Frontier is an Ethereum compatibility layer for Substrate. Prior to commit d3beddc6911a559a3ecc9b3f08e153dbe37a8658, the worst case weight was always accounted as the block weight for all cases. In case of large EVM gas refunds, this can lead to block spamming attacks -- the adversary can construct blocks with transactions that have large amount of refunds or unused gases with reverts, and as a result inflate up the chain gas prices. The impact of this issue is limited in that the spamming attack would still be costly for any adversary, and it has no ability to alter any chain state. This issue has been patched in commit d3beddc6911a559a3ecc9b3f08e153dbe37a8658. There are no known workarounds.