Protocol Upgrade Compatibility Review: Hyperliquid Bridge

Target Protocol: Hyperliquid Bridge (TVL: $6481.2M)

Hyperliquid Bridge – Protocol Upgrade Compatibility Review

TVL: ≈ $6.48 B (Ethereum + L2)

Date of Review: 2 September 2026

Prepared by: [Your Name], Senior DeFi Security Researcher & Smart‑Contract Auditor

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1. Executive Summary

The Hyperliquid Bridge is the primary cross‑chain liquidity conduit for the Hyperliquid ecosystem, enabling the transfer of ERC‑20 tokens, native assets, and custom LP shares between Ethereum L1 and multiple L2 roll‑ups (Optimism, Arbitrum, zkSync, StarkNet). The bridge is built around a proxy‑based upgradable contract system (UUPS pattern) with a multi‑signer governance module that authorises upgrades via an off‑chain DAO vote.

Our Upgrade Compatibility Review focused on the next scheduled protocol upgrade (v2.3 → v2.4), which introduces:

• A new fee‑splitting module for L2‑specific gas rebates.
• Support for “instant‑finality” withdrawals on zkSync.
• Refactored storage layout for the BridgeState struct to accommodate additional asset metadata.

The review examined the upgrade path, storage compatibility, governance controls, cross‑chain message handling, and interaction with external adapters.

Key Findings

Area	Finding	Severity
Storage Layout Migration	The new BridgeStateV2 adds three new fields before existing ones, breaking the storage slot order used by the current implementation.	Critical
Upgrade Governance	The DAO’s timelock is set to 24 h, but the upgrade can be executed by any of the 5‑of‑9 signers without a secondary on‑chain challenge period.	High
Cross‑Chain Replay Protection	The bridge relies on a single nonce per source chain; the new instant‑finality flow re‑uses the same nonce space, opening a potential replay attack if an upgrade is reverted.	High
External Adapter Compatibility	The L2 adapters (Optimism, Arbitrum) are compiled against the old ABI; the upgrade changes function selectors for deposit() and withdraw().	Medium
Testing & Simulation	No formal upgrade simulation (e.g., using Foundry’s fork + upgrade scripts) was found in the repo; only unit tests for the new module exist.	Medium
Event Emission Consistency	New events (InstantWithdrawalRequested) reuse the same topic0 hash as the legacy WithdrawalRequested, breaking off‑chain indexers.	Low

Overall, the upgrade introduces significant compatibility risks that could lead to fund loss, state corruption, or permanent bridge downtime if deployed without remediation.

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2. Identified Attack Vectors

#	Vector	Description	Potential Impact
1	Storage Slot Collision	Adding new variables before existing ones shifts the storage layout. Existing state (e.g., totalLiquidity, paused) will be overwritten by the new fields (assetMetadata, feeCollector). This can corrupt balances, disable pausing, and allow unauthorized fee withdrawals.	Total loss of user funds, bridge freeze, loss of trust.
2	Governance Execution Race	The upgrade can be executed immediately after the DAO vote (24 h timelock). An attacker controlling 5 of 9 signers could push a malicious upgrade that re‑routes withdrawals to a controlled address. No on‑chain challenge period exists.	Unauthorized asset siphoning, protocol takeover.
3	Replay of Cross‑Chain Messages	The bridge uses a per‑chain nonce to prevent replay. The new instant‑finality flow re‑uses the same nonce range for both classic and instant withdrawals. If the upgrade is rolled back, a previously processed instant withdrawal could be replayed on the old contract, resulting in double‑spend.	Double withdrawal of assets, loss of up to TVL‑proportional value.
4	Adapter ABI Mismatch	L2 adapters are linked at deployment time to the bridge’s ABI. Changing function selectors (deposit(address,uint256) → deposit(address,uint256,bytes)) without updating adapters will cause calls to revert, halting deposits/withdrawals on affected L2s.	Service outage on specific L2s, liquidity lock‑up.
5	Event Collision & Indexer Failure	Re‑using the same topic0 for new events will cause off‑chain indexers (TheGraph, Covalent) to mis‑classify events, leading to inaccurate balance reporting and potential user panic.	Reputation damage, increased support overhead.
6	Insufficient Upgrade Simulation	Lack of end‑to‑end upgrade simulation on a forked mainnet means hidden bugs (e.g., re‑entrancy in the new fee module) may go undetected until after live deployment.	Unexpected contract failures, possible re‑entrancy exploits.
7	Access‑Control Mis‑configuration in New Module	The new fee‑splitting contract inherits Ownable but the owner is set to msg.sender during upgrade, which will be the proxy admin (the DAO). If the admin key is compromised, the attacker can change the fee collector address arbitrarily.	Gradual fee exfiltration.

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3. Prioritized Technical Recommendations

Priority	Recommendation	Rationale & Implementation Details
P1 – Immediate	Fix Storage Layout – Refactor BridgeStateV2 to append new variables after the existing ones, preserving slot order. Use OpenZeppelin’s StorageSlot library to explicitly map new fields to unused slots if expansion is required.	Prevents catastrophic state corruption. Must be verified with a storage layout diff (forge inspect BridgeV2 storage-layout).
P1 – Immediate	Add Upgrade Timelock & Challenge Period – Extend the DAO timelock to 72 h and introduce an on‑chain challenge function that allows any holder to veto an upgrade within the timelock by depositing a bond (e.g., 10 k ETH).	Mitigates rushed malicious upgrades.
P2	Separate Nonce Spaces – Introduce distinct nonce counters: classicNonce and instantNonce. Update the message‑verification logic to include the nonce type in the signed payload.	Eliminates replay risk across withdrawal modes.
P2	Adapter ABI Upgrade – Deploy new L2 adapter contracts with the updated ABI and perform a proxy‑upgrade of the adapter registry to point to them. Include a fallback that forwards old selectors to the new implementation for a grace period (e.g., 30 days).	Guarantees continuity of service during migration.
P3	Event Namespace Change – Define a new topic0 for InstantWithdrawalRequested (e.g., keccak256("InstantWithdrawalRequested(address,uint256,bytes32)")). Update off‑chain indexers accordingly and emit a deprecation notice for the old event.	Prevents indexer confusion and preserves analytics integrity.
P3	Formal Upgrade Simulation Suite – Build a CI pipeline that forks mainnet at the latest block, deploys the current proxy, runs the upgrade transaction, and executes a full suite of state‑integrity checks (balance invariants, paused flag, fee collector). Use Foundry/Hardhat scripts with forge test --fork-url.	Detects hidden bugs before live deployment.
P4	Secure Ownership Transfer for New Modules – In the new fee‑splitting contract, set the owner to the DAO’s multi‑sig address via an explicit initialize(address dao) call, not msg.sender. Add a renounceOwnership() guard that can only be called after a successful DAO vote.	Reduces risk of owner key compromise.
P4	Comprehensive Documentation & Migration Guide – Publish a versioned spec that details storage layout, nonce handling, and adapter upgrade steps. Include a step‑by‑step migration checklist for operators.	Improves operational security and community confidence.
P5	External Audits & Formal Verification – Engage a third‑party audit firm to perform a full contract audit of the upgraded codebase, focusing on the new fee module and instant‑finality logic. Consider formal verification of the storage layout using tools like Certora or Echidna.	Provides an independent security guarantee.

All recommendations should be tracked in a dedicated **upgrade ticket* (e.g., JIRA EPIC‑HBR‑001) with clear owners, deadlines, and test‑coverage acceptance criteria.*

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4. Risk Score

Metric	Score (1‑10)	Weight	Weighted Score
Technical Complexity (storage migration, cross‑chain logic)	8	0.30	2.40
Potential Financial Impact (TVL exposure)	9	0.35	3.15
Likelihood of Exploit (given current controls)	6	0.20	1.20
Mitigation Readiness (existing safeguards)	4	0.15	0.60
Overall Risk Score	–	–	7.35 ≈ 7

Risk Rating: High (7/10) – The upgrade presents a high probability of severe financial loss if the identified issues are not remediated before deployment.

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5. Conclusion

The Hyperliquid Bridge is a cornerstone of the Hyperliquid ecosystem, handling billions of dollars in cross‑chain liquidity. The upcoming v2.4 upgrade introduces valuable functionality but also critical compatibility hazards—most notably an unsafe storage layout change and insufficient governance safeguards.

If the P1–P5 recommendations are implemented and verified through rigorous on‑chain simulation, the upgrade can be executed safely, preserving bridge integrity and user confidence. Failure to address these issues could result in state corruption, fund loss, and prolonged service outages, jeopardizing the protocol’s reputation and the broader Hyperliquid ecosystem.

We advise the Hyperliquid development and governance teams to pause the upgrade deployment until the storage layout is corrected, the governance timelock is extended, and a full upgrade‑simulation pipeline is in place. Once these mitigations are verified, a staged rollout (starting on a low‑value testnet, followed by a limited‑scope mainnet pilot) will further reduce risk.

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Prepared for the Hyperliquid DAO and Bridge Development Team

Confidential – Do not distribute without prior written consent.

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