Why ERC-4337 Bundler Logic Demands Its Own Audit Framework

Bundlers must parse a public mempool of UserOperations, exposing them to DoS via malformed or gas-guzzling ops. A single bad actor can cripple bundler profitability by forcing it to simulate invalid transactions.

• Risk: ~$0 cost for attacker vs. unbounded simulation gas for bundler.
• Solution: Requires robust mempool filtering and reputation scoring akin to Ethereum's txpool management.

Bundler logic for fee estimation and paymaster validation depends on external data feeds (e.g., ETH/USD price). A manipulated oracle can cause systemic underpayment or bundler insolvency.

• Risk: Centralized oracle failure propagates to the entire Account Abstraction stack.
• Solution: Audit must verify redundant oracle design and circuit-breaker logic, similar to protocols like MakerDAO or Aave.

A bundler's ability to order, censor, and insert its own transactions creates inherent MEV. Without transparent rules, this leads to user value extraction and chain congestion.

• Risk: Opaque bundling turns a public good into a rent-seeking intermediary.
• Solution: Audits must scrutinize ordering logic for fairness, pushing towards credible neutrality models seen in CowSwap or Flashbots SUAVE.

Bundlers must simulate each UserOperation's validity, but their view of chain state can lag. A race condition between simulation and execution can allow invalid ops to be included, forcing the bundler to pay for reverted txns.

• Risk: Front-running and state-based attacks directly hit the bundler's wallet.
• Solution: Audit framework must test simulation logic against adversarial chain reorgs and mempool timing attacks.

Bundlers stake ETH to sponsor gas for paymaster transactions. If a paymaster contract is malicious or buggy, the bundler's stake can be slashed or locked. This is a smart contract risk transferred to the infrastructure layer.

• Risk: Indirect financial exposure to unaudited, third-party paymaster code.
• Solution: Requires bundlers to implement paymaster whitelisting and coverage limits, with audits verifying these safeguards.

High-performance bundling favors centralized, optimized sequencers. This recreates the L2 sequencer centralization problem at the application layer, threatening censorship resistance.

• Risk: A few dominant bundlers (e.g., Stackup, Alchemy) become de facto gatekeepers.
• Solution: Audit criteria should include decentralization roadmaps and forced inclusion mechanisms, learning from Arbitrum and Optimism.

The bundler is a privileged, off-chain actor that can censor, front-run, or reorder UserOperations. Traditional audits focus on on-chain contract logic, but the bundler's mempool logic and economic incentives are a new, untested attack vector.

• Risk: Centralization of power in a few bundler implementations (e.g., Stackup, Alchemy).
• Blind Spot: MEV extraction logic and transaction ordering are opaque.

Audits must simulate the full UserOperation lifecycle, from the client to the EntryPoint. This requires testing the bundler's validation logic, paymaster sponsorship checks, and aggregation rules against malicious or edge-case inputs.

• Focus: Ensure atomicity of the validateUserOp -> handleOps flow.
• Tooling Gap: Existing fuzzers like Foundry need extensions for off-chain component simulation.

Paymasters (like Biconomy, Candide) reimburse bundlers for gas, creating a trust assumption. A malicious or buggy bundler could drain a paymaster's deposit by spamming invalid ops, or a paymaster could refuse payment post-execution.

• Systemic Risk: A compromised paymaster can brick dependent accounts.
• Audit Gap: Economic security models and slashing conditions are not formally verified.

Model the bundler-paymaster-account factory network as a multi-agent game. Use agent-based simulation to stress-test for liveness failures and extractable value under adversarial conditions, similar to analyses done for Cosmos or Polkadot validators.

• Output: Quantify the minimum staking/penalty required for safety.
• Benchmark: Measure censorship resistance against a cartel of bundlers.

The ERC-4337 mempool is public by default, exposing UserOperation intent. This recreates the same MEV issues seen on Uniswap and Ethereum L1, but now with more complex transactions. Bundlers become the new block builders.

• Exploit: Sandwich attacks on token approvals within a UserOperation.
• Audit Blind Spot: Privacy-preserving tech (e.g., SUAVE, Flashbots) integration is not audited.

Audit the bundler's compliance with intent standards and its integration with privacy layers. Evaluate if the system's design allows for future integration with CowSwap-style solvers or Across-like relayers without breaking security guarantees.

• Criteria: Can user intent be fulfilled without leakage?
• Future-Proofing: Ensure architecture doesn't preclude encrypted mempools like Shutter Network.