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Background

The upcoming Istanbul Ethereum hardfork contains the EIP-1884. EIP-1884 will, besides other things, raise the gas cost of the EVM SLOAD instruction from 200 to 800 gas.

Issue Description

When the TotlePrimary contract executes the performSwap function, it might execute the minimumRateFailed function, which internally calls getDecimals on the target token contract. As part of this call, getDecimals only passes 5000 gas:

If this the target token is Augur's REP token, then the Delegator will be called. This call currently consumes 4511 gas and successfully returns 18.

However, this call also contains three invocations of SLOAD, so it will cost 6311 after the activation of the Istanbul hard fork. Then, the call will fail (as it is out-of-gas). TotlePrimary will interpret the falling call as 0 decimals:

As the decimals are used to calculate token quantities, incorrect token quantities will be computed. Hence, users would be as the security check for the minimum rate could be circumvented.

Example

The scenario described above recently took place in the following transaction.
This trade converted 0.06978 Ether into 1.497 Reputation (REP) tokens.

Additional Information

ChainSecurity has conducted some analysis on the effects of EIP-1884. The intermediate results can be found here.

Martin Holst Swende has collected information on the effects of EIP-1884 here.

Contact

Feel free to contact [email protected] or @HRitzdorf on Telegram in case of questions or comments.