nft: N-08 Easily bypass-able requirement (#3493)

packages/contracts-periphery/contracts/L1/L1ERC721Bridge.sol

@@ -90,9 +90,11 @@ contract L1ERC721Bridge is Semver, CrossDomainEnabled, Initializable {
     }
 
     /**
-     * @notice Initiates a bridge of an NFT to the caller's account on L2. Note that the current
-     *         owner of the token on this chain must approve this contract to operate the NFT before
-     *         it can be bridged.
+     * @notice Initiates a bridge of an NFT to the caller's account on L2. Note that this function
+     *         can only be called by EOAs. Smart contract wallets should use the `bridgeERC721To`
+     *         function after ensuring that the recipient address on the remote chain exists. Also
+     *         note that the current owner of the token on this chain must approve this contract to
+     *         operate the NFT before it can be bridged.
      *
      * @param _localToken  Address of the ERC721 on this domain.
      * @param _remoteToken Address of the ERC721 on the remote domain.
@@ -109,8 +111,12 @@ contract L1ERC721Bridge is Semver, CrossDomainEnabled, Initializable {
         uint32 _minGasLimit,
         bytes calldata _extraData
     ) external {
-        // Modifier requiring sender to be EOA. This check could be bypassed by a malicious
-        // contract via initcode, but it takes care of the user error we want to avoid.
+        // Modifier requiring sender to be EOA. This prevents against a user error that would occur
+        // if the sender is a smart contract wallet that has a different address on the remote chain
+        // (or doesn't have an address on the remote chain at all). The user would fail to receive
+        // the NFT if they use this function because it sends the NFT to the same address as the
+        // caller. This check could be bypassed by a malicious contract via initcode, but it takes
+        // care of the user error we want to avoid.
         require(!Address.isContract(msg.sender), "L1ERC721Bridge: account is not externally owned");
 
         _initiateBridgeERC721(

packages/contracts-periphery/contracts/L2/L2ERC721Bridge.sol

@@ -104,9 +104,11 @@ contract L2ERC721Bridge is Semver, CrossDomainEnabled, Initializable {
     }
 
     /**
-     * @notice Initiates a bridge of an NFT to the caller's account on L1. Note that the current
-     *         owner of the token on this chain must approve this contract to operate the NFT before
-     *         it can be bridged.
+     * @notice Initiates a bridge of an NFT to the caller's account on L1. Note that this function
+     *         can only be called by EOAs. Smart contract wallets should use the `bridgeERC721To`
+     *         function after ensuring that the recipient address on the remote chain exists. Also
+     *         note that the current owner of the token on this chain must approve this contract to
+     *         operate the NFT before it can be bridged.
      *
      * @param _localToken  Address of the ERC721 on this domain.
      * @param _remoteToken Address of the ERC721 on the remote domain.
@@ -123,8 +125,12 @@ contract L2ERC721Bridge is Semver, CrossDomainEnabled, Initializable {
         uint32 _minGasLimit,
         bytes calldata _extraData
     ) external {
-        // Modifier requiring sender to be EOA. This check could be bypassed by a malicious
-        // contract via initcode, but it takes care of the user error we want to avoid.
+        // Modifier requiring sender to be EOA. This prevents against a user error that would occur
+        // if the sender is a smart contract wallet that has a different address on the remote chain
+        // (or doesn't have an address on the remote chain at all). The user would fail to receive
+        // the NFT if they use this function because it sends the NFT to the same address as the
+        // caller. This check could be bypassed by a malicious contract via initcode, but it takes
+        // care of the user error we want to avoid.
         require(!Address.isContract(msg.sender), "L2ERC721Bridge: account is not externally owned");
 
         _initiateBridgeERC721(