Vega's validators able to submit duplicate transactions

A vulnerability exists that allows a malicious validator to trick the Vega network into re-processing past Ethereum events from Vega’s Ethereum bridge. For example, a deposit to the collateral bridge for 100USDT that credits a party’s general account on Vega, can be re-processed 50 times resulting in 5000USDT in that party’s general account. This is without depositing any more than the original 100USDT on the bridge.

Despite this exploit requiring access to a validator's Vega key, a validator key can be obtained at the small cost of 3000VEGA, the amount needed to announce a new node onto the network.

The steps to carry out this exploit are as follows:

1. Cause an Ethereum event on one of the bridge contracts e.g a deposit to the collateral bridge, or the staking bridge
2. This will result in the Ethereum-event-forwarder of each node to submit a ChainEvent transaction to the Vega network corresponding to that event
3. Scrape the valid chain event transaction from the Tendermint block data using a node’s Tendermint API
4. Change the value of the txId field of the ChainEvent to any valid, but different, value
5. Bundle the tweaked ChainEvent into a new transaction, sign it with a validator key and resubmit to the Vega network
6. The fraudulent ChainEvent will be processed by Vega as if it were a new ChainEvent even though it did not occur on Ethereum

The key to this exploit is in step 4. The txId field of the ChainEvent is used when checking for ChainEvent resubmission, but NOT during the subsequent on-chain verification of the event. Therefore changing the txId of an existing ChainEvent is enough to by-pass the duplication check and for it to still be verified as a real event.

Impact

The impact of this exploit is dependent on the ChainEvent being manipulated. The below table describes each one:

Chain Event	Allows	Consequence
Deposit	Generation of unlimited funds of any asset	Withdrawal of all assets
Stake Deposit	Delegate unlimited Vega to a single node	A single node has controlling amount of voting power
Stake Removed	Force a Validator node to drop below self-stake requirements	Prevents reward payouts
Bridge Stop	The Vega network to think the bridge is stopped	Prevent anyone from withdrawing funds
Signer Removed	The Vega network to think a validator nodes is not on the multisig contract