This is a fairly simple bug that let anyone mint rewardTokens.

Background: https://x.com/storming0x/status/1473321779250802693

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Description

  • The deposit function lets anyone mint rewardTokens if
    • the from address is a contract and has an owner and where msg.sender == owner
  • It is intended that the from address sends the underlying token to this contract
    • But it does not check whether delegatedTransferERC20 is implemented correctly
    • it can be a noop
  • The attacker can mint rewardTokens to himself

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Solution

While this is an obvious bug, an invariant could have prevented an exploit of the bug.

Invariant:

function assertionIsSolvent() public view {
  require(visr.balanceOf(address(assertionAdopter)) >= vvisr.totalSupply(), "Reward tokens are undercollateralized");
}

Alternatively, the assertion could test the correct behavior of the previous transaction. The assertion could be triggered by a call to the deposit function.

function assertion_triggerDeposit_remainsSolvent() public view {

  uint256 postBalanceCollateral = visr.balanceOf(address(adopter));
  uint256 postTotalSupplyRewards = vvisr.totalSupply();
  uint256 postRatio = postBalanceCollateral / postTotalSupplyRewards;
  ph.forkPreState();

  uint256 preBalanceCollateral = visr.balanceOf(address(adopter));
  uint256 preTotalSupplyRewards = vvisr.totalSupply();
  uint256 preRatio = preBalanceCollateral / preTotalSupplyRewards;

  require(preRatio == postRatio, "Reward tokens are undercollateralized");
}
Cream Finance 2First Depositor Bug