Bitcoin Wallet Vulnerability Turned Faster Payments Into a Potential Fund Theft Risk

A flaw in the Electrum Bitcoin wallet allowed attackers operating malicious servers to redirect payments to their own addresses when users attempted to speed up pending transactions. Developers confirmed at least one incident involving stolen bitcoin before releasing a security update.

Electrum, one of the longest-running software wallets in the Bitcoin ecosystem, revealed the vulnerability on Saturday and said some servers had already been exploiting it. The team received two reports in October, including one from a user who lost funds after approving what appeared to be a routine transaction-fee adjustment.

The vulnerability affected how Electrum processed Bitcoin transactions that had not yet been confirmed by the network.

Unlike a full Bitcoin node, Electrum relies on external servers to retrieve transaction histories and broadcast payments. Before the vulnerability was fixed, the wallet accepted information about unconfirmed transactions without verifying their signatures.

This allowed an attacker controlling a malicious server to intercept a payment before it reached the Bitcoin network and replace the transaction displayed in the wallet with a fraudulent version. The altered transaction directed the user’s bitcoin to an attacker-controlled address, while Electrum failed to detect the invalid signatures.

The wallet then showed the fraudulent payment as pending, creating an opportunity for attackers to exploit users who wanted to accelerate the transaction.

Bitcoin’s Replace-by-Fee (RBF) mechanism allows users to replace an unconfirmed transaction with another that offers a higher fee. In affected Electrum versions, choosing this option could lead the wallet to sign a replacement transaction that sent the funds to the attacker rather than the intended recipient.

In one reported incident, the fraudulent transaction carried no fee, making it appear stuck and encouraging the user to increase the fee. Attackers also generated destination addresses with the same final four characters as the legitimate recipient’s address, making the substitution more difficult to notice at a glance.

The first report, submitted on October 11, involved a two-of-three multisignature wallet secured with two Trezor hardware devices. The user verified the intended payment on both devices but later discovered a different outgoing transaction in Electrum’s history. No financial loss was reported in that case.

A second report followed on October 18, when a user said they had increased the fee on a substituted transaction and lost funds. Developers have not revealed the amount stolen or identified the malicious servers responsible.

Electrum Releases a Security Patch

Electrum addressed the problem with version 4.8.3, released on October 24. The update introduced signature verification for unconfirmed transactions spending coins from the user’s own wallet.

Older versions remain susceptible to the reported attack when connected to malicious servers.

Developers recommend upgrading to the patched release and carefully checking recipient addresses and transaction amounts before signing payments. Users who connected exclusively to trusted servers, including servers they operated themselves, were not exposed to this particular exploit.

The fix arrived six days after the first reported incident, highlighting the security risks that can emerge when cryptocurrency wallets rely on external servers without properly verifying transaction data.