In many crypto apps, the user does not send tokens directly to a protocol each time. Instead, the user grants a smart contract permission to move tokens on the user’s behalf. That permission is called an approval, and it is built on the ERC-20 allowance model.
Under this model, a wallet calls approve(spender, amount). The token contract stores an allowance for that spender. Later, the spender can call transferFrom(user, to, amount) to pull tokens up to the approved limit. OpenZeppelin’s ERC-20 documentation notes that setting allowance to the maximum uint256 behaves like an infinite approval.
Approvals exist because they enable smooth UX for swaps, vault deposits, and recurring actions. The same mechanism also creates a long-lived risk surface.
A malicious approval is an approval that gives a bad contract or attacker-controlled spender more power than the user intended, in a way that can be exploited to move assets later. The danger is not the approval transaction itself. The danger is what the approval enables after it is mined.
MetaMask defines a malicious token approval as granting excessive or dangerous permissions to a bad dapp or contract, which can lead to unauthorized spending.
Malicious approvals are common because they are easy to hide inside normal-looking flows. A fake DEX can ask for an approval as part of a swap. A fake airdrop claim can ask for a signature that authorizes spending. The user sees a familiar prompt, approves quickly, and the real drain happens later when the spender calls transferFrom.
The most common version is a request for unlimited allowance. Unlimited approvals are not always malicious. Many reputable protocols request them to avoid repeated approvals and gas costs. The risk is that unlimited allowance becomes catastrophic if the spender is malicious, compromised, upgraded to unsafe logic, or routed through a vulnerable adapter.
This is why “approve only what is needed” is a security best practice. If the spender only has 25 USDC allowance, only 25 USDC can be drained. If the spender has unlimited allowance, the entire balance can be drained.
Some token standards allow approvals via signatures rather than on-chain approve transactions. A widely used pattern is EIP-2612, often referred to as “permit,” which lets a user sign an approval off-chain and have it submitted on-chain by someone else.
Permit can be safe and convenient, but it changes user expectations. The wallet prompt may look like “Sign message,” not “Approve spending.” A malicious site can exploit that mismatch and obtain a signature that grants allowance.
The EIP-2612 specification explains how permit-based approvals work, including nonces and deadlines that are meant to prevent replay and limit the window of risk.
For NFTs, the equivalent danger is often “setApprovalForAll,” which grants an operator the right to transfer many NFTs in a collection. A malicious operator approval can lead to fast NFT draining, because the attacker does not need to prompt the user again.
Users often miss this because the UI can present it as a harmless “enable trading” step.
The sequence is usually simple.
First, the user approves a spender or signs an approval-like message. Second, the allowance or operator permission persists on-chain. Third, at any later time, the spender calls transferFrom, or the operator transfers NFTs, up to the permission level. Tokens move out without another approval prompt, because permission already exists.
This is why malicious approvals are so effective. The “moment of consent” is separated from the “moment of loss,” sometimes by hours or weeks. That separation makes the user less likely to connect the drain to the earlier approval.
A wallet prompt can show signals that are easy to miss in a rush.
A spender address or contract name is unfamiliar, or does not match the site’s brand. The amount is “unlimited,” “infinite,” or a huge number that does not match the intended action. The request appears during an action that should not need an approval, such as viewing an NFT, checking eligibility, or opening a dashboard. The site pushes urgency, time pressure, or rewards that expire quickly.
Even when a protocol is reputable, approvals should align with intent. A one-time swap does not require unlimited approvals for every token in the wallet. A mint does not require approval for unrelated ERC-20 balances.
Approvals do not expire by default. Revocation requires an on-chain transaction that sets the allowance back to zero, which costs gas.
A practical way to review permissions is to use an explorer-based tool such as the Etherscan Token Approval Checker, which lists token allowances and lets users revoke them by sending a new approve transaction with a value of zero.
The key habit is to treat approvals like open tabs. If a spender is no longer needed, closing that permission reduces the attack surface. Revoking is especially important after interacting with new dapps, short-lived farming campaigns, and airdrop claim sites.
Risk does not go to zero, but it can drop sharply with a few defaults. Limited approvals matter most for stablecoins and high-value tokens. Separate wallets help because a “testing” wallet can interact with new dapps while a “vault” wallet stays mostly approval-free. Routine revocation reduces the blast radius after experimentation.
Message signatures deserve extra skepticism. If the prompt cannot be understood, the safest choice is to refuse. Reputable dapps can explain why a signature is needed, and many safe flows show typed data fields rather than an opaque blob.
For teams and treasuries, multi-sig controls and minimal operational allowances reduce single-click failure modes. For developers, the safest pattern is to request the smallest allowance needed, explain why it is needed in the UI, and avoid “approve unlimited” as a default when a precise amount will work.
Malicious approvals weaponize a legitimate feature: permission to spend. By tricking users into granting excessive allowances or operator rights, attackers can drain tokens or NFTs later without another prompt. Safer approvals, routine revocation, and skepticism toward unknown signing requests sharply reduce the risk.
The post What Are Malicious Approvals? appeared first on Crypto Adventure.
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