The Initial Coin Offering (ICO) boom has been powered by smart contracts, making them a hotter and more lucrative target than ever before. One might expect that being well-funded by security-sensitive investors would be enough to save this technology from the security sins of its forebears. Unfortunately, smart contract security is poorly understood, even by some of the field’s most prolific and successful developers. However, the wider Ethereum community is working to change that. An amalgam of academics, security enthusiasts, and industry professionals have created the first generation of audit and development practices focused on defense of smart contracts, along with a nascent tool suite to augment them. This talk presents a digestible but robust set of tools and practices used by the authors to find real vulnerabilities in real contracts during the course of their work as security consultants. Audience members who develop smart contracts will leave with a strong understanding of development and test best practices to make sure they are not the next to be “popped”, while those interested in auditing them will learn a battle-tested practicum for finding vulns in the wild.

3. Ethereum is a computer
— GavinWood

12. what are
smart contracts
?

17. what are
smart contracts
used for?

18. IPO

19. IPO
ICO

59. Reentrancy
Race-to-empty
• Sending ether to an address might lead to code execution
• Changing the state after calling another contract

77. Randomness
Nothing is secret

82. NOTHING IS SECRET

88. • Using the Timestamp has the same issue, also miners can influence it

92. Randomness
Nothing is secret
• Be wary of ”randomness”
• Strong reliance on time is bad

93. Front-running
also known as race condition,TOCTOU,TOD

99. Front-running
also known as race condition,TOCTOU,TOD
• Can transactions re-ordering strongly affect the outcome?
• Do some transactions contain secret information?

100. Arithmetic Errors
Integer overflows and underflows

101. uint8
+1
1 1 1 1 1 1 1 1

102. uint8
+1
1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0

103. batchOverflow

111. _value = 115792089237316195423570985008687907853269984665640564039457584007913129639936
amount = 2 * _value = 0

112. _value = 115792089237316195423570985008687907853269984665640564039457584007913129639936
amount = 2 * _value = 0

113. _value = 115792089237316195423570985008687907853269984665640564039457584007913129639936
amount = 2 * _value = 0

114. _value = 115792089237316195423570985008687907853269984665640564039457584007913129639936
amount = 2 * _value = 0

115. • User input are integer values
• Unchecked arithmetic operations (OpenZeppelin library)
Arithmetic Errors
Integer overflows and underflows

116. • Oyente, Mythril, Manticore, …
• Run them in the beginning, but don’t expect much unless there
is an evident issue
• Remix and Linters
• really useful to catch quick mistakes and bad behaviors that
might lead to issues.Awesome to test and debug quickly.
• Truffle:
• is OK
• Disassembler and decompilers
Tools

117. Future
• Formal verification
• Frameworks and re-usable code
• Ethereum rapidly evolves
• Solidity, Mist, Ethereum Wallet still have experimental versions.
• Audit process is immature
• Lots of money put in lightly-audited smart contracts
• Unknown unknowns ?