Electrum patches high-severity bug after attackers steal Bitcoin by hijacking pending transactions
Electrum has rushed out version 4.8.3 to close a high-severity vulnerability that attackers abused to substitute outgoing payments and steal funds. The wallet’s developers confirmed at least two privately reported incidents — including one case in which a user lost funds after trying to raise a transaction fee — and warned users to upgrade immediately.
What happened
- Electrum is a lightweight Bitcoin wallet that relies on external Electrum servers for transaction history, broadcasting and other network data. Unlike a full node, it does not independently validate all aspects of unconfirmed transaction data.
- The bug stemmed from Electrum’s historical trust in a single “main” server when displaying unconfirmed (pending) outgoing transactions. Malicious servers intercepted users’ submitted transactions, blocked the legitimate broadcast, and returned a substituted transaction that redirected funds to an attacker-controlled address.
- Although the replacement transactions carried invalid signatures, affected Electrum versions displayed them in the wallet history without verifying those signatures. The deception became critical when users used Replace-by-Fee (RBF) to increase a transaction fee: by approving the fee bump, users could unknowingly produce a newly signed, valid transaction that sent funds to the attacker.
Timeline and confirmed incidents
- On October 11 a user (dannydeezy) reported a mismatch between the transaction shown in Electrum’s history and the payment they had submitted.
- On October 18 a second user (Raju-K12) reported losing funds while attempting to increase a transaction fee. Developers say both reports involved attacker-controlled receiving addresses deliberately designed to match the final four characters of the intended recipient’s address. In the second case the fake transaction had a zero fee, likely to entice the victim to initiate an RBF fee increase.
- The first report involved a two-of-three 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 fake redirected nearly all funds to a lookalike address and returned a small amount to the wallet. The original transaction itself remained valid but was never broadcast to the Bitcoin network.
The fix
- Electrum released v4.8.3 (binaries posted Oct. 24) which, via commit 90fe308, adds signature verification for unconfirmed transactions that spend inputs owned by the user’s wallet. Electrum now checks those signatures before accepting such transactions into local history, preventing the specific attack that made fraudulent outgoing payments appear legitimate.
- The update does not convert Electrum into a full node. Incoming unconfirmed transactions are still not covered by the new checks, and confirmed transactions continue under existing verification rules.
- Electrum’s advisory also notes that Windows Defender may flag ZXing.dll as suspicious — a false positive to be ignored — and that a full description of the vulnerability will be published later.
What users should do
- Upgrade to Electrum 4.8.3 immediately from the official Electrum website.
- If you cannot upgrade right away, always verify the entire receiving address and amounts before signing any transaction — including when approving RBF fee increases.
- Connecting exclusively to trusted Electrum servers (including your own) removes exposure to this particular malicious-server attack.
Broader context
- Electrum’s reliance on external servers has been targeted before. In December 2018 malicious servers displayed fake update instructions and other scams, with losses estimated near $1 million. A 2019 campaign used fake wallet updates and denial-of-service to push users toward compromised software.
- Those earlier attacks primarily used deceptive downloads. This vulnerability is notable because it manipulated pending transaction information inside an otherwise legitimate wallet interface, exploiting how lightweight clients trade off full local verification for convenience and reduced resource use.
- Electrum’s documentation explains the client typically connects to ~10 servers but selects one main server for wallet history and broadcasting — a design choice that made trusting unconfirmed transaction data from that main server a critical weak point.
Electrum developers say some servers were performing this attack in the wild but have not disclosed how many servers were involved, the total amount stolen, or whether additional victims have been identified. For now, upgrading and double-checking addresses before signing remain the best defenses.
Electrum has rushed out version 4.8.3 to close a high-severity vulnerability that attackers abused to substitute outgoing payments and steal funds. The wallet’s developers confirmed at least two privately reported incidents — including one case in which a user lost funds after trying to raise a transaction fee — and warned users to upgrade immediately.
What happened
- Electrum is a lightweight Bitcoin wallet that relies on external Electrum servers for transaction history, broadcasting and other network data. Unlike a full node, it does not independently validate all aspects of unconfirmed transaction data.
- The bug stemmed from Electrum’s historical trust in a single “main” server when displaying unconfirmed (pending) outgoing transactions. Malicious servers intercepted users’ submitted transactions, blocked the legitimate broadcast, and returned a substituted transaction that redirected funds to an attacker-controlled address.
- Although the replacement transactions carried invalid signatures, affected Electrum versions displayed them in the wallet history without verifying those signatures. The deception became critical when users used Replace-by-Fee (RBF) to increase a transaction fee: by approving the fee bump, users could unknowingly produce a newly signed, valid transaction that sent funds to the attacker.
Timeline and confirmed incidents
- On October 11 a user (dannydeezy) reported a mismatch between the transaction shown in Electrum’s history and the payment they had submitted.
- On October 18 a second user (Raju-K12) reported losing funds while attempting to increase a transaction fee. Developers say both reports involved attacker-controlled receiving addresses deliberately designed to match the final four characters of the intended recipient’s address. In the second case the fake transaction had a zero fee, likely to entice the victim to initiate an RBF fee increase.
- The first report involved a two-of-three 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 fake redirected nearly all funds to a lookalike address and returned a small amount to the wallet. The original transaction itself remained valid but was never broadcast to the Bitcoin network.
The fix
- Electrum released v4.8.3 (binaries posted Oct. 24) which, via commit 90fe308, adds signature verification for unconfirmed transactions that spend inputs owned by the user’s wallet. Electrum now checks those signatures before accepting such transactions into local history, preventing the specific attack that made fraudulent outgoing payments appear legitimate.
- The update does not convert Electrum into a full node. Incoming unconfirmed transactions are still not covered by the new checks, and confirmed transactions continue under existing verification rules.
- Electrum’s advisory also notes that Windows Defender may flag ZXing.dll as suspicious — a false positive to be ignored — and that a full description of the vulnerability will be published later.
What users should do
- Upgrade to Electrum 4.8.3 immediately from the official Electrum website.
- If you cannot upgrade right away, always verify the entire receiving address and amounts before signing any transaction — including when approving RBF fee increases.
- Connecting exclusively to trusted Electrum servers (including your own) removes exposure to this particular malicious-server attack.
Broader context
- Electrum’s reliance on external servers has been targeted before. In December 2018 malicious servers displayed fake update instructions and other scams, with losses estimated near $1 million. A 2019 campaign used fake wallet updates and denial-of-service to push users toward compromised software.
- Those earlier attacks primarily used deceptive downloads. This vulnerability is notable because it manipulated pending transaction information inside an otherwise legitimate wallet interface, exploiting how lightweight clients trade off full local verification for convenience and reduced resource use.
- Electrum’s documentation explains the client typically connects to ~10 servers but selects one main server for wallet history and broadcasting — a design choice that made trusting unconfirmed transaction data from that main server a critical weak point.
Electrum developers say some servers were performing this attack in the wild but have not disclosed how many servers were involved, the total amount stolen, or whether additional victims have been identified. For now, upgrading and double-checking addresses before signing remain the best defenses.