💰Paymasters
How paymasters enable gas abstraction in ERC-4337.
A Paymaster is a smart contract that can sponsor gas fees on behalf of users. This allows users to send transactions without needing ETH, improving onboarding and UX.
🔧 What Do Paymasters Do?
Paymasters interact with the EntryPoint during the validation phase of a UserOperation. They are responsible for:
Verifying that the operation meets their criteria
Covering gas costs if validation succeeds
Having sufficient stake and deposit with the EntryPoint
They enable gas abstraction by allowing:
ERC-20-based fee payments
Third-party gas sponsorship
Business logic gating (e.g. ad views, subscriptions)
🧪 How Are Paymasters Invoked?
If a UserOperation includes a non-empty paymasterAndData field, the EntryPoint calls:
validatePaymasterUserOp(userOp, requestId, maxCost)
This method is expected to:
Check if it’s willing to sponsor the op
Optionally return context data
Revert if the op should be rejected
Later, if the operation succeeds, the EntryPoint calls:
postOp(mode, context, actualGasCost)
This allows the Paymaster to finalize accounting or take post-op actions.
💸 Staking and Security
To prevent griefing (e.g., spamming ops that fail), Paymasters must:
Deposit ETH to cover potential costs
Stake additional ETH as a safety requirement
Wait through an unstake delay if they want to withdraw
If a sponsored op fails validation or execution, the Paymaster pays the gas. Thus, it's critical to simulate carefully and enforce strict checks in validatePaymasterUserOp.
⚠️ Common Attacks and Design Considerations
Replay abuse: ensure ops are only valid once
Gas griefing: require off-chain checks before sponsorship
Whitelist bypass: signatures or preconditions must be enforced securely
Stake draining: validate preconditions tightly, and monitor gas costs
🔄 Stateless vs Contextual Paymasters
Some Paymasters simply check a whitelist or signature and return — these are stateless. Others encode server-verified rules, access passes, or custom logic using:
Rate-limiting APIs
Game logic
DApp identity protocols
These typically return context to postOp() to complete the flow.
✅ Summary
Paymasters are essential for enabling gasless UX in Account Abstraction. They let dApps or wallets pay for users’ gas under custom conditions. But to do this safely, they must be staked, carefully simulate all sponsored ops, and implement robust validation logic.
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