Fix BSQ swap buyer tx fee theft vulnerability #5889
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(Actual tolerance by maker of agreed tx fee is 50%, not 10%.)
Prevent the seller from stealing the combined tx fee as change by lying about the value of one or more of his BTC inputs, which are passed to the buyer as raw inputs in the 'BsqSwapFinalizeTxRequest' message. To this end, add a 'RawTransactionInput::validate' method to check the 'value' field against the output value of the respective spending tx and run it on every seller input in 'ProcessBsqSwapFinalizeTxRequest', so that the buyer is no longer just trusting those numbers. Additionally, check that the spending txIds from the raw BTC inputs supplied by the seller actually match those of his signed inputs in the accompanying partially signed tx, thus tying the raw input values to the seller's tx.
Add a check of 'scriptTypeId' field, against the output of the spending tx, to the 'RawTransactionInput::validate' method. Also make the seller as well as the buyer validate each raw BSQ/BTC input received from the peer. This prevents either peer from claiming that any of their non-segwit inputs are segwit in order to underpay the tx fee.
ripcurlx
approved these changes
Dec 4, 2021
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Prevent the seller from stealing the combined tx fee as change by lying about the value of one or more of his BTC inputs, which are passed to the buyer as raw inputs in the
BsqSwapFinalizeTxRequestmessage.To this end, add a
RawTransactionInput::validatemethod to check thevaluefield against the output value of the respective spending tx and run it on every raw seller input inProcessBsqSwapFinalizeTxRequest, so that the buyer is no longer just trusting those numbers.Additionally, check that the spending txIds from the raw BTC inputs supplied by the seller actually match those of his signed inputs in the accompanying partially signed tx, thus tying the raw input values to the seller's tx.
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This should block an attack by the BTC seller that I was able to carry out on regtest. If the seller is the taker, the attack can be made worse by creating a swap trade with the tx fee set at just under twice that seen by the maker, so that it is still within tolerance. So the seller-as-taker could wait until the tx fee spikes to, say, 50 sat/vbyte (but the mempool is not too full) and then take an offer with the tx fee set to 99 sat/vbyte. In this way, they could steal around 12000 sat or so as change and still leave 1 sat/vbyte for the fee so that the transaction eventually confirms. The attack is made worse the more inputs the maker uses or the higher the broadcast tx fee.