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 fuel prices at the moment have been even drastically completely different from what they’re now. Moreover, a few of the early design choices have been taken over from Serpent. Over the past couple of months, examples and patterns that have been initially thought-about best-practice have been uncovered to actuality and a few of them truly turned out to be anti-patterns. On account of that, we lately up to date a few of the Solidity documentation, however as most individuals in all probability don’t observe the stream of github commits to that repository, I want to spotlight a few of the findings right here.

I can’t discuss concerning 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 can be crucial 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;
  perform 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 improve the stack measurement to 1023 after which name bid() which can 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 strategy to test whether or not ship was profitable is to test 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 unhealthy thought, as a result of the recipient, e.g. by implementing the fallback perform as 
perform() { throw; }
 can at all times drive the Ether switch to fail and this is able to have the impact that no one can overbid her.

The one strategy 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;
  perform bid() {
    if (msg.worth < highestBid) throw;
    if (highestBidder != 0)
      refunds[highestBidder] += highestBid;
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
  perform withdrawRefund() {
    if (msg.sender.ship(refunds[msg.sender]))
      refunds[msg.sender] = 0;
  }
}
 

Why does it nonetheless say “unfavourable instance” above the contract? Due to fuel mechanics, the contract is definitely high quality, however it’s nonetheless not a very good instance. The reason being that it’s not possible to forestall code execution on the recipient as a part of a ship. Which means that whereas the ship perform remains to be in progress, the recipient can name again into withdrawRefund. At that time, the refund quantity remains 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 fuel stipend (2100 fuel) and it’s not possible to carry out one other ship with this quantity of fuel. The next code, although, is susceptible to this assault: msg.sender.name.worth(refunds[msg.sender])().

Having thought-about all this, the next code ought to be high quality (in fact it’s nonetheless not a whole instance of an public sale contract):

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

Notice that we didn’t use throw on a failed ship as a result of we’re capable of revert all state modifications manually and never utilizing throw has quite a bit much less side-effects.

Utilizing Throw

The throw assertion is commonly fairly handy to revert any modifications made to the state as a part of the decision (or entire transaction relying on how the perform known as). You must remember, although, that it additionally causes all fuel to be spent and is thus costly and can doubtlessly stall calls into the present perform. Due to that, I want to suggest to make use of it solely within the following conditions:

1. Revert Ether switch to the present perform

If a perform shouldn’t be meant to obtain Ether or not within the present state or with the present arguments, you must use throw to reject the Ether. Utilizing throw is the one strategy to reliably ship again Ether due to fuel and stack depth points: The recipient might need an error within the fallback perform that takes an excessive amount of fuel and thus can not obtain the Ether or the perform might need been referred to as in a malicious context with too excessive stack depth (even perhaps previous the calling perform).

Notice that unintentionally sending Ether to a contract shouldn’t be at all times a UX failure: You possibly can by no means predict through which order or at which period 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 must be rejected.

2. Revert results of referred to as capabilities

In case you name capabilities on different contracts, you possibly can by no means understand how they’re carried out. Which means that the results of those calls are additionally not know and thus the one strategy to revert these results is to make use of throw. In fact you must at all times write your contract to not name these capabilities 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 fuel will be spent in a single block. This restrict is versatile, however it’s fairly onerous to extend it. Which means that each single perform in your contract ought to keep beneath a specific amount of fuel in all (cheap) 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;
  perform voteYes() { yesVotes.push(msg.sender); }
  perform 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 example). This implies that you could create an arbitrary variety of clones of your self. Creating such clones will improve the size of the loop within the tallyVotes perform till it takes extra fuel than is offered inside a single block.

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

As a facet observe, 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 solid, to forestall the truth that the loop will get caught, and if the vote weight wouldn’t be mounted till the top of the voting interval, you would solid a second vote by simply transferring your tokens after which voting once more.

Receiving Ether / the fallback perform

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