Bitcoin User Claims Hack Triggered Record $3M Bitcoin Fee, 139 BTC Lost

A Bitcoin user has come forward claiming they were the victim of a hack that led to the record-breaking $3 million Bitcoin transaction fee paid last week.

The anonymous user, going by the handle @83_5BTC on X, said they had created a new cold wallet and transferred 139 BTC to it, worth over $5 million. The funds then were immediately transferred out to another wallet.

It was my BTC that paid the high fee.

I created a new cold wallet, transferred 139BTC to it and it got transferred out to another wallet immediately. 😵

I can only imagine that someone was running a script on that wallet and that the script had a weird fee calculation. ☹️

— Hackers_paid_83.5BTC_fee_with_my_money (@83_5BTC) November 24, 2023

“I can only imagine that someone was running a script on that wallet and that the script had a weird fee calculation,” @83_5BTC tweeted on Friday.

The Record-Breaking $3M Transaction Fee

On Thursday, a transaction on the Bitcoin blockchain paid a fee of 83.65 BTC, worth $3.1 million, setting a new record high in US dollar terms for a single transaction on the network.

The transaction, first spotted by the Bitcoin developer @Mononautical, sent 55.77 BTC while paying the abnormally large mining fee.

@83_5BTC posted on X the sending address used for the transaction, with cryptography expert Jameson Lopp also verifying the signature checks out.

The signature checks out, @83_5BTC apparently controls the key that paid that 83.7 BTC fee.

1/🧵 https://t.co/vmZFn6sozN pic.twitter.com/rFcxmxOCwO

— mononaut (tx/acc) (@mononautical) November 27, 2023

Bitcoin Hack Fee Hits 60% of Stolen Funds

According to @Mononaut, the most likely cause of the monumental fee was a low-entropy wallet vulnerable to hacking.

The transaction was swiftly bumped using replace-by-fee, a Bitcoin protocol feature allowing senders to increase the fee on pending transactions to have them confirmed faster.

@Mononaut theorized that multiple hackers could have been targeting the vulnerable wallet, competing to swiftly steal the funds first. This could explain the astronomical fee as scripts were configured to spend a large portion of the transaction to hinder other attackers.

I just noticed that the ~83.7 BTC fee was exactly 60% of the stolen UTXO value.

(60% × 139.42495946 = 83.65497568)

And the attacker *also* swept a 100k sat UTXO from the same address, paying exactly 60k sats in fees 🧐https://t.co/b88xsi2iFk

— mononaut (tx/acc) (@mononautical) November 27, 2023

The transaction fee was exactly 60% of the total 139.42 BTC balance swept from the wallet, providing further evidence of an automated script preset to pay out 60% of the funds as a fee.

According to blockchain explorer Blockchair, the record fee transaction was mined by the pool AntPool in block 818,087, surpassing the previous high set in September, when a $500,000 overpayment was made by Paxos to F2Pool and identified as an error. In that case, the mining pool l refunded the fee to Paxos after facilitating the transaction.

Whether AntPool would be open to a similar reimbursement agreement for this latest record fee is unclear.

While the individual behind @83_5BTC has claimed ownership of the funds, exactly how the hack transpired is still uncertain. The Bitcoin community is actively investigating the addresses and transactions involved for more clues.

Blockchain analysis could help track the destination of the stolen funds and potentially identify patterns linking the hacker(s). Exchanging or tumbling the Bitcoin through mixers may obscure the trail, however.