Submitted by berndartmueller
https://github.com/code-423n4/2023-11-zetachain/blob/b237708ed5e86f12c4bddabddfd42f001e81941a/repos/node/x/crosschain/keeper/msg_server_add_to_intx_tracker.go#L49-L53 
A single observer has too much power over the ZetaChain network, allowing them to halt outbound cctxs and steal funds.
Both the MsgAddToInTxTracker and MsgAddToOutTxTracker messages allow observers to add a transaction to the tracker without providing Merkle proof to verify if the transaction is valid.
This is a real risk to ZetaChain as a single malicious observer is able to cause significant harm.
For example, the MsgAddToInTxTracker message can be misused in the following ways:
Similarly, trusting observers to not misbehave with MsgAddToOutTxTracker messages  imposes a trust issue as well.
Moreover, a single malicious observer can mess with the external chain nonces by using the (deprecated) MsgNonceVoter message, halting outbound cctxs due to nonce mismatches.
Re-consider the trust model of the MsgAddToInTxTracker and MsgAddToOutTxTracker messages and possibly also require a Merkle proof from observers.
lumtis (ZetaChain) confirmed
