Smart Contract Security | Ethereum Foundation Blog

Solidity was began in October 2014 when neither the Ethereum community nor the digital machine had any real-world testing, the gasoline prices at the moment had been even drastically totally different from what they’re now. Moreover, a number of the early design selections had been taken over from Serpent. Over the last couple of months, examples and patterns that had been initially thought of best-practice had been uncovered to actuality and a few of them truly turned out to be anti-patterns. Resulting from that, we just lately up to date a number of the Solidity documentation, however as most individuals most likely don’t comply with the stream of github commits to that repository, I want to spotlight a number of the findings right here.

I cannot discuss in regards to the minor points right here, please learn up on them within the documentation.

Sending Ether

Sending Ether is meant to be one of many easiest issues in Solidity, nevertheless it seems to have some subtleties most individuals don’t realise.

It is necessary that at finest, the recipient of the ether initiates the payout. The next is a BAD instance of an public sale contract:

// THIS IS A NEGATIVE EXAMPLE! DO NOT USE!
contract public sale {
  handle highestBidder;
  uint highestBid;
  operate bid() {
    if (msg.worth  highestBid) throw;
    if (highestBidder != 0)
      highestBidder.ship(highestBid); // refund earlier bidder
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
}

Due to the maximal stack depth of 1024 the brand new bidder can at all times enhance the stack measurement to 1023 after which name bid() which is able to trigger the ship(highestBid) name to silently fail (i.e. the earlier bidder won’t obtain the refund), however the brand new bidder will nonetheless be highest bidder. One approach to verify whether or not ship was profitable is to verify its return worth:

/// THIS IS STILL A NEGATIVE EXAMPLE! DO NOT USE!
if (highestBidder != 0)
  if (!highestBidder.ship(highestBid))
    throw;
The 
throw
 assertion causes the present name to be reverted. This can be a dangerous concept, as a result of the recipient, e.g. by implementing the fallback operate as 
operate() { throw; }
 can at all times power the Ether switch to fail and this may have the impact that no person can overbid her.

The one approach to forestall each conditions is to transform the sending sample right into a withdrawing sample by giving the recipient management over the switch:

/// THIS IS STILL A NEGATIVE EXAMPLE! DO NOT USE!
contract public sale {
  handle highestBidder;
  uint highestBid;
  mapping(handle => uint) refunds;
  operate bid() {
    if (msg.worth  highestBid) throw;
    if (highestBidder != 0)
      refunds[highestBidder] += highestBid;
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
  operate withdrawRefund() {
    if (msg.sender.ship(refunds[msg.sender]))
      refunds[msg.sender] = 0;
  }
}
 

Why does it nonetheless say “damaging instance” above the contract? Due to gasoline mechanics, the contract is definitely fantastic, however it’s nonetheless not a superb instance. The reason being that it’s unattainable to stop code execution on the recipient as a part of a ship. Because of this whereas the ship operate continues to be in progress, the recipient can name again into withdrawRefund. At that time, the refund quantity continues to be the identical and thus they might get the quantity once more and so forth. On this particular instance, it doesn’t work, as a result of the recipient solely will get the gasoline stipend (2100 gasoline) and it’s unattainable to carry out one other ship with this quantity of gasoline. The next code, although, is weak to this assault: msg.sender.name.worth(refunds[msg.sender])().

Having thought of all this, the next code needs to be fantastic (after all it’s nonetheless not a whole instance of an public sale contract):

contract public sale {
  handle highestBidder;
  uint highestBid;
  mapping(handle => uint) refunds;
  operate bid() {
    if (msg.worth  highestBid) throw;
    if (highestBidder != 0)
      refunds[highestBidder] += highestBid;
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
  operate withdrawRefund() {
    uint refund = refunds[msg.sender];
    refunds[msg.sender] = 0;
    if (!msg.sender.ship(refund))
     refunds[msg.sender] = refund;
  }
}

Be aware that we didn’t use throw on a failed ship as a result of we’re in a position to revert all state modifications manually and never utilizing throw has lots much less side-effects.

Utilizing Throw

The throw assertion is usually fairly handy to revert any modifications made to the state as a part of the decision (or complete transaction relying on how the operate known as). It’s important to bear in mind, although, that it additionally causes all gasoline to be spent and is thus costly and can doubtlessly stall calls into the present operate. Due to that, I want to advocate to make use of it solely within the following conditions:

1. Revert Ether switch to the present operate

If a operate is just not meant to obtain Ether or not within the present state or with the present arguments, it’s best to use throw to reject the Ether. Utilizing throw is the one approach to reliably ship again Ether due to gasoline and stack depth points: The recipient may need an error within the fallback operate that takes an excessive amount of gasoline and thus can not obtain the Ether or the operate may need been known as in a malicious context with too excessive stack depth (maybe even previous the calling operate).

Be aware that unintentionally sending Ether to a contract is just not at all times a UX failure: You’ll be able to by no means predict by which order or at which era transactions are added to a block. If the contract is written to solely settle for the primary transaction, the Ether included within the different transactions needs to be rejected.

2. Revert results of known as features

In case you name features on different contracts, you may by no means understand how they’re applied. Because of this the results of those calls are additionally not know and thus the one approach to revert these results is to make use of throw. In fact it’s best to at all times write your contract to not name these features within the first place, if you recognize you’ll have to revert the results, however there are some use-cases the place you solely know that after the actual fact.

Loops and the Block Gasoline Restrict

There’s a restrict of how a lot gasoline might be spent in a single block. This restrict is versatile, however it’s fairly arduous to extend it. Because of this each single operate in your contract ought to keep beneath a specific amount of gasoline in all (affordable) conditions. The next is a BAD instance of a voting contract:

/// THIS IS STILL A NEGATIVE EXAMPLE! DO NOT USE!
contract Voting {
  mapping(handle => uint) voteWeight;
  handle[] yesVotes;
  uint requiredWeight;
  handle beneficiary;
  uint quantity;
  operate voteYes() { yesVotes.push(msg.sender); }
  operate tallyVotes() {
    uint yesVotes;
    for (uint i = 0; i  yesVotes.size; ++i)
      yesVotes += voteWeight[yesVotes[i]];
    if (yesVotes > requiredWeight)
      beneficiary.ship(quantity);
  }
}

The contract truly has a number of points, however the one I want to spotlight right here is the issue of the loop: Assume that vote weights are transferrable and splittable like tokens (consider the DAO tokens for instance). This implies you could create an arbitrary variety of clones of your self. Creating such clones will enhance the size of the loop within the tallyVotes operate till it takes extra gasoline than is obtainable inside a single block.

This is applicable to something that makes use of loops, additionally the place loops usually are not explicitly seen within the contract, for instance if you copy arrays or strings inside storage. Once more, it’s fantastic to have arbitrary-length loops if the size of the loop is managed by the caller, for instance should you iterate over an array that was handed as a operate argument. However by no means create a scenario the place the loop size is managed by a celebration that will not be the one one affected by its failure.

As a facet word, this was one motive why we now have the idea of blocked accounts contained in the DAO contract: Vote weight is counted on the level the place the vote is forged, to stop the truth that the loop will get caught, and if the vote weight wouldn’t be fastened till the tip of the voting interval, you would forged a second vote by simply transferring your tokens after which voting once more.

Receiving Ether / the fallback operate

If you would like your contract to obtain Ether through the common ship() name, it’s a must to make its fallback operate low cost. It may solely use 2300, gasoline which neither permits any storage write nor operate calls that ship alongside Ether. Principally the one factor it’s best to do contained in the fallback operate is log an occasion in order that exterior processes can react on the actual fact. In fact any operate of a contract can obtain ether and isn’t tied to that gasoline restriction. Features truly must reject Ether despatched to them if they don’t wish to obtain any, however we’re fascinated about doubtlessly inverting this behaviour in some future launch.