Basket becomes unusable if everybody burns their shares

[code423n4]

Handle

kenzo

Vulnerability details

While handling the fees, the contract calculates the new ibRatio by dividing by totalSupply. This can be 0 leading to a division by 0.

Impact

If everybody burns their shares, in the next mint, totalSupply will be 0, handleFees will revert, and so nobody will be able to use the basket anymore.

Proof of Concept

Vulnerable line:
https://github.com/code-423n4/2021-09-defiProtocol/blob/52b74824c42acbcd64248f68c40128fe3a82caf6/contracts/contracts/Basket.sol#L124
You can add the following test to Basket.test.js and see that it reverts:
it("should divide by 0", async () => {
await basket.connect(addr1).burn(await basket.balanceOf(addr1.address));
await basket.connect(addr2).burn(await basket.balanceOf(addr2.address));

  await UNI.connect(addr1).approve(basket.address, ethers.BigNumber.from(1));
  await COMP.connect(addr1).approve(basket.address, ethers.BigNumber.from(1));
  await AAVE.connect(addr1).approve(basket.address, ethers.BigNumber.from(1));
  await basket.connect(addr1).mint(ethers.BigNumber.from(1));

});

Tools Used

Manual analysis, hardhat.

Recommended Mitigation Steps

Add a check to handleFees: if totalSupply= 0, you can just return, no need to calculate new ibRatio / fees.
You might want to reset ibRatio to BASE at this point.

[GalloDaSballo]

I feel like this can also happen right after the contract was created
mintTo will call handleFees which will revert due to division by 0

Finding is valid