The developers released version 4.8.3 on 24 October and urged users of earlier versions to upgrade. Their security advisory classified the vulnerability as high severity and said some servers had exploited it against customers. Two users reported suspicious activity during October, including one who lost funds after approving a higher fee.
The flaw affected the way Electrum, a lightweight bitcoin wallet, handled payments awaiting confirmation on the Bitcoin network. Attackers operating a server connected to the wallet could suppress a legitimate outgoing transaction and substitute a fraudulent entry directing the same coins to an address under their control.
Although the substituted transaction carried invalid digital signatures, vulnerable wallet versions displayed it as a pending payment. The software did not check those signatures before accepting information about certain unconfirmed transactions from its main server.
The theft occurred when a user tried to accelerate the apparent payment through Replace-by-Fee, or RBF, a Bitcoin feature allowing an unconfirmed transaction to be replaced by another offering a higher processing fee. Electrum could then sign a valid version of the substituted transaction, authorising payment to the attacker rather than the intended recipient.
Developers said the attack required the user to approve the replacement. Merely displaying the forged transaction did not transfer funds. The vulnerability therefore combined a weakness in server-provided transaction information with a subsequent signing action by the wallet owner.
The first complaint arrived on 11 October, when a user noticed that a transaction appearing in the wallet’s history differed from the payment originally submitted. The user said the genuine transaction had not been propagated across Bitcoin’s peer-to-peer network, indicating that the server had withheld it. No loss was reported in that case.
A second complaint, submitted on 18 October, involved a user who increased the fee on a substituted transaction and lost bitcoin. Electrum has not disclosed the amount stolen, the identity of the servers involved or the total number of people potentially affected.
Both complainants described a further deceptive detail: the attackers generated destination addresses whose final four characters matched those of the legitimate receiving addresses. That similarity could make the alteration difficult to notice if someone checked only the shortened address displayed by their software.
The fraudulent transaction described in the second complaint also carried a zero fee. Developers suggested this might have been intended to encourage the victim to increase the fee, thereby completing the attacker’s attempt to obtain a valid signature.
Electrum’s patch changes how the wallet processes pending transactions that spend coins belonging to its user. Before recording those transactions in local history, the software now verifies the signatures associated with the wallet’s own inputs. This prevents the specific invalid substitutions described in the advisory from being accepted as genuine pending payments.
The update does not mean the wallet independently verifies every piece of information supplied by outside servers. Electrum continues to use its lightweight architecture, which avoids downloading and validating the entire Bitcoin blockchain locally. Its security checks for confirmed transactions operate differently from its handling of unconfirmed payments.
Developers advised customers to install the corrected version and examine destination addresses and payment amounts before signing any transaction, including one intended only to increase a fee. Users who connect exclusively to servers they trust, such as servers they operate themselves, were not exposed to this particular attack.
The attack did not require the criminal to obtain the user’s private keys directly. Instead, it exploited the wallet’s transaction approval process, turning a request to adjust network fees into authorisation for a different payment. This distinction matters because the user’s initial payment could be correctly signed while the wallet subsequently displayed a different transaction supplied by the server.
Electrum’s advisory describes the main connected server as a trusted source of information about transactions awaiting confirmation. That design assumption was central to the exploit: the server could present an invalid replacement before the Bitcoin network had confirmed the genuine payment. The patch addresses that verification gap.
The vulnerability was documented in a public GitHub security advisory dated 24 October. The project identified the corrective code change as commit 90fe308 and listed all Electrum versions before 4.8.3 as affected. The advisory did not identify the operators of the malicious servers.
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