If the security of a blockchain is compromised, pausing bridge withdrawals will mitigate damage until the issue is resolved.
The `OptimismPortal` is a bridge contract that makes it possible to send messages between your L1 and your L2 OP Stack chain.
The `OptimismPortal` is pausable as a backup safety mechanism that allows a specific `GUARDIAN` address to temporarily halt deposits and withdrawals to mitigate security issues if necessary.
An OP Stack chain does not have to specify a usable `GUARDIAN` address if it does not want to make the `OptimismPortal` contract pausable, it can specify an address such as zero.
## Who can do it?
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@@ -20,9 +22,8 @@ That address can call [`pause`](https://github.com/ethereum-optimism/optimism/bl
The guardian created by the setup script is the admin account.
This is sufficient for testing, but for a production system you would want the guardian to be a multisig with trusted security council.
The guardian is immutable, there is no way to change it while using the same contract.
Luckily, it isn't supposed to be called directly, but through a proxy.
[You can tell the proxy to go to a new contract](../build/tutorials/new-precomp.md).
The `GUARDIAN` variable is immutable, but the `OptimismPortal` contract sits behind a proxy, so the `GUARDIAN` can be modified by changing the `OptimismPortal` proxy to point to a new implementation contract.
You do this using the L1 [`ProxyAdmin`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/universal/ProxyAdmin.sol) contract.
