avoiding_common_attacks.md

Avoiding Common Attack Vectors

Integer arithmetic and overflow

• OraclizeUtils library offers safe addition function used in ImageSeller for any addition. Throws on overflow.

Re-entrancy attacks

• malicious code cannot call back into ImageSeller since "transfer" is used to send Ether to contract owner (locked down via a modifier on withdraw function)
• recursive calls back into ImageSeller from external contract are thus prohibited

Poison Data

• validation done before state mutated in ImageSeller via "check effects interactions" pattern
• Specifically, price is required to be greater than 0 for adding an image to seller's registry
• Require Price > sale price before returned unencrypted IPFS hash to buyer

Malicious Admins

• This attack is prevented by the onlyOwner modifier in ImageSeller
• Only one admin per deployed ImageSeller, the contract deployer.

TX.origin

• all contracts use msg.sender which is safer since it does not follow call stack up to original tx in an embedded call scenario.

Gas Limits

• utility function in OraclizeUtils library used to ensure enough gas is sent by buyers in ImageSeller "buyFromRegistry" function.

Check Effects Interactions

• this pattern is used in any function in ImageSeller that deals with value. This includes payable function for buying an image and withdraw function.
• First, sanity checks are made. Did buyer send enough ether to cover image cost ?
• Then, contract state members are mutated. Registry updated for new sale. Balance updated.
• Finally, external side effects if any are executed. msg.sender.transfer on withdraw function.