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avoiding_common_attacks.md

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Avoiding Common Attacks

See: https://consensys.github.io/smart-contract-best-practices/recommendations/

External Calls

The only external call made by a contract is from the Stores to Marketplace contract (in onlyStoreOwner). The address of the Marketplace contract is set by the owner (i.e. the address that deploys the contract) in the constructor, and there is no way to change it. If a user trusts the addresses that have deployed the contracts, then the external call should be reasonably safe.

Avoid state changes after external calls

The only state change after an external call is in createStorefront in Stores.sol, because it is necessary to check in the Marketplace contract whether an address is marked as a storeOwner. For security purposes, this function (as well as all others that modify state) is pausable by the owner of the contract because of the whenNotPaused modifier.

Be aware of the tradeoffs between send(), transfer(), and call.value()()¶

Only transfer() is used to transfer value. It is used in withdrawStorefrontBalance, removeStorefront and purchaseProduct.

Handle errors in external calls

Not applicable.

Favor pull over push for external calls

Not applicable.

Don't assume contracts are created with zero balance¶

This is not assumed anywhere, and withdrawals from Stores.sol are based on each storefront's balance value, which is stored separately from the total contract balance.

Remember that on-chain data is public

No issues here, as all data is meant to be visible in the UI. The only somewhat sensitive data are storefront balances, but these could be seen anyways once a store owner withdraws their balance.

In 2-party or N-party contracts, beware of the possibility that some participants may "drop offline" and not return

Unless the products on a storefront require physical delivery, this is not a problem. The worst case scenario would be that a storeowner does not return and does not claim their storefront balance.

Use assert() and require() properly

assert() is not used, but require() is used to validate input. See design_pattern_decisions.md for more details.

Explicitly mark visibility in functions and state variables

All function visibility is explicitly marked in Marketplace.sol and Stores.sol. All variables except contract addresses are set to private.

Lock pragmas to specific compiler version

Version 0.4.24 is used for the Marketplace, Stores and Migrations contracts.

Differentiate functions and events

Functions and events are differentiated in two ways:

  1. Events will start with an uppercase character (ex: AdminAdded) and functions with a lowercase one (ex: addAdmin)
  2. Events will use past tense verbs (ex: BalanceWithdrawn) and functions will use present tense verbs (ex: withdrawStorefrontBalance)

Prefer newer Solidity constructs

  • selfdestruct is used over suicide (see Destructible.sol)
  • transfer is used over require(msg.sender.send()) (see Stores.sol)
  • keccak256 is used over sha3 (see: Stores.sol)

Avoid using tx.origin

tx.origin is not used.

Timestamp Dependence

Timestamps are only used to generate IDs for products and storefronts. The goal is for these to be unique, and they do not have any security implications.

Multiple Inheritance Caution

While the contracts do have multiple inheritances, they are inherited from Most General to Most Specific (and the compiler will not work otherwise). Also, the functionality of the inherited contracts is tested.