Add note about arbitrary message sending from OptimismPortal

Author: mdehoog

specs/withdrawals.md

@@ -48,6 +48,7 @@ finalization.
 - [Security Considerations](#security-considerations)
   - [Key Properties of Withdrawal Verification](#key-properties-of-withdrawal-verification)
   - [Handling Successfully Verified Messages That Fail When Relayed](#handling-successfully-verified-messages-that-fail-when-relayed)
+  - [OptimismPortal can send abitrary messages on L1](#optimismportal-can-send-abitrary-messages-on-l1)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
@@ -216,3 +217,15 @@ contracts if desired.
 
 [`WithdrawalTransaction` type]: https://github.com/ethereum-optimism/optimism/blob/08daf8dbd38c9ffdbd18fc9a211c227606cdb0ad/packages/contracts-bedrock/src/libraries/Types.sol#L62-L69
 [`OutputRootProof` type]: https://github.com/ethereum-optimism/optimism/blob/08daf8dbd38c9ffdbd18fc9a211c227606cdb0ad/packages/contracts-bedrock/src/libraries/Types.sol#L25-L30
+
+### OptimismPortal can send abitrary messages on L1
+
+The `L2ToL1MessagePasser` contract's `initiateWithdrawal` function accepts a `_target` address and `_data` bytes,
+which is passed to a `CALL` opcode on L1 when `finalizeWithdrawalTransaction` is called after the challenge
+period. This means that, by design, the `OptimismPortal` contract can be used to send arbitrary transactions on
+the L1, with the `OptimismPortal` as the `msg.sender`.
+
+This means users of the `OptimismPortal` contract should be careful what permissions they grant to the portal.
+For example, any ERC20 tokens mistakenly sent to the `OptimismPortal` contract are essentially lost, as they can
+be claimed by anybody that pre-approves transfers of this token out of the portal, using the L2 to initiate the
+approval and the L1 to prove and finalize the approval (after the challenge period).