Why Gas Sponsorship Is a Backdoor for Account Takeover

A compromised or rogue paymaster can censor, front-run, or arbitrarily modify the user's transaction before execution. This is a single point of failure for any dApp that integrates it.

• Vector: Paymaster can replace the calldata or to address in the user's transaction.
• Impact: Funds are drained to an attacker's address, not the intended dApp.
• Prevalence: A single paymaster often serves millions of users across hundreds of dApps.

ERC-4337 user operations are signed off-chain, creating a signature that a paymaster can replay on a different chain or in a different context for which the user did not give consent.

• Vector: Paymaster submits the signed userOp to an unintended network or with modified parameters.
• Impact: User authorizes a $10 swap on Polygon, but the paymaster executes a $10,000 transfer on Arbitrum.
• Mitigation Gap: Native EIP-155 chainID protection does not exist for userOp hashes.

Paymasters offer "gasless" transactions by sponsoring fees, but can embed arbitrary logic to revoke sponsorship based on market conditions, leaving users with failed transactions or unexpected costs.

• Vector: Paymaster's validation logic checks an oracle and reverts if ETH price drops, causing user's transaction to fail post-signature.
• Impact: User's transaction is stuck or they are forced to pay 100% of the gas in a volatile market.
• Trust Assumption: Users must audit paymaster logic, an impossible burden for most.

The standard's Paymaster model is the baseline. It's a double-edged sword: it enables sponsorship but requires explicit user approval for each new Paymaster contract. This creates a critical UX friction versus seamless onboarding. Security relies entirely on users understanding and approving a new, potentially malicious, contract for each session.

• Key Benefit: Explicit user consent for each sponsor.
• Key Flaw: Creates onboarding friction; users may blindly approve.

Wallets like Privy embed gas sponsorship into embedded wallet onboarding, using pre-authorized session keys with strict limits. The sponsor pays, but the user's core signing key remains cold. This shifts risk from total account control to the scope of the session (e.g., max spend, time window).

• Key Benefit: Isolates sponsor risk to a limited session.
• Key Flaw: Still requires trust in the session key signer logic; malicious dApp frontends can abuse scope.

Infrastructure providers like Pimlico implement bundler-level policy rules to reject malicious UserOperations. They can blacklist known malicious paymasters or require certain verification steps before including a sponsored tx in a bundle. This is a network-level guardrail.

• Key Benefit: Protocol-level defense against known attack vectors.
• Key Flaw: Reactive, not proactive; cannot stop novel paymaster exploits.

The most dangerous pattern. Protocols like OpenZeppelin's ERC-2771 (meta-transactions) combined with a trusted forwarder allow a relayer to impersonate the user for any call. If the relayer is compromised or malicious, it's a direct ATO. This architecture inherently centralizes trust in the relay service.

• Key Problem: Relayer holds ultimate power; a breach = total loss.
• Industry Response: Moving towards ERC-4337, but legacy systems remain.

Relayers are centralized bottlenecks. A malicious or compromised relay can censor, front-run, or reorder your user's transactions, breaking the trustless promise.\n- Key Risk: Single point of failure for millions of dollars in user assets.\n- Solution Pattern: Use decentralized relay networks like Gelato or Biconomy, or implement a permissionless, open relay specification.

ERC-4337 Paymasters require users to grant token spending approvals. A buggy or malicious paymaster contract can drain the entire approved balance, not just the gas fee.\n- Key Risk: Unlimited approval to a complex, upgradeable contract.\n- Solution Pattern: Implement spending limits or use ERC-20 permit for single-use signatures. Audit paymaster logic as rigorously as your core protocol.

User signatures for sponsored transactions must be tightly scoped to a specific UserOperation. Weak construction leads to replay attacks across chains or sessions.\n- Key Risk: A signature for a $1 gas payment reused to steal an NFT or ERC-20s.\n- Solution Pattern: Enforce EIP-712 typed data with explicit chain IDs, nonces, and deadline fields. Treat the signature like a blank check.

Dynamic gas sponsorship relies on oracles for exchange rates and policy. Manipulating the price feed or rule engine can bankrupt the sponsor or deny service.\n- Key Risk: $10M+ subsidy pools drained via manipulated ETH/USD price.\n- Solution Pattern: Use decentralized oracles (Chainlink, Pyth) and implement circuit breakers. Never let a single data source control the treasury spigot.

Public mempools expose pending sponsored transactions. Bots can copy the UserOperation, replace the beneficiary, and pay a higher fee to steal the execution right and the subsidy.\n- Key Risk: Sponsor pays for a malicious actor's transaction.\n- Solution Pattern: Use private transaction pools (e.g., Flashbots Protect, BloXroute) or commit-reveal schemes to hide intent until execution.

ERC-4337's security model depends on honest bundlers. A bundler can exploit its privileged position to extract MEV, steal gas, or censor. Your smart account's safety is only as strong as the weakest bundler it accepts.\n- Key Risk: Systemic failure if a major bundler (e.g., Stackup, Alchemy) is compromised.\n- Solution Pattern: Design for bundler decentralization. Use reputation systems and allow users to specify trusted bundler endpoints.