Does revoking an allowance cost gas?

Yes. Revoking an allowance requires an on-chain transaction that sets the approved amount to zero. Each revocation costs a small gas fee, similar to a simple token transfer.

How do I know if an approval is unlimited?

Blockchain explorers and revocation tools display the approved amount. If it shows a very large number (often displayed as 'Unlimited' or a number with dozens of digits), it is an unlimited approval.

Can a contract spend my tokens without an active allowance?

No. Without an active allowance, a contract cannot call transferFrom to move your tokens. However, native assets like ETH do not use the approval pattern and are sent directly in transactions.

How to Check ERC-20 Token Allowance

5 min readCrypto

Every time you use a DeFi protocol, you typically approve it to spend your tokens via an ERC-20 allowance. Many approvals are set to unlimited by default, creating a security risk if the approved contract is later compromised. This guide explains how allowances work, how to check them, and when to revoke them.

Quick Steps

1
Learn about ERC-20 allowances
Open the ERC-20 Allowance Checker on Toolin to understand how token approvals work.
2
Review the approve/transferFrom pattern
Understand that every DeFi interaction typically requires an approval transaction.
3
Check your active approvals
Use Revoke.cash or Etherscan to view all contracts authorized to spend your tokens.
4
Identify risky allowances
Flag unlimited approvals and approvals to contracts you no longer interact with.
5
Revoke unnecessary approvals
Send a zero-amount approval transaction to revoke permissions for each risky contract.
6
Adopt safe approval habits
Going forward, approve only the exact amount needed and review allowances regularly.

ERC-20 Allowance Checker

Learn about token approvals and understand allowance risks

Open Tool

What Is an ERC-20 Allowance?

An ERC-20 allowance is a permission recorded on the blockchain that lets a smart contract spend a specific amount of your tokens on your behalf. When you click 'Approve' in your wallet before a swap or deposit, you are setting this allowance. The approve function takes two parameters: the spender address (the contract) and the amount. If the amount is set to the maximum uint256 value, the approval is effectively unlimited.

Why Unlimited Approvals Are Risky

If the approved contract has a vulnerability, attackers can drain all your approved tokens, not just the amount you intended to use.
Malicious dApps can request unlimited approval and then steal tokens at any time.
Old approvals for contracts you no longer use remain active indefinitely unless revoked.
Phishing sites disguised as legitimate dApps can trick you into approving their malicious contract.
Even audited contracts can be upgraded to malicious versions if they use proxy patterns.

Checking Your Allowances

Open the ERC-20 Allowance Checker

Navigate to the ERC-20 Allowance Checker on Toolin to learn about token approval security concepts.

Understand the approval model

Review how the ERC-20 approve/transferFrom pattern works and why it creates security exposure.

Use a blockchain explorer or revocation tool

Tools like Revoke.cash or Etherscan's token approval checker let you view all active allowances for your wallet address.

Identify high-risk approvals

Look for unlimited approvals, approvals to contracts you no longer use, and approvals to unverified contracts.

Revoke unnecessary allowances

Set the allowance to zero for any contract you no longer need. This requires a small gas fee for each revocation transaction.

Best Practices for Token Approvals

Approve only the exact amount you need for each transaction when your wallet supports custom approval amounts.
Review and revoke old approvals regularly, especially after high-profile DeFi exploits.
Use hardware wallets and verify approval transactions on the device screen before confirming.
Be cautious with new or unaudited protocols that request token approvals.

How Revocation Works On-Chain

// Solidity: ERC-20 approve function
token.approve(spenderAddress, 0);

// Setting the allowance to 0 revokes permission.
// This is a standard transaction that costs gas.
// After revocation, the spender can no longer call
// transferFrom on your tokens.

Try ERC-20 Allowance Checker Now — Free, No Sign-up

Frequently Asked Questions

Does revoking an allowance cost gas?: Yes. Revoking an allowance requires an on-chain transaction that sets the approved amount to zero. Each revocation costs a small gas fee, similar to a simple token transfer.
How do I know if an approval is unlimited?: Blockchain explorers and revocation tools display the approved amount. If it shows a very large number (often displayed as 'Unlimited' or a number with dozens of digits), it is an unlimited approval.
Can a contract spend my tokens without an active allowance?: No. Without an active allowance, a contract cannot call transferFrom to move your tokens. However, native assets like ETH do not use the approval pattern and are sent directly in transactions.