Replay Attack Danger: If a minority chain appears this weekend, buyers could replay signed fork-coin sales on bitcoin itself, making inaction the safest move until the chains can be separated.

A planned Bitcoin fork tied to the controversial BIP-110 proposal could create duplicate balances on two chains, tempting holders to sell the new coins for what looks like free money. Because both chains would initially accept identical transactions, selling the forked coins could trigger a replay attack that also spends the seller's real bitcoin on the main chain.

Developers warn that, without built-in replay protection until at least early September, the safest course for non-experts is to avoid moving coins during the potential split.

How the replay attack works

Bitcoin may split into two chains in the coming days. If it does, every bitcoin holder would end up with the same balance on each chain. Someone might then offer to buy the new coins at an attractive price — they appear to be free money, so selling them can seem like an easy win.

But accepting such a deal could cost the seller their real bitcoin. Both chains initially accept identical transactions, meaning a transaction signed to send fork coins can also be broadcast on the main Bitcoin network. The buyer receives the same amount in actual BTC at the same destination address.

This is known as a replay attack. A replay does not drain the entire wallet — only the coins put up for sale move, and they leave as real bitcoin rather than the fork version, with a transaction fee paid on both chains.

Replay attacks are a well-known risk when a blockchain splits without built-in protection. During Bitcoin's 2017 fork that produced Bitcoin Cash, the spin-off chain added replay protection specifically so that transactions on one chain could not be replayed on the other. BIP-110 does not include equivalent safeguards at launch, which is why the danger period exists.

The safest move for anyone unable to separate the two balances is to leave the coins untouched. Bitcoin developer Kevin Loaec, who flagged the risk on X this week, said large holders could be targeted first. Doing nothing is the safer option, he stated, because coins that never move cannot be replayed — there is no signed transaction to copy.

⚠️IMPORTANT⚠️In the next couple of days, a new shitcoin will fork off Bitcoin. It is a big security risk for people who just believe they will get an "airdrop" and want to sell it, to get more bitcoin. I will write more about it, but here is the TLDR: 👇 — Kevin Loaec 🧙‍♂️🐟 (@KLoaec) August 6, 2026

How BIP-110 makes this possible

BIP-110 is a proposal to keep pictures, text, and other non-payment data out of Bitcoin transactions for a period of one year. The push stems from a broader debate within the Bitcoin community over inscriptions and ordinals — mechanisms that have let users embed images and other data on-chain since 2023, increasing transaction sizes and fueling disagreement over whether Bitcoin should serve purely as a settlement network for payments.

Changing Bitcoin's rules requires miners to signal agreement by marking the blocks they produce. BIP-110 needs 1,109 marked blocks out of a 2,016-block stretch — or 55% — to activate through the standard signalling route.

That route is effectively closed given current signalling levels, but the proposal contains a second activation mechanism. From block 961,632, expected this weekend, computers running BIP-110 software will reject any block that does not carry the mark, regardless of whether miners agreed.

Almost every block being mined at present does not carry it. Those BIP-110 nodes will therefore begin rejecting the chain that nearly all of Bitcoin's mining power is building. If some miners continue extending a BIP-110-compatible branch while the rest keep mining Bitcoin as usual, two competing versions of the transaction history could emerge. However, if nobody sustains the minority branch, it stalls.

This imbalance makes a split possible rather than certain. Miner signalling was running near 2.6% as of Friday, according to trackers. That node share does not translate into mining power, and with signalling this low, a minority branch could produce blocks very slowly or stop advancing altogether.

Nevertheless, if a minority chain does emerge, every Bitcoin holder would initially have the same balance on both chains. The second copy may carry little or no value, yet someone may still offer to buy it.

Separating the two balances is harder at first because the fork provides no automatic replay protection. BIP-110's actual restrictions on transaction data do not switch on until block 965,664, expected around the start of September. Before then, holders would need to deliberately create coins that exist on only one branch before spending safely.

Timing depends on how quickly blocks are found, so the mandatory-signalling window could begin a day earlier or later than current estimates.