oracle: wrong price for asset if underlying aggregator hits minAnswer

[cvetanovv]

Summary

Chainlink aggregators have a built in circuit breaker if the price of an asset goes outside of a predetermined price band. The result is that if an asset experiences a huge drop in value (i.e. LUNA crash) the price of the oracle will continue to return the minPrice instead of the actual price of the asset. This would allow user to continue borrowing with the asset but at the wrong price. This is exactly what happened to Venus on BSC when LUNA imploded.

Vulnerability Detail

ChainlinkAdapterOracle uses the ChainlinkFeedRegistry to obtain the price of the requested tokens.

    function latestRoundData(
      address base,
      address quote
    )
      external
      view
      override
      checkPairAccess()
      returns (
        uint80 roundId,
        int256 answer,
        uint256 startedAt,
        uint256 updatedAt,
        uint80 answeredInRound
      )
    {
      uint16 currentPhaseId = s_currentPhaseId[base][quote];
      //@audit this pulls the Aggregator for the requested pair
      AggregatorV2V3Interface aggregator = _getFeed(base, quote);
      require(address(aggregator) != address(0), "Feed not found");
      (
        roundId,
        answer,
        startedAt,
        updatedAt,
        answeredInRound
      ) = aggregator.latestRoundData();
      return _addPhaseIds(roundId, answer, startedAt, updatedAt, answeredInRound, currentPhaseId);
    }

ChainlinkFeedRegistry latestRoundData() pulls the associated aggregator and requests round data from it. ChainlinkAggregators have minPrice and maxPrice circuit breakers built into them. This means that if the price of the asset drops below the minPrice, the protocol will continue to value the token at minPrice instead of it's actual value. This will allow users to take out huge amounts of bad debt and bankrupt the protocol.

Example: TokenA has a minPrice of $1. The price of TokenA drops to $0.10. The aggregator still returns $1 allowing the user to borrow against TokenA as if it is $1 which is 10x it's actual value.

Impact

In the event that an asset crashes (i.e. LUNA) the protocol can be manipulated to give out loans at an inflated price

Code Snippet

https://github.com/Fujicracy/fuji-v2/blob/1b939ec84af137db430fc2aa1b4c6f15e5254003/packages/protocol/src/FujiOracle.sol#L113-L115

Recommendation

FujiOracle should check the returned answer against the minPrice/maxPrice and revert if the answer is outside of the bounds:

113: (, int256 latestPrice,,,) = IAggregatorV3(usdPriceFeeds[asset]).latestRoundData();

114:  if (latestPrice >= maxPrice or answer <= minPrice) revert();