Submitted by xiaoming90, also found by shenwilly
A third party sponsor would need to implement a SponsorVault contract that is aligned with the ISponsorVault interface.
Assume that a SponsorVault contract has been defined on Optimism chain. All cross-chain communications are required to call the BridgeFacet.execute, which in turn will trigger the BridgeFacet._handleExecuteTransaction internal function.
However, if there is an error within SponsorVault contract in Optimism causing a revert when s.sponsorVault.reimburseLiquidityFees or s.sponsorVault.reimburseRelayerFees is called, the entire execute transaction will revert. Since execute transaction always revert, any cross-chain communication between Optimism and other domains will fail.
BridgeFacet.sol#L819
It will result in denial of service. The SponsorVault contract, which belongs to a third-party, is a single point of failure for a domain.
This is a problem commonly encountered whenever a method of a smart contract calls another contract  we cannot rely on the other contract to work 100% of the time, and it is dangerous to assume that the external call will always be successful. Additionally, external smart contract might be vulnerable and compromised by an attacker. Even if the team has audited or review the SponsorVault before whitelisting them, some risk might still exist.
Therefore, it is recommended to implement a fail-safe design where failure of an external call to SponsorVault will not disrupt the cross-chain communication. Consider implementing a try-catch block as shown below. If there is any issue with the external SponsorVault contract, no funds are reimbursed to the users in the worst case scenario, but the issue will not cause any impact to the cross-chain communication.
LayneHaber (Connext) confirmed and resolved:
0xleastwood (judge) decreased severity to Medium and commented:
