Electrum malicious-server RBF fee-bump theft via unverified mempool tx (CVE-2026-117601)
Severity: critical · Patch status: patched · Since 2026-10-28 · CVE-2026-117601
A malicious Electrum server can swallow a client's just-broadcast transaction (tx1) and replace it in the client's history with a conflicting, invalid transaction (tx2) whose outputs pay the attacker; signatures are not verified due to light-client design. If the user then RBF fee-bumps the displayed tx2, they sign and pay the attacker. Attackers grinded output addresses to mimic the last four chars of the legit address and used zero-fee tx2 to bait fee bumps; some servers performed this in the wild, with two user reports including a confirmed loss. Fix in commit 90fe308 (v4.8.3, tagged 2026-10-23, released 2026-10-24) verifies signatures of ismine inputs for mempool transactions; incoming txs remain unvalidated. Users connecting only to trusted/self-run servers are unaffected.
What to do
- Upgrade Electrum to 4.8.3 or later immediately
- Until upgraded, manually verify output addresses and amounts before signing any transaction, including fee bumps
- Connect only to trusted servers, ideally run your own Electrum server
- Check wallet history for unexpected replacement transactions after broadcasting
- Watch for addresses whose last characters mimic your intended payee address
Fixed versions
Sources
All live incidents on Threat Watch
