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The OptimismPortal is the low-level L1 message passing contract present on all standard OP Stack chains. This contract handles the L1 side of the communication channel between an OP Stack chain and its L1 parent chain. As a safety mechanism, a privileged GUARDIAN address can pause withdrawals. When paused, the OptimismPortal contract will prevent L2-to-L1 transactions from being executed. This is a backup safety mechanism that can be used to help mitigate potential active security concerns. Pause functionality were introduced to the OP Stack to mitigate the risk of withdrawal bugs that have led to exploits in other bridging systems.

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Pause functionality

The OptimismPortal points to a SuperchainConfig smart contract which has a privileged GUARDIAN address that can pause and unpause L2-to-L1 transactions from being executed. The SuperchainConfig contract is a shared implementation across the Superchain. All Optimism-governed chains point to it, and any OP Stack chain can point its SuperchainConfigProxy to this shared implementation. L2-to-L1 transactions allow users and smart contracts on the OP Stack chain to send messages to the L1 parent chain. Pause functionality allows a GUARDIAN to halt L2-to-L1 transaction execution for the OP Stack chain in question. L1-to-L2 transactions are not affected by pause functionality. Pauses by the GUARDIAN impact all L2-to-L1 transactions for the OP Stack chain in question and cannot be targeted to specific users, smart contracts, or transactions. Pauses are designed to be a backup safety mechanism and are expected to be used only in the event of an active pressing security concern.

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Pause and unpause functions

The GUARDIAN can pause and unpause L2-to-L1 transactions at any time by calling the pause and unpause functions on the SuperchainConfig contract.

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Guardian address

The GUARDIAN address is configured in the SuperchainConfig contract and can be modified by the network’s administrative address or smart contract. To learn more about the privileged role, see documentation. The GUARDIAN address is set as an immutable variable inside of the SuperchainConfig contract. To change the GUARDIAN address, the SuperchainConfig proxy contract must be upgraded to a new implementation contract that has a different GUARDIAN address.

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Additional information

While this functionality is available for the OP Stack protocol, it does not cover the security of centralized exchanges and third party bridges. If you operate a centralized exchange or third party bridge, you should monitor this contract and pause withdrawals from the Superchain if you see that it has been paused. If you’d like to learn more about the privileged roles in the OP Stack, see the privileged roles documentation.