Electrum wallet: malicious servers swap just-sent transactions and trick users into fee-bumping a payment to the attacker
Electrum Bitcoin wallet
Electrum before 4.8.3 trusted its server for unconfirmed transactions without checking their signatures. A malicious Electrum server can swallow a transaction the user just broadcast and show a conflicting fake (with invalid signatures) that pays an attacker address ground to match the last four characters of the real one. If the user then bumps the fee with Replace-By-Fee, the wallet signs a valid payment to the attacker. Electrum says some servers are doing this in the wild, and one user lost money. Upgrade to Electrum 4.8.3; until then, check every output address and amount before signing, including fee bumps. Users connecting only to a server they run or trust are not affected.
Why this severity: Electrum rates it sev-high; exploited in the wild by malicious servers, one confirmed loss
- Started
- Oct 24, 2026
Affected
- Electrum < 4.8.3
Fixed in
- Electrum 4.8.3
Timeline3
Electrum 4.8.3 binaries released and GHSA-6jp4-8v48-cc5v published; the client now verifies signatures of its own inputs in mempool transactions.
Second private report: a user loses funds after fee-bumping the substituted transaction.
First private report: a user sees a different transaction in their wallet history after broadcasting.
Advisories2
News & write-ups2
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