GHSA-6jp4-8v48-cc5v: High severity pip/electrum vulnerability
Summary
The main Electrum server the client connects to is trusted with mempool ("unconfirmed") transactions. This is documented design. [0]
However now someone figured out a way to practically exploit it: - when the client broadcasts a new tx1, a malicious server can swallow it, - and update the client's history with a conflicting (invalid) tx2 that replaces the outputs with the attacker's output address. - Tx2 has invalid signatures, but this is not checked (due to light client design). - The wallet will display tx2 as an unconfirmed transaction - tx1 will not be displayed. - If the client then manually RBF fee-bumps tx2, the client will sign it with a higher fee, and hence pay the attacker.
Fixes
Commit 90fe30822 contains the fix [1], included in version 4.8.3. The client now verifies the signatures of wallet.ismine inputs for mempool txs (before adding them to its local history).
Users should upgrade (and regularly do so in any case). As a workaround, without upgrading, when signing any transaction, for bumping the fee or not, they should verify the output addresses and amounts are as expected.
Clients that connect to servers they trust (e.g. run their own) are not affected.
Remarks
- some servers are performing this attack in the wild - electrum received two user-reports of this. user1 noticed after broadcasting tx1 the wallet history contained a different tx. user2 bumped the fee of tx2 and lost money. - user1 reported tx1 was not propagated over the bitcoin p2p, so the server held it - both reported the attacker's output address in tx2 was grinded to match the last four chars of the legit address from tx1 - based on user2's report, the server constructed a tx2 with zero fee (presumably in hope of the user bumping its fee) - even with the fix, we still only validate "ismine" inputs (i.e. paying from addresses that the wallet owns). So e.g. "incoming" txs are not validated. The client lacks a full consensus engine / script interpreter, though even with that, to cover non-ismine inputs, it would have to validate the unconfirmed parents recursively until reaching mined txs. Mined txs we already SPV.
Timeline
- 2026-10-11: user1 sends private disclosure, but the bumpfee vector is not realised - 2026-10-18: user2 (Raju-K12) reports privately that they lost money after bumpfee - 2026-10-23: version 4.8.3 git tagged, with the fix - 2026-10-24: version 4.8.3 binaries released - 2026-10-24: public disclosure
Credits
Electrum thanks user1 and Raju-K12 for finding and responsibly disclosing this issue to us.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/electrumto a version that resolves this vulnerability.Fixed in 4.8.3 - Upgrade
Upgrade
Electrumto a version that resolves this vulnerability.Fixed in 4.8.3Patch 90fe30822 - Compensating control
When signing any transaction, including fee-bumped transactions, verify that the output addresses and amounts are as expected.
- Compensating control
Connect to trusted Electrum servers, such as running your own server.
Event History
Frequently Asked Questions
Which users are most exposed to this issue?
Users are exposed when their Electrum client connects to a malicious server, broadcasts a transaction, and later manually fee-bumps the transaction using RBF. The malicious server can hide the original transaction and show a conflicting unconfirmed transaction instead.
What must an attacker do to exploit this?
An attacker needs to operate or control the server the client is using. The server swallows the newly broadcast transaction and supplies a conflicting transaction with attacker-controlled outputs; exploitation completes only if the user manually RBF fee-bumps that displayed transaction.
How can the risk be reduced before upgrading?
When signing any transaction, including a fee bump, verify the destination output addresses and amounts. This workaround is necessary because the displayed unconfirmed transaction may contain invalid signatures that were not checked.
What version contains the fix?
The fix is included in Electrum version 4.8.3 and in commit 90fe30822. It verifies signatures of wallet-owned inputs in mempool transactions before adding them to local history.