Briefly describe
On October 24, the Bitcoin wallet Electrum released version 4.8.3 to patch a critical vulnerability. This vulnerability allowed malicious servers to inject forged transactions, tricking users into sending funds to addresses controlled by attackers.
At least one user confirmed losing funds on October 18 due to paying an additional fee for a transaction that had been altered by the server.
This fix will verify the signature of the wallet’s own inputs before displaying unconfirmed transactions; however, incoming transactions from external senders will still not be verified.
This weekend, the Bitcoin wallet Electrum patched a critical vulnerability. The flaw allowed malicious servers to manipulate pending Bitcoin transactions and redirect funds to addresses controlled by attackers. All versions prior to 4.8.3 were affected; the updated version is now available for download.
At least one user has confirmed losing funds due to this vulnerability.
Electrum is one of the earliest Bitcoin software wallets. This attack exploited the wallet’s trust model for unconfirmed transactions. When a user broadcasts a payment through a compromised server, the server can block the legitimate transaction and inject a forged conflicting transaction—displaying an attacker’s output address and zero fee—to make the payment appear stuck.
Since Electrum does not verify the signatures of unconfirmed transactions received from the server, this forged transaction will appear in the wallet history as a legitimate transaction.
When the user performs a fee bump on this seemingly stalled payment, they sign and broadcast a brand-new valid transaction whose output address belongs to the attacker. The original payment was never sent; the fee-bumped transaction is the first one to enter the network—and it pays the attacker.
The announcement by Electrum developer SomberNight states that attackers can modify the output address of a forged transaction to match the last four characters of the user’s intended address. Anyone who only checks the end of a long Bitcoin address will see what appears to be a match.
The announcement stated that "some servers are actively carrying out this attack," a claim supported by two user reports. User dannydeezy first flagged suspicious server behavior on October 11; no funds were lost because the altered transactions did not propagate beyond the local server. The second user, Raju-K12, reported a confirmed loss on October 18 after paying additional fees to rectify an altered transaction.
The patch in version 4.8.3 closes a major attack vector by verifying cryptographic signatures on the wallet’s own inputs before adding any unconfirmed transactions to the local history. The server can no longer inject forged transactions involving the user’s own address without providing valid signatures, which attackers cannot forge. This fix was marked in the code repository on October 23, and the binary was released the following day.
Inbound transactions from external senders—those with no inputs belonging to the user’s wallet—are still accepted without signature verification. SomberNight acknowledged this gap in the announcement and stated that additional work is needed to close it.
This is not the first time Electrum has faced such an incident. In 2018, a malicious attack used forged servers to push fake update notifications; by 2020, these attacks were estimated to have stolen $25 million from Electrum users. Electrum’s open-server model—where anyone can run a node—makes such attacks possible.
Users connected only to their own servers are unaffected. Any users unable to update immediately should manually verify the output address and amount before signing any transaction; Electrum 4.8.3 is available at electrum.org and on the project's GitHub.