Protocol Upgrade Compatibility Review: Sky Lending

Sky Lending is a cross-chain lending platform with a TVL of approximately $5.88 billion, split between Ethereum L1 and L2 roll-ups like Optimism, Arbitrum, and zkSync. The protocol uses an upgradeable Transparent Proxy (EIP-1967) controlled by a multi-sig DAO consisting of 4 out of 7 members. The review's objective was to evaluate upgrade compatibility, ensuring that future contract upgrades won't break existing state, introduce new attack vectors, or compromise the protocol's economic guarantees.

The review identified seven critical findings, ranging from low to medium severity, that could impact the protocol's safety during upgrades:

1. Storage Slot Collision in InterestRateModelV2: A new variable added in the upgraded InterestRateModelV2 contract overwrites the existing multiplierPerYear slot, leading to incorrect interest calculations. If exploited, it could inflate borrowing capacity and drain reserves, potentially causing losses of up to $200 million.

2. Uninitialized Storage Gap in RewardsDistributor: The upgraded RewardsDistributor contract reduces the reserved slot count from 50 to 30, leaving the remaining slots uninitialized. Malicious upgrades could write unintended data into these slots, redirecting reward emissions to an attacker's address, leading to reward token misallocation worth approximately $15 million.

3. Missing OnlyGovernor Guard on EmergencyPause: The EmergencyPause function, designed to be callable only by the DAO governor, can be invoked by any address with the PROPOSER_ROLE due to a missing onlyGovernor modifier. This oversight could allow malicious relayers or compromised keys to pause deposits and withdrawals during an upgrade, creating a temporary denial-of-service situation and enabling state skew attacks.

4. Replay-Attack on Bridge Messages: The BridgeExecutor contract validates incoming messages based on the msg.sender but fails to verify the chainId embedded in the payload. An attacker could exploit this by replaying a historic L2-to-L1 message after an upgrade, causing double-minting of aTokens and inflating the token supply by approximately 0.5% of the total TVL, or $30 million.

5. Upgradeable Proxy Admin Set to Upgradeable Contract: The DAO can change the proxy admin to any address, potentially leading to a "self-destruct-upgrade" path if an attacker sets the admin to a contract that itself is upgradeable. This could result in the attacker replacing the admin with a malicious contract that calls proxy.selfdestruct() or proxy.upgradeTo(address(0)), effectively disabling the protocol's upgradeability and freezing it permanently.

6. Missing Invariant Test for Total Supply Consistency: The protocol's test suite does not verify that totalSupply equals the sum of userDeposits plus accruedInterest after any upgrade. This oversight could allow subtle rounding behavior changes to occur, leading to hidden accounting discrepancies and "dust" accumulation or loss of funds over time.

In conclusion, the protocol's upgradeability model is fundamentally sound, but the identified storage-layout mismatches and governance guard omissions pose a material risk that could be exploited during or right after a scheduled upgrade. With an overall risk score of 7 out of 10 (elevated), addressing these concerns should be a top priority to maintain the platform's security and integrity.