Electrum fixes an exploited malicious-server trick that turns a fee bump into an attacker payment

The malicious server withholds a newly broadcast payment and inserts a conflicting transaction with attacker-controlled outputs into the wallet’s pending history. Its old signatures are invalid for those changed outputs. If the user then signs a replacement to increase the fee, the replacement authorizes the attacker payment.

This needs a malicious main server and a further user signature. It is not automatic seed theft. The original payment may never have reached Bitcoin peers. Even a transaction previously checked on a hardware wallet must be checked again when signing a fee bump.

Version 4.8.3 rejects invalid signatures on inputs belonging to the wallet before accepting mempool history. It does not turn this light client into a full consensus validator: incoming unconfirmed payments remain outside that added check.

The project received two reports and says one reporter lost money. It gives no total loss or wider victim count. Active exploitation in a widely used wallet warrants L4; no broader loss estimate is inferred.

Update, October 28: The maintainer advisory now identifies this same vulnerability as CVE-2026-117601, also tracked as GHSA-6jp4-8v48-cc5v. The October 24 disclosure date and Electrum 4.8.3 fix are unchanged; the advisory does not specify when the CVE was added.