-None of Bitcoin’s signature hash types protect the signature script, leaving the door open for a limited denial of service attack called :term:`transaction malleability <Transaction malleability>`\ {:.term}{:#term-transaction-malleability}. The signature script contains the `secp256k1 <http://www.secg.org/sec2-v2.pdf>`__ signature, which can’t sign itself, allowing attackers to make non-functional modifications to a transaction without rendering it invalid. For example, an attacker can add some data to the signature script which will be dropped before the previous pubkey script is processed.
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