Electrum: Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing it

Summary The main Electrum server the client connects to is trusted with mempool ("unconfirmed") transactions. This is documented [design](https://electrum.readthedocs.io/en/latest/faq.html#does-electrum-trust-servers). [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…

curl https://cve.zerocrew.in/api/cve/CVE-2026-117601