Electrum fixes a flaw that could redirect a payment after a fee increase

A high-severity advisory published on October 24

Electrum published a high-severity security advisory on October 24, 2026, covering versions earlier than 4.8.3. Developer SomberNight describes an attack by malicious servers against the display of unconfirmed payments. The team says it received two reports, including one from a user who lost funds after increasing a transaction’s fee on October 18. The advisory does not specify the amount lost.

The wallet’s operation explains the exposure. Electrum uses servers to obtain address histories and broadcast payments. Its documentation distinguishes confirmed transactions, verified using SPV proofs, from pending transactions, for which the main server plays a trusted role. It also states that the client never sends its private keys to servers. Protecting these secrets, however, does not guarantee that the supplied history always displays the correct recipient.

The redirection requires a new signature

In the scenario described, the server holds back the legitimate payment sent by the client and displays a conflicting transaction instead, with invalid signatures and an attacker-controlled destination. If the user then manually increases the fee on this deceptive transaction, the wallet signs a new payment that could send the bitcoins to that wrong address. This is therefore not a server spending funds on its own using stolen private keys: the user’s new signature plays a decisive role.

The fix checks signatures on inputs belonging to the wallet before adding an unconfirmed transaction to its local history. The advisory highlights a limitation: this verification does not cover inputs outside the wallet, including in certain incoming transactions. The corrected version therefore does not turn Electrum into a full Bitcoin node. Clients using only a server they control or trust are not affected by this scenario, according to the team.

Update from the official distribution

The release notes date version 4.8.3 to October 23, while the advisory’s timeline places distribution of the executables on October 24. This difference reflects the software’s preparation and distribution stages. The official website provides downloads and the corresponding verification signatures. It instructs users to obtain the software exclusively from electrum.org and verify GPG signatures. It also states that the executables are reproducible and independently signed by several people involved in building them.

The documentation explains that a fee increase concerns a replaceable transaction and produces a new transaction to sign and then broadcast. This normal mechanism is used to speed up a pending payment; it requires checking again what will be signed. The fix here addresses manipulation of the history presented by a server. Checking the full destination and amount remains relevant before any signature, rather than treating the action as a purely technical change to the sending cost.