If a script is automatically valid when an OP_SUCCESS opcode is called, what mechanism prevents a malicious actor from taking a random UTXO and creating a fake scriptsig with an OP_SUCCESS included?

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The UTXO's scriptPubKey commits to the script(s) that are permitted for spending, so you can't just pick an OP_SUCCESSx script and spend with that - the coins have to be sent to an address that permits that (and no rational receiver would ever create such an address).

This is no different really from the fact that when spending an old style P2PKH address, you can't just substitute your own public key when spending. You must use the public key which has was committed to by the address.

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