Electrum Patches Critical Security Flaw After Hackers Steal Bitcoin

CoinDesk reports that Electrum has released version 4.8.3 to fix a critical vulnerability in its Bitcoin wallet. Developers confirmed that malicious servers exploited the flaw to redirect user payments, and at least one theft of funds has occurred. Attackers could replace pending Bitcoin payments with fraudulent transactions. Users might mistake the replacement for a legitimate payment and authorize it when raising the transaction fee. Two users reported incidents. One lost Bitcoin after increasing the fee. The vulnerability affects all Electrum versions before 4.8.3. Developers have not disclosed total losses. The patch verifies transaction signatures for coins controlled by the wallet that remain unconfirmed. In an October 24 security advisory, Electrum said attackers abused its handling of unconfirmed Bitcoin transactions to swap legitimate outgoing payments for fraudulent ones. Users may unknowingly authorize the swap when increasing fees. Electrum is a lightweight Bitcoin wallet. It connects to external servers to retrieve transaction history, broadcast transactions, and obtain other network data. Unlike a full Bitcoin node, it does not independently process every aspect of blockchain activity. Documentation says Electrum uses Simple Payment Verification (SPV) for confirmed transactions, but historically trusted its main server when displaying unconfirmed transactions. Attackers exploited that gap. A malicious server intercepted submitted transactions and altered them before they reached the Bitcoin network. After blocking a legitimate broadcast, the attacker could send an alternative transaction to the wallet, changing the recipient address to one the attacker controls. The fake transaction carries invalid signatures. The vulnerable Electrum version did not check those signatures before showing the transaction in wallet history. The ruse turns dangerous when users try to raise fees with Replace-by-Fee (RBF), a Bitcoin feature that lets a higher-fee transaction replace an unconfirmed one. If the user approves the fee bump, they may sign a valid transaction that pays the attacker. Electrum has confirmed real-world attacks that caused Bitcoin losses. Developers disclosed two privately reported incidents, showing the flaw is no longer theoretical. On October 11, a user named dannydeezy reported that a transaction in Electrum history did not match the payment originally submitted. On October 18, another user, RajuK12, lost funds while trying to increase a trading fee and contacted the developer. Both incidents involved receiving addresses controlled by attackers. The addresses were designed to match the last four characters of the Bitcoin address of the intended recipient. In the second case, the fraudulent transaction had a zero fee, leading developers to suspect it was meant to lure the victim into initiating a fee increase. The first incident involved a 2-of-3 multisignature wallet using two Trezor devices. The legitimate transaction had 31 inputs and one output. Investigators found all 62 signatures were valid for the original payment but invalid for the substituted transaction. The forged transaction moved nearly all the bitcoins to a similar address and returned only a small portion to the wallet. The original transaction remains valid but never entered the public Bitcoin network. Electrum acknowledges that some servers are indeed carrying out the attack. Developers have not disclosed how many servers are involved, how much was stolen, or whether more victims have been identified. Version 4.8.3 blocks malicious fee-increasing transactions. The fix landed in version 4.8.3 on October 23, and downloadable binaries were made available on October 24. With commit 90fe308, a developer added signature verification for unconfirmed transactions whose inputs the wallet controls. Electrum now checks signatures before adding those transactions to local history, preventing the fraudulent outbound payments described in the advisory from appearing as legitimate. The update does not turn Electrum into a full Bitcoin node. Developers note that incoming unconfirmed transactions are still not covered by the new checks, and confirmed payments continue to use the existing verification process. Users on versions before 4.8.3 should download the update from the official Electrum website. Those who cannot upgrade immediately should verify the full recipient address and amount before signing, including fee-replacement transactions. Users who connect only to trusted servers, including their own, will not be affected by this specific malicious-server attack. Past attacks on Electrum show that server risks recur. The incident continues earlier attempts to exploit Electrum reliance on external servers. In December 2018, crypto.news reported that malicious servers showed forged software update notices to users. The report estimated losses near $1 million. Another round of malicious server attacks in 2019 used fake wallet updates and denial-of-service activity to steer users to compromised software. Those attacks relied on deceptive downloads. The newly patched vulnerability instead manipulates pending transaction information inside an otherwise legitimate wallet. A 2020 article featuring an interview about Electrum servers discussed the tradeoffs of lightweight wallets, including their reliance on external infrastructure for transaction history. Electrum documentation states that its client typically connects to about ten servers but selects one primary server to retrieve wallet history and broadcast transactions. The documentation also notes that this primary server is trusted for unconfirmed transactions.