CoinDesk
At least one Bitcoin user lost funds after a malicious Electrum server substituted a fake pending transaction and later induced the user to approve a higher-fee replacement. The attack exploited Replace-by-Fee, which lets users speed up unconfirmed Bitcoin payments. Electrum released version 4.8.3 on Oct. 24 to address the flaw.
Why it matters
Electrum relies on outside servers to retrieve transaction histories and broadcast payments. Before the fix, the wallet did not verify signatures on certain unconfirmed transactions reported by those servers. A server controlled by an attacker could replace a legitimate payment with a fake transaction directing the coins to an attacker-controlled address.
The fake transaction could appear stuck, particularly when it carried no fee. When a user selected Replace-by-Fee, Electrum could sign a valid replacement of the fraudulent transaction, completing the theft. In one reported case, attackers also generated a receiving address whose last four characters matched the legitimate destination, making a quick review less likely to catch the substitution.
Market impact
The incident is a direct wallet-security risk rather than a change to Bitcoin's underlying protocol. Users on older Electrum versions should upgrade to 4.8.3 and verify payment addresses and amounts before signing transactions, especially when increasing a fee on an unconfirmed payment.
The reported attack required a connection to a malicious server. Electrum users connected only to trusted servers, including their own, were not exposed to this specific flaw. Developers received two reports in October, and the amount stolen and identities of the malicious servers have not been disclosed.
Frequently asked questions
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How did the Electrum vulnerability enable theft?
A malicious server could replace a legitimate pending Bitcoin transaction with a fake one directing funds to an attacker. If the user then raised the fee, Electrum could sign a valid replacement of that fraudulent transaction.
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What Electrum version fixes the vulnerability?
Electrum version 4.8.3 adds signature checks for relevant unconfirmed transactions and was released on Oct. 24.
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Were all Electrum users exposed to the attack?
No. The reported attack required a connection to a malicious server. Users connected only to trusted servers, including their own, were not exposed to this specific flaw.
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Why did Replace-by-Fee make the attack possible?
Replace-by-Fee lets users speed up unconfirmed Bitcoin payments by signing a higher-fee replacement. In the vulnerable versions, that action could turn the server's fake pending transaction into a valid payment to the attacker.
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What should Electrum users do after the disclosure?
Users should upgrade to version 4.8.3, use trusted servers, and carefully verify the full receiving address and payment amount before signing or increasing a transaction fee.
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