Chromia - Postchain

In `withdrawBySnapshot()`, the `ERC20StateHeader` is validated to confirm that the withdrawal snapshot is intended for the bridge contract. A snapshot can either be authorized for any bridge on a given network, or a specific bridge on the network. The relevant code is reproduced below: ```js // assume networkId must fit in 96 bits uint256 allowedDiscriminator1 = networkId << 160; // discriminator allows any bridge contract on the network uint256 allowedDiscriminator2 = allowedDiscriminator1 + uint160(address(this)); require((header.discriminator == allowedDiscriminator1) || (header.discriminator == allowedDiscriminator2), "TokenBridge: invalid bridge contract"); ``` <br> `hbridge/snapshot.rell` indicates that the open discriminator is used as a default: ```js function network_id_to_discriminator(network_id: integer): big_integer { return big_integer(network_id) * big_integer.from_hex("1" + "00".repeat(20)); } function maybe_create_local_erc20_state_slot(account: ft4.accounts.account, erc20_asset): state_slot? { .... if (not exists(existing_slot)) { existing_slot = create state_slot ( id = get_next_state_slot_id(), network_id, recipient_address = account_address, type = state_slot_type.erc20, discriminator = network_id_to_discriminator(network_id), // Open discriminator local = true ); } .... } ``` <br> If a mass exit is triggered across multiple bridges on the same network, then `header.discriminator == allowedDiscriminator1` would allow users to call `withdrawBySnapshot()` across different bridges.

In `TokenBridgeWithSnapshotWithdraw.triggerMassExit()`, the owner assigns the `massExitBlock` as the last valid block from which users can call `withdrawBySnapshot()`. The block is required to have occurred within the last three days: ```js require(header.timestamp >= (block.timestamp - 3 days) * 1000, "TokenBridge: mass exit block is too old"); ``` <br> Given that the requirement is hardcoded to 3 days, this duration may be longer than the dispute period if `withdrawOffset` is shorter than 3 days. This would allow users who executed their withdrawals to withdraw additional funds via the older snapshot.

The entity `evm_block` has three keys: 1. `network_id` 1. `evm_block_height` 1. `evm_block_hash` <br> This would allow a given `<network_id, evm_block_height>` pair to be associated with multiple `evm_block_hashes`, which could cause inconsistencies when processing blocks.

The `TokenBridge` contract has had its storage layout refactored since the previous audit, with some variables removed/added across the `ChromiaTokenBridge` and `TokenBridgeWithSnapshotWithdraw` contracts. This makes the new `TokenBridge` incompatible for upgrading previous versions. After speaking with the Chromia team, none of the deployed instances of the `TokenBridge` contract are scheduled for upgrades, and thus the severity of the issue has been marked as Informational.