The Unlimited Token Approval That Drained a DeFi Portfolio
Six months after using a DEX aggregator, a vulnerability in the contract was exploited. Because the user had granted unlimited token approval, the attacker drained every approved token -$28K across 4 assets.
The Incident
The victim had used a lesser-known DEX aggregator six months prior for a single token swap. During the swap, the dApp requested unlimited token approval for USDC, WETH, LINK, and UNI -a common pattern in DeFi interfaces that optimizes for convenience. The victim approved all four without modifying the amounts, completed the swap, and never used the protocol again. Six months later, a vulnerability was discovered in the aggregator's smart contract that allowed arbitrary transferFrom calls. An attacker exploited this vulnerability to drain approved tokens from every wallet that had outstanding unlimited approvals. The victim lost $28,000 across the four approved tokens.
Attack Vector: Unlimited ERC-20 Approvals
The ERC-20 approve() function allows a contract to spend tokens on your behalf up to a specified amount. When you approve MAX_UINT256 (the maximum possible value -approximately 1.15 x 10^77 tokens), you're granting permanent, unlimited permission. The contract can call transferFrom to move any amount of that token from your wallet at any time. This approval persists until you explicitly revoke it. The vulnerability in the aggregator contract allowed the attacker to call the contract's swap function with crafted parameters that redirected the transferFrom output to their own address instead of the intended liquidity pool.
On-Chain Trail
The attacker executed the drain in a single block using a custom contract that batched transferFrom calls across hundreds of victim wallets. This is a common pattern in approval exploits -batch everything into one transaction to minimize the window for response. The drained tokens were immediately swapped to ETH through Uniswap, then bridged across multiple chains. An MEV bot attempted to front-run some of the recovery transactions initiated by white hat researchers, further complicating the situation. The total exploit across all victims exceeded $3 million, of which the victim in this case study lost $28,000.
Outcome
The $28,000 was not recovered. The aggregator protocol had no insurance fund, bug bounty program, or treasury to compensate affected users. The protocol team published a post-mortem acknowledging the vulnerability but stated they had no means to make users whole. Some affected users joined a class action lawsuit against the protocol's development company, but the company was incorporated in a jurisdiction with limited legal recourse. The victim had to manually check and revoke all remaining approvals across their wallets to prevent similar attacks from other approved contracts.
Prevention
Always modify token approval amounts to the exact amount needed for the transaction -not unlimited. Most modern DeFi interfaces show an 'Edit' button on the approval prompt where you can enter a specific amount. After completing a swap or interaction, revoke the approval using revoke.cash or Etherscan's token approval checker. Use a hardware wallet so every approval requires physical confirmation -this forces you to pause and consider what you're approving. Consider using a separate wallet for interacting with unaudited or new protocols, keeping your main holdings in a wallet with zero outstanding approvals.