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code-423n4 / 2021-12-mellow-findings

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The Contract Should Approve(0) first #115

Open code423n4 opened 2 years ago

code423n4 commented 2 years ago

Handle

defsec

Vulnerability details

Impact

Some tokens (like USDT L199) do not work when changing the allowance from an existing non-zero allowance value. They must first be approved by zero and then the actual allowance must be approved.

IERC20(token).approve(address(operator), 0);
IERC20(token).approve(address(operator), amount);

Proof of Concept

  1. Navigate to the following contracts.
https://github.com/code-423n4/2021-12-mellow/blob/6679e2dd118b33481ee81ad013ece4ea723327b5/mellow-vaults/contracts/YearnVault.sol#L106

https://github.com/code-423n4/2021-12-mellow/blob/6679e2dd118b33481ee81ad013ece4ea723327b5/mellow-vaults/contracts/UniV3Vault.sol#L228

https://github.com/code-423n4/2021-12-mellow/blob/6679e2dd118b33481ee81ad013ece4ea723327b5/mellow-vaults/contracts/AaveVault.sol#L103

https://github.com/code-423n4/2021-12-mellow/blob/6679e2dd118b33481ee81ad013ece4ea723327b5/mellow-vaults/contracts/ERC20Vault.sol#L104
  1. When trying to re-approve an already approved token, all transactions revert and the protocol cannot be used.

Tools Used

None

Recommended Mitigation Steps

Approve with a zero amount first before setting the actual amount.

MihanixA commented 2 years ago

Duplicates https://github.com/code-423n4/2021-12-mellow-findings/issues/14 IMO