for the state of unconfirmed (mempool) transactions. When a user requests a fee bump, the wallet asks the server for a replacement transaction that spends the same inputs but includes a higher fee. A compromised server can:

1. Hide the genuine transaction from the network.
2. Return a counterfeit transaction that redirects the output to an attacker‑controlled address.
3. Omit a fee on the fake transaction, hoping the user will increase it.

Because Electrum did not verify the signatures of mempool transactions that spent the wallet’s own coins, the forged transaction appeared legitimate. When the user accepted the fee bump, the wallet signed a valid replacement that paid the attacker.

The fix in Electrum 4.8.3

Version 4.8.3 introduces a signature check for any mempool transaction that spends the wallet’s funds before it is added to the local history. This prevents a server from injecting an unsigned or improperly signed transaction. The update also clarifies that incoming transactions are still not validated, so users should remain vigilant.

Mitigation steps for users who cannot upgrade

If you are unable to move to version 4.8.3 immediately, follow these best practices:

• Inspect every output on the confirmation screen, even when you are only adjusting the fee.
• Connect only to trusted servers (e.g., run your own Electrum server or use a reputable private node).
• Avoid fee‑bump operations on unconfirmed transactions unless you are certain of the server’s integrity.
• Monitor transaction IDs after broadcasting; a mismatch between the displayed ID and the one shown on a block explorer can indicate tampering.

Conclusion

The Electrum fee‑bump exploit underscores the trade‑off of delegating transaction state to a remote server. Upgrading to version 4.8.3 eliminates the signature‑verification gap, while diligent output verification and trusted server usage protect users in the interim. Stay current with wallet updates and always double‑check where your Bitcoin is going.