Advanced Token Approval Auditing: Building a Multi-Chain Security Workflow to Prevent Drain Attacks After the 1inch Fusion Exploit

The March 2025 1inch Fusion v1 exploit — which leveraged a calldata corruption vulnerability to drain $5 million from resolver contracts — serves as a technical inflection point for anyone managing significant value across DeFi protocols. While the attack targeted resolver market maker contracts rather than end users directly, it exposed a systemic weakness in how DeFi protocols manage deprecated code and, by extension, how users manage their exposure to potentially vulnerable contracts. This tutorial walks through building a comprehensive, multi-chain token approval auditing workflow that goes far beyond basic approval revocation.

The Objective

The goal is to construct a repeatable, automated security workflow that audits all token approvals across every chain where you hold assets, identifies approvals to potentially compromised or deprecated contracts, prioritizes them by risk level, and enables rapid revocation. This workflow should be executable on a regular cadence — weekly for active DeFi users, monthly for more passive holders — and should generate a documented audit trail for your records.

The 1inch exploit demonstrates why this matters. The vulnerability existed in deprecated Fusion v1 code that remained active on-chain. Users who had granted token approvals to the old resolver contracts maintained those permissions even after 1inch migrated to newer versions. Any approval to a deprecated, unaudited, or potentially vulnerable contract represents an attack surface that should be systematically identified and eliminated.

Prerequisites

Before starting, ensure you have the following tools and access configured. You need a Web3 wallet with transaction capability on each chain you use — MetaMask, Rabby, or Frame are recommended for their advanced transaction simulation features. Install Revoke.cash as a bookmark or browser extension for quick approval management. Set up a spreadsheet or database to track your approval audit history — Google Sheets with timestamp columns works well for most users.

For automated scanning, you will need basic familiarity with Etherscan API or equivalent block explorers for each chain (Arbiscan for Arbitrum, BscScan for BNB Chain, and so on). A free API key from each explorer is sufficient for personal auditing purposes. If you use a portfolio tracker like Zapper or Zerion, ensure it is connected to all your wallets and chains for a unified view of your positions.

Most importantly, have your hardware wallet accessible. Revoking approvals requires an on-chain transaction, which must be signed. Using a hardware wallet for this step ensures that even if your computer is compromised, the approval revocation transactions cannot be tampered with.

Step-by-Step Walkthrough

Step 1: Enumerate all active wallets and chains. List every wallet address you use for DeFi, including hardware wallets, browser wallets, and any smart contract wallets like Safe (formerly Gnosis Safe). For each wallet, list every chain where you have active positions — Ethereum, Arbitrum, Optimism, Polygon, Base, BNB Chain, Avalanche, and any others. Missing a chain means missing potential exposure.

Step 2: Export current approvals for each wallet-chain combination. For each wallet address on each chain, visit Revoke.cash and connect your wallet. The tool displays all active token approvals — the contract addresses you have authorized to spend your tokens, the token types, and the approval amounts. Export this data for each chain. Alternatively, use the Etherscan API’s token approval endpoint to programmatically pull this data: call the token approval module endpoint with your address.

Step 3: Cross-reference approvals against known vulnerable contracts. Maintain a list of contracts associated with known exploits — security firms like SlowMist, CertiK, and BlockSec regularly publish lists of compromised contract addresses. Cross-reference your active approvals against these lists. For the 1inch exploit specifically, identify any approvals to the deprecated Fusion v1 settlement contract address (available in the post-mortem reports from Decurity and SlowMist). Any match should be flagged as critical priority.

Step 4: Classify approvals by risk tier. Categorize each approval into risk tiers. Tier 1 (Critical): approvals to deprecated, exploited, or unaudited contracts. Tier 2 (High): unlimited approvals to any contract. Tier 3 (Medium): approvals to audited, active protocol contracts with reasonable limits. Tier 4 (Low): approvals to well-known, frequently audited contracts with limited spend amounts. Focus revocation efforts on Tier 1 and Tier 2 approvals first.

Step 5: Execute targeted revocations. For each Tier 1 approval, revoke immediately using Revoke.cash or by sending a zero-amount approval transaction directly to the token contract. For Tier 2 unlimited approvals, either revoke entirely or reduce to the specific amount needed for your next intended transaction. Most DeFi protocols function perfectly well with limited approvals — the unlimited approval is a convenience feature, not a requirement.

Step 6: Document and schedule recurring audits. Record all findings and actions in your audit spreadsheet, including the date, wallet, chain, contract address, token, approval amount, risk tier, and action taken. Set a calendar reminder for your next audit cycle. For users with significant DeFi exposure, consider building a simple script that queries the Etherscan API weekly and emails you a summary of any new approvals.

Troubleshooting

If Revoke.cash fails to load approvals for a specific chain, try the block explorer’s native token approval interface directly. Some newer chains may not be fully supported by third-party tools immediately after launch. For approvals that fail to revoke — which can happen with proxy contracts or unusual token standards — you may need to interact with the token contract directly using a tool like Cast (from the Foundry suite) or by calling the approve function with a zero amount through your wallet’s custom contract interaction feature.

If you encounter a gas estimation error when revoking, the target contract may have a non-standard approval function or may have been designed to prevent revocation. In extreme cases, you may need to move your tokens to a fresh wallet address that has no approvals granted — effectively starting with a clean slate. This is a last resort but provides absolute certainty that no hidden approvals remain active.

For multi-signature wallets like Safe, approval revocation requires coordination among all signers. Plan these operations in advance and ensure quorum availability before initiating. The extra coordination overhead is a worthwhile trade-off for the enhanced security that multi-sig provides.

Mastering the Skill

To move beyond basic approval auditing, consider implementing transaction simulation before every significant DeFi interaction. Tools like Tenderly and Blocknative’s transaction preview simulate exactly what a transaction will do before you sign it, including which contracts it interacts with and what approvals it requests. This pre-flight check can prevent you from granting approvals to unexpected or malicious contracts in the first place.

For power users, building a custom monitoring dashboard that tracks approvals in real-time provides the highest level of security. Using tools like The Graph to index approval events for your addresses, combined with automated alerts via Telegram or Discord, creates an early warning system that notifies you the moment a new approval is granted — whether by you or by any unauthorized interaction with your wallet.

The 1inch Fusion exploit cost $5 million and was caused by code that should have been deactivated. Your token approvals are the permissions that make such exploits possible. By building a systematic auditing workflow, you transform approval management from a reactive panic response into a proactive security practice. In a space where a single unchecked approval can result in total fund loss, this discipline is not optional — it is the cost of doing business in DeFi.

Disclaimer: This article is for educational purposes only and does not constitute financial or security advice. Always verify contract addresses and perform your own due diligence before executing on-chain transactions.