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

I cannot speak 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, however it seems to have some subtleties most individuals don’t realise.

It will be significant that at greatest, 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 {
  deal with 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 all the time 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 technique 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. It is a dangerous concept, as a result of the recipient, e.g. by implementing the fallback operate as 
operate() { throw; }
 can all the time power the Ether switch to fail and this might have the impact that no person can overbid her.

The one technique to stop 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 {
  deal with highestBidder;
  uint highestBid;
  mapping(deal with => 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 “adverse instance” above the contract? Due to fuel mechanics, the contract is definitely wonderful, however it’s nonetheless not a very good instance. The reason being that it’s inconceivable 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 fuel stipend (2100 fuel) and it’s inconceivable to carry out one other ship with this quantity of fuel. The next code, although, is weak to this assault: msg.sender.name.worth(refunds[msg.sender])().

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

contract public sale {
  deal with highestBidder;
  uint highestBid;
  mapping(deal with => 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;
  }
}

Observe 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 commonly 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 remember, although, that it additionally causes all fuel to be spent and is thus costly and can probably 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, you must use throw to reject the Ether. Utilizing throw is the one technique to reliably ship again Ether due to fuel and stack depth points: The recipient may need an error within the fallback operate that takes an excessive amount of fuel and thus can’t obtain the Ether or the operate may need been referred to as in a malicious context with too excessive stack depth (maybe even previous the calling operate).

Observe that unintentionally sending Ether to a contract is just not all the time a UX failure: You possibly can 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 referred to as capabilities

Should you name capabilities on different contracts, you’ll be able to by no means understand how they’re carried out. Because of this the consequences of those calls are additionally not know and thus the one technique to revert these results is to make use of throw. After all you must all the time write your contract to not name these capabilities within the first place, if you realize you’ll have to revert the consequences, however there are some use-cases the place you solely know that after the very 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 laborious to extend it. Because of this each single operate in your contract ought to keep under a certain quantity of fuel 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(deal with => uint) voteWeight;
  deal with[] yesVotes;
  uint requiredWeight;
  deal with 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 example). 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 fuel 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 wonderful to have arbitrary-length loops if the size of the loop is managed by the caller, for instance if you happen to 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 might not be the one one affected by its failure.

As a facet notice, 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 possibly can forged a second vote by simply transferring your tokens after which voting once more.

Receiving Ether / the fallback operate

If you need your contract to obtain Ether through the common ship() name, you must make its fallback operate low-cost. It could solely use 2300, fuel which neither permits any storage write nor operate calls that ship alongside Ether. Mainly the one factor you must do contained in the fallback operate is log an occasion in order that exterior processes can react on the very fact. After all any operate of a contract can obtain ether and isn’t tied to that fuel restriction. Features truly should reject Ether despatched to them if they don’t wish to obtain any, however we’re fascinated by probably inverting this behaviour in some future launch.