Exploring Smart Contract Vulnerabilities: The King of Ether

Now we will go deeper in the world of intelligent contract vulnerabilities, focusing in a intriguing weakness that came into prominence due to Ethereum-based auction sport. This attack occurred due to misallocation of gasoline. Before we enter the facts of the assault, let us look in the match itself and the notion of gasoline.

Exploring Smart Contract Vulnerabilities: The King of Ether

Therefore, the”King of the Ether Throne” (KotET) is a sport in which players compete with each other to the title of”King of the Ether.” So, how can this work?

By easy bidding. This is how the bidding process works based on their own site:

  • Suppose the current claim price for the throne is 10 ether.
  • You want to be King/Queen, so you send 10 ether to the contract.
  • The contract sends your 10 ether (less a 1% commission) to the previous King/Queen, as a “compensation payment”.
  • The contract makes you the new King/Queen of the Ether Throne.
  • The new claim price for the throne goes up by 50%, to 15 ether in this case.
  • If a usurper comes along who is willing to pay 15 ether, they depose you and become King/Queen, and you receive their payment of 15 ether as your “compensation payment”.

Basically, it’s a Ponzi Scheme.

Before we proceed with exactly what this ties in with intelligent contract vulnerabilities, let us know how gas functions.

What is Ethereum Gas?

“gas” is the lifeblood of this Ethereum ecosystem, there’s not any other method of putting this. Gas is a device that measures the total amount of computational effort it will require to perform specific operations.

Each and every operation that participate in Ethereum, make it a very easy trade, or even a wise contract, or an ICO takes a little quantity of gasoline. Gas is what’s used to figure out the amount of charges which have to be compensated into the community so as to execute a procedure.

All of the wise contracts which operate in the EVM are coded with solidity (Ethereum will proceed to Vyper in Solidity later on.) Every single line of code inside solidity demands a specific quantity of gasoline to receive computed.

To understand how gasoline functions in Ethereum, let us use an analogy. Suppose you’re moving to get a road trip. Before you do this that you go through these measures:

  • You go to the gas station and specify how much gas you want to fill up in your car.
  • You get that gas filled up in your car.
  • You pay the gas station the amount of money you owe them for the gas.

Now, let’s draw parallels with Ethereum.

  • The car is the operation that you want to execute, like a gas or a smart contract.
  • The gas station is your miner.
  • The money that you paid them is the miner fees.

All the operations that users want to execute in there must provide gas for the following:

  • To cover its data aka intrinsic gas.
  • To cover its entire computation.

What is Gas Limit?

To be able to find an operation performed in Ethereum, the performance generator (i.e. that the individual initiating the trade or the wise contract founder ) must define a gasoline limitation before they apply it to the miners. When a gasoline limitation was given only afterward will the miners begin executing the surgery.

After submitting a gasoline limitation, these points should be considered:

  • Different operations will have different gas costs (as has been shown before).
  • The miners will stop executing the moment the gas runs out.
  • If there is any gas left over, it will be immediately refunded to the operation generator.

Let us see this in performance at a hypothetical situation:

Suppose, We’re adding two amounts and therefore, the arrangement has to perform the following activities:

  • Storing 10 in a variable. Let’s say this operation costs 45 wei gas.
  • Adding two variables, let’s say this costs 10 wei gas.
  • Storing the result which again costs 45 wei gas.

Suppose the gas limit is 120 wei.

The total gas used by the miner is (45+10+45) 100 wei.

The fees that is owed to them assuming 1 wei costs 0.02 micro ETH is (100*0.02 micro ETH) = 0.000002 ETH.

Now, how much gas is left over?

120 – 100 = 20 wei.

The 20 wei is refunded back to the operation generator.

So what went wrong with KotET?

KotET contracts acted fairly normally except during a situation. As soon as the KotET contract delivered a payment into a”contract accounts”, it only allocated a little bit of gasoline…2300 to become precise. That’s obviously insufficient gasoline to pay the costs.

As KotET puts it,

“After a pocket contract neglected to process the charge delivered to it from the KotET contract, then the ether paid has been returned into the KotET contract. The KotET wasn’t conscious that the charge had neglected and it lasted processing, which makes the caller King regardless of the reparation payment never having been shipped into the preceding monarch.”

Let’s try and breakdown everything that went wrong here:

  • The mist wallet contract which was insufficient for the payment to be accepted by the wallet contract.
  • KotET contract developer was unaware of the 2300 gas limit which was included when sending the payment.
  • KotET contract developer was unaware of that any part of the transaction could fail and revert without the whole chain failing and going back.
  • Minimal beta testing in the real-world by the KotET contract developers.
  • The fallback function in the wallet contracts used by the Ethereum Mist Wallet requires more gas than 2300 available during a <address>.send(<amount>) call.

Looking at the Code

This contract is a simplified representation of the game:

Exploring Smart Contract Vulnerabilities: The King of Ether

According to the contract, even as soon as a player sends ether into the contract (msg.value), they also activate KotET’s fallback from default. Even the KotET fallback first tests if the shipped ether is sufficient to acquire the name.

Otherwise, it reverts the ether move. If so, the player is selected as the new king. When that comes to pass, that the compensations is delivered to the preceding king as well as the player is crowned. The gap between the cash delivered by the king as well as the reimbursement to the older king is retained from the contract. The KotET proprietor can draw all these ethers gathered from the contract via sweepCommission.

The vulnerability lies the ship function in the event the function has insufficient gasoline, the king.send(reimbursement ) will fail if the king’s speech is due to a contract having a costly fallback. In cases like this, the settlement is only kept from the contract rather than the preceding king.


We expect that you learnt from KotET’s error. It’s little mistakes such as those which can result in a great deal of havoc if not fixed in time. If it comes to intelligent contract programming, focus on detail is of utmost importance to prevent clever contract vulnerabilities.


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