Bitcoin Wallet Bug Exploited to Steal Funds via Accelerated Fees

On October 26, 2026, a critical flaw in the Electrum Bitcoin wallet allowed malicious servers to divert funds by exploiting the accelerated fee mechanism. According to CoinDesk, at least one user confirmed losing bitcoins, highlighting the persistent risks associated with the security of third-party infrastructures in an ecosystem where trust relies on decentralized protocols. Context Electrum, one of the oldest and most widely used Bitcoin software wallets, relies on a network of external servers to broadcast transactions and retrieve payment history. Unlike a full Bitcoin node, which verifies all blockchain data itself, Electrum delegates this task to third parties—an approach that optimizes speed but introduces vulnerabilities. The Bitcoin network, for its part, was experiencing a period of moderate congestion at the time of the incident, with over 43,000 transactions pending in the mempool and average fees of 0.135 satoshi per byte (sat/vB) for confirmation within three blocks. The flaw revealed on October 26 occurs within a context of growing tensions surrounding wallet security. European regulators, under the MiCA (Markets in Crypto-Assets) regulation, have recently tightened requirements regarding transparency and user protection. Meanwhile, the digital asset market showed relative stability, with Bitcoin trading around 61,000 dollars according to initial CoinDesk data, before CryptoScore Pro revised this figure to 80,053 dollars on the day of publication. The Facts The vulnerability, identified and patched on October 24, 2026, with the release of Electrum version 4.8.3, exploited a flaw in the management of unconfirmed transactions. Here are the key elements, attributed to their sources: Attack Method : Malicious servers intercepted a legitimate transaction before it was broadcast to the Bitcoin network. They replaced it with a fraudulent version, spending the funds to an address controlled by the attacker, with an invalid signature that the wallet did not verify. When the user attempted to accelerate the transaction via the Replace-by-Fee (RBF) mechanism, Electrum validated the fraudulent version, sending the funds to the attacker (source: CoinDesk, October 26, 2026). Victims and Amounts : Two reports were received by developers in October. The first, on October 11, concerned a multisignature wallet (2 of 3) using two Trezor hardware devices. Although the user verified the legitimate transaction on their devices, a different outgoing transaction appeared in the wallet history, with no loss of funds reported. The second case, reported on October 18, resulted in a theft, but neither the amount nor the identity of the malicious servers were disclosed (source: CoinDesk). Fix and Recommendations : Electrum version 4.8.3 added signature verification for unconfirmed transactions spending funds from the wallet. Developers advise users to update their software and systematically verify receiving addresses and amounts before signing a transaction. Those connected only to trusted servers, including their own, were not exposed (source: Electrum, October 24, 2026). According to CryptoScore Pro data as of October 26, 2026, at 05:19 UTC, the price of Bitcoin stood at 80,053 dollars, down 0.96% over 24 hours and 2.97% over seven days, with a market capitalization of 1.61 trillion dollars. The Crypto Fear & Greed index showed a neutral score of 50/100. No significant price variation was observed in the hours following the revelation of the flaw (source: CryptoScore Pro). Understanding Key Concepts Replace-by-Fee (RBF) Replace-by-Fee is a Bitcoin protocol feature that allows a user to replace an unconfirmed transaction with a new version offering higher fees. This option is useful when the network is congested and miners prioritize the most profitable transactions. For example, if a user sends 0.01 BTC with fees of 10 sat/vB but the transaction remains stuck, they can create a new transaction for the same amount with fees of 20 sat/vB. Miners, attracted by the higher reward, will include the latter in the next block. However, this flexibility introduces risks. In the case of Electrum, attackers exploited RBF to trick users into validating a fraudulent transaction by displaying a modified version as "stuck" and requiring acceleration. The wallet, not verifying the authenticity of the initial transaction, allowed the attacker to divert the funds. Mempool and Network Congestion The mempool (or "memory pool") is a temporary area where Bitcoin transactions wait to be confirmed by miners. Each network node maintains its own mempool, which can vary in size depending on congestion. When the mempool is saturated, as was the case on October 26, 2026, with 43,420 pending transactions, users must increase fees for their transaction to be processed quickly. For example, if the average fees for confirmation in three blocks are 0.135 sat/vB, a transaction with lower fees could remain pending for hours or even days. This dynamic creates pressure on users to adjust their fees, which was exploited in the attack against Electrum: attackers presented a fraudulent transaction as "stuck" to encourage acceleration via RBF. Multisignature (multisig) Wallet A multisignature (or "multisig") wallet requires multiple signatures to authorize a transaction, which enhances security by distributing control among several parties. For example, a 2-of-3 wallet requires two of the three private keys to sign a transaction for it to be valid. This configuration is commonly used by companies or groups to prevent a single person from being able to move funds. In the first case reported to Electrum, a 2-of-3 multisig wallet using two Trezor devices was compromised. Although the user verified the legitimate transaction on both devices, the substitution performed by the malicious server bypassed this protection, illustrating the limits of multisig solutions when the underlying software has vulnerabilities. Analysis: what is at stake The…