Electrum Bitcoin wallet fixes the theft vulnerability with a fee increase

On October 24, the Bitcoin lightweight wallet Electrum disclosed a malicious server attack targeting transaction fee increases and fixed it in version 4.8.3, corresponding to GitHub security advisory GHSA-6jp4-8v48-cc5v. Users may inadvertently sign funds over to the attacker when performing regular fee increases on transactions. Developers stated that the issue arose after users reported losing funds after signing fee increases.

The results of two reports differed: one person discovered that transactions in the wallet history were replaced after broadcasting a payment and claimed that the original payment did not enter the Bitcoin peer-to-peer network and was withheld by the server; another person, Raju-K12, reported losing funds due to the fee increase on the replaced transaction. In the attack, the malicious server withheld new transactions broadcasted by the client and inserted a conflicting transaction into the history, replacing the original output with the attacker's address. The replacement transaction was signed invalidly at that time, but the lightweight client did not verify these signatures before displaying unconfirmed transactions; after the user manually increased the fee and signed through RBF, the attacker received the funds. Both individuals claimed that the last four digits of the attacker's address were the same as those of the legitimate receiving address. The announcement did not disclose the amount stolen, transaction identifiers, or the attacker's address.

The patch corresponds to commit 90fe308, which checks the signatures of the wallet's own inputs before writing memory pool transactions to local history, but does not overwrite inputs from external addresses of the wallet and does not perform complete consensus verification. The announcement suggests that users who have not upgraded should verify the output address and amount each time they sign; clients connected to their own or trusted servers are not affected by this attack. On the timeline, the issue was first privately disclosed on October 11, received reports of fee loss on October 18, the fixed version was tagged on October 23, and the binary and public disclosure occurred on October 24.