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Since users cannot deploy contracts, this maximum cpu execution limit is the only one enforced on contracts and each contract that exists on the chain is thoroughly audited so that there is no possible scenario that would allow a single action to create runaway/unlimited CPU execution scenario.
Every user authorized action has a maximum potential size. Transactions that are too large will be rejected. If an actions size is not restricted by this method, it is by default restricted by default_max_transaction_net_usage.
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