Electrum patches high-severity bug that let servers hijack unconfirmed payments
Security & Exploits ·
The Bitcoin wallet shipped version 4.8.3 to close a flaw that allowed malicious servers to swap in fraudulent transactions and divert user funds.
Electrum released the fix on October 24, addressing a vulnerability present in every prior version of the wallet, according to Decrypt. At least one user has confirmed a financial loss tied to the issue, and a second had earlier spotted suspicious activity without losing any coins.
The weakness centered on how Electrum handled transactions that hadn't yet confirmed on-chain. A dishonest server could intercept a user's outgoing payment, hide it, and substitute a bogus version showing a zero fee and an output address belonging to the attacker — making the legitimate payment appear stalled. Because the wallet did not check signatures on these pending transactions, the fake entry looked like a normal part of the user's history. If that person then tried to "speed up" the seemingly stuck payment by attaching a higher fee, they would unknowingly sign and send a brand-new transaction — the first one to actually reach the network — with the money going straight to the attacker instead of the intended recipient. Developer SomberNight, who wrote up the disclosure, noted that attackers could also tailor the fraudulent address so its final four characters matched the real destination, defeating casual visual checks.
The advisory says the technique has already been used by some servers operating in real conditions, citing two separate user reports. One person flagged odd server behavior on October 11 before any transaction actually went through, while another user lost funds after fee-bumping a manipulated transaction on October 18. The update was tagged in Electrum's code repository on October 23, with installable builds following a day later.
The new version closes the main avenue of attack by requiring valid signatures on any unconfirmed transaction touching the wallet's own inputs before it's recorded locally — something an outside attacker cannot fabricate. However, the patch does not fully resolve the trust problem: incoming payments from external senders, where none of the inputs come from the user's own wallet, still aren't signature-checked, a gap the developer acknowledged needs further work. CoinDesk likewise reported that the bug specifically struck during fee-bumping actions.
Electrum's decentralized server structure, which lets anyone operate a node, is what enables this category of exploit; the wallet previously suffered a multi-year phishing campaign tied to fake update prompts that was estimated to have cost users around $25 million by 2020. Users running their own dedicated server were not exposed to this particular bug. It remains unclear how many servers actively attempted the attack or how much total value was diverted before the patch, and whether Electrum will extend signature verification to externally sourced transactions is still an open question.