tl;dr: Transactions with SIGHASH_ANYONECANPAY-using inputs can be DoS
attacked by attackers adding extra inputs to them that make the fee/byte
paid unfavorable to miners, while still being high enough to be relayed.
While just a nuisance DoS attack, this is a serious obstacle towards
using ANYONECANPAY.


Background: What uses ANYONECANPAY?
-----------------------------------

1) Crowdfunds/assurance contracts: e.g. Hearn's upcoming Lighthouse, as
well as Armory's implementation.

2) Fee bumping: receiver or sender can add inputs w/ ANYONECANPAY to get
a tx confirmed without the (expensive) overhead of a second CPFP tx.

3) Privacy: inputs are more deniable in some cases, e.g. dust used for
fees, which anyone could have added.

4) Replace-by-fee scorched earth: best implementations(1) depend on fee
bumping.


Partial defense: replace-by-fee
-------------------------------

The attacker's modified transaction will usually, but not always, be
replaced by the intended one as the latter will have higher fees.
However replace-by-fee implementations must charge adequately for
network bandwidth consumed, so there will be edge-cases where the
replacement does not happen.


Transaction fee/byte optimization
---------------------------------

Each input that does not use SIGHASH_ALL can be evaluated in terms of
whether or not it increases the fees/byte paid by the transaction. Thus
we can optimize a transaction to pay the highest fees/byte by doing the
following:

    def optimize_tx(tx):
        tx2 = CTransaction(vin=[], vout=tx.vout, nLockTime=tx.nLockTime)

        for txin in <tx.vin sorted by fees/byte>:
            if <txin depends on other txins>:
                continue

            if <tx2 is valid>:
                prev_fee_per_byte = tx2.fees / len(tx2.serialized())
                tx2.vin.append(txin)
                if tx2.fees / len(tx2.serialized()) < prev_fee_per_byte:
                    # adding txin decreased fees/byte
                    tx2.vin.pop()
                    return tx2

            else:
                tx2.vin.append(txin)

        return tx

Essentially txin's that reduce the profitability of the transaction are
dropped, including the attacker's added txins. Meanwhile txins that
increase the profitability can be added by anyone.


1) "[Bitcoin-development] Replace-by-fee scorched-earth without child-pays-for-parent",
   Apr 28th 2014, Peter Todd,
   https://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg05211.html