Electrum has released a security fix for a Bitcoin wallet flaw that attackers were using to turn a routine transaction fee increase into a payment to a different address.
The developers' October 24 advisory identifies versions before 4.8.3 as affected and says malicious servers have exploited the weakness. One user reported losing funds after approving a replacement transaction. The disclosure does not establish a total loss across users.
Version 4.8.3 adds a signature check before certain unconfirmed transactions enter the wallet's history. The release was tagged on October 23, with binaries and the public disclosure following on October 24.
Why increasing a fee became the final step
A Bitcoin wallet signs instructions specifying which coins to spend and where they should go. Increasing a fee through replace-by-fee involves authorizing a replacement, rather than merely changing a number on an already settled payment.
In the attack described by the developers, a hostile Electrum server withholds the user's original payment and substitutes a conflicting transaction in the wallet's displayed history. That substitute directs the funds to the attacker but initially lacks valid signatures.
The dangerous step occurs if the user manually increases its fee. Signing that replacement supplies the authorization the attacker previously lacked. The incident therefore concerns what the wallet asks a person to approve, rather than a server independently possessing the person's private keys.
Confirmed and pending payments have different checks
Electrum is a lightweight wallet. Its documentation says it uses servers for transaction history and broadcasting, while verifying confirmed transactions through simplified payment verification. One connected server acts as the main server and supplies information about unconfirmed transactions.
That distinction matters because a payment visible in a wallet's pending history has not necessarily been accepted into a mined block. A server's account of a pending payment and the network's eventual confirmed record are different things.
The documentation also describes fee replacement as a process that opens an unsigned transaction for the user to sign and broadcast. Reviewing the destination and amount at that stage is consequently part of authorizing the payment, even when the task began as an attempt to speed up an earlier transfer.
A targeted fix, with limits
The advisory says clients connected to a trusted server, such as one they operate themselves, are unaffected by this particular malicious-server attack. It recommends upgrading and checking the intended output addresses and amounts when signing.
The patch checks signatures on inputs belonging to the wallet. It does not turn the application into a full Bitcoin consensus engine or establish the validity of every incoming unconfirmed payment. Those limits should not be confused with a failure to close the reported outgoing-payment attack.
The release notes label 4.8.3 a security update and list additional wallet, interface and Lightning fixes. Users should distinguish the specific advisory from the broader release rather than assume every listed change describes another theft.
Our report on Bitcoin Core's revised vulnerability-disclosure policy concerns a separate project. Electrum's finding does not, by itself, establish a flaw in Bitcoin Core or in Bitcoin's consensus rules.