This is an opinionated list of issues that can be improved if they are found in Cadence code intended for production.
Some may choose to authenticate or perform operations for their users by using the users' account addresses.
In order to do this, a commonly seen case would be to pass the user's AuthAccount
object
as a parameter to a contract function to use for querying the account or storing objects directly.
This is problematic, as the AuthAccount
object allows access to ALL private areas of the account,
for example, all of the user's storage, authorized keys, etc.,
which provides the opportunity for bad actors to take advantage of.
1...2// BAD CODE3// DO NOT COPY45// Imagine this code is in a contract that uses AuthAccount to authenticate users6// To transfer NFTs78// They could deploy the contract with an Ethereum-style access control list functionality910pub fun transferNFT(id: UInt64, owner: AuthAccount) {11assert(owner(id) == owner.address)1213transfer(id)14}1516// But they could upgrade the function to have the same signature17// so it looks like it is doing the same thing, but they could also drain a little bit18// of FLOW from the user's vault, a totally separate piece of the account that19// should not be accessible in this function20// BAD2122pub fun transferNFT(id: UInt64, owner: AuthAccount) {23assert(owner(id) == owner.address)2425transfer(id)2627// Sneakily borrow a reference to the user's Flow Token Vault28// and withdraw a bit of FLOW29// BAD30let vaultRef = owner.borrow<&FlowToken.Vault>(/storage/flowTokenVault)!31let stolenTokens <- vaultRef.withdraw(amount: 0.1)3233// deposit the stolen funds in the contract owners vault34// BAD35contractVault.deposit(from: <-stolenTokens)36}37...
Projects should find other ways to authenticate users, such as using resources and capabilities as authentication objects. They should also expect to perform most storage and linking operations within transaction bodies rather than inside contract utility functions.
There are some scenarios where using an AuthAccount
object is necessary, such as a cold storage multi-sig,
but those cases are extremely rare and AuthAccount
usage should still be avoided unless absolutely necessary.
Authorized references allow downcasting restricted
types to their unrestricted type and should be avoided unless necessary.
The type that is being restricted could expose functionality that was not intended to be exposed.
If the auth
keyword is used on local variables they will be references.
References are ephemeral and cannot be stored.
This prevents any reference casting to be stored under account storage.
Additionally, if the auth
keyword is used to store a public capability, serious harm
could happen since the value could be downcasted to a type
that has functionality and values altered.
A commonly seen pattern in NFT smart contracts is including a public borrow function that borrows an auth reference to an NFT (eg. NBA Top Shot). This allows anyone to access the stored metadata or extra fields that weren't part of the NFT standard. While generally safe in most scenarios, not all NFTs are built the same. Some NFTs may have privileged functions that shouldn't be exposed by this method, so please be cautious and mindful when imitating NFT projects that use this pattern.
When we create a public capability for our FungibleToken.Vault
we do not use an auth capability:
1// GOOD: Create a public capability to the Vault that only exposes2// the balance field through the Balance interface3signer.link<&FlowToken.Vault{FungibleToken.Balance}>(4/public/flowTokenBalance,5target: /storage/flowTokenVault6)
If we were to use an authorized type for the capability, like so:
1// BAD: Create an Authorized public capability to the Vault that only exposes2// the balance field through the Balance interface3// Authorized referenced can be downcasted to their unrestricted types, which is dangerous4signer.link<auth &FlowToken.Vault{FungibleToken.Balance}>(5/public/flowTokenBalance,6target: /storage/flowTokenVault7)
Then anyone in the network could take that restricted reference that is only supposed to expose the balance field and downcast it to expose the withdraw field and steal all our money!
1// Exploit of the auth capability to expose withdraw2let balanceRef = getAccount(account)3.getCapability<auth &FlowToken.Vault{FungibleToken.Balance}>(/public/flowTokenBalance)4.borrow()!56let fullVaultRef = balanceRef as! &FlowToken.Vault78// Withdraw the newly exposed funds9let stolenFunds <- fullVaultRef.withdraw(amount: 1000000)
Public functions in a contract can be called by anyone, e.g. any other contract or any transaction. If that function creates a resource, and that resource has functions that emit events, that means any account can create an instance of that resource and emit those events. If those events are meant to indicate actions taken using a single instance of that resource (eg. admin object, registry), or instances created through a particular workflow, it's possible that events from other instances may be mixed in with the ones you're querying for - making the event log search and management more cumbersome.
To fix this, if there should be only a single instance of the resource,
it should be created and link()
ed to a public path in an admin account's storage
during the contracts's initializer.
Be sure to keep track of access modifiers when structuring your code, and make public only what should be public. Accidentally exposed fields can be a security hole.
When writing your smart contract, look at every field and function and make sure
that they are all access(self)
, access(contract)
, or access(account)
, unless otherwise needed.
This is a specific case of "Public Functions And Fields Should Be Avoided", above.
The values of public fields can be copied. Capabilities are value types, so if they are used as a public field, anyone can copy it from the field and call the functions that it exposes. This almost certainly is not what you want if a capability has been stored as a field on a contract or resource in this way.
For public access to a capability, place it in an accounts public area so this expectation is explicit.
This anti-pattern has been addressed with FLIP #703
This is a specific case of "Public Functions And Fields Should Be Avoided", above. Public array or dictionary fields are not directly over-writable, but their members can be accessed and overwritten if the field is public. This could potentially result in security vulnerabilities for the contract if these fields are mistakenly made public.
Ex:
1pub contract Array {2// array is intended to be initialized to something constant3pub let shouldBeConstantArray: [Int]4}
Anyone could use a transaction like this to modify it:
1import Array from 0x0123transaction {4execute {5Array.shouldbeConstantArray[0] = 10006}7}
Make sure that any array or dictionary fields in contracts, structs, or resources
are access(contract)
or access(self)
unless they need to be intentionally made public.
1pub contract Array {2// array is inteded to be initialized to something constant3access(self) let shouldBeConstantArray: [Int]4}
This is a specific case of "Public Functions And Fields Should Be Avoided", above.
A public function on a contract that creates a resource can be called by any account. If that resource provides access to admin functions then the creation function should not be public.
To fix this, a single instance of that resource should be created in the contract's init()
method,
and then a new creation function can be potentially included within the admin resource, if necessary.
The admin resource can then be link()
ed to a private path in an admin's account storage during the contract's initializer.
1// Pseudo-code23// BAD4pub contract Currency {5pub resource Admin {6pub fun mintTokens()7}89// Anyone in the network can call this function10// And use the Admin resource to mint tokens11pub fun createAdmin(): @Admin {12return <-create Admin()13}14}1516// This contract makes the admin creation private and in the initializer17// so that only the one who controls the account can mint tokens18// GOOD19pub contract Currency {20pub resource Admin {21pub fun mintTokens()2223// Only an admin can create new Admins24pub fun createAdmin(): @Admin {25return <-create Admin()26}27}2829init() {30// Create a single admin resource31let firstAdmin <- create Admin()3233// Store it in private account storage in `init` so only the admin can use it34self.account.save(<-firstAdmin, to: /storage/currencyAdmin)35}36}
This is another example of the risks of having publicly accessible parts to your smart contract.
Data structure definitions in Cadence currently must be declared as public so that they can be used by anyone. Structs do not have the same restrictions that resources have on them, which means that anyone can create a new instance of a struct without going through any authorization.
Any contract state-modifying operations related to the creation of structs should be contained in restricted resources instead of the initializers of structs.
This used to be a bug in the NBA Top Shot smart contract, so we'll use that as an example. Before, when it created a new play, it would initialize the play record with a struct, which increments the number that tracks the play IDs and emits an event:
1// Simplified Code2// BAD3//4pub contract TopShot {56// The Record that is used to track every unique play ID7pub var nextPlayID: UInt3289pub struct Play {1011pub let playID: UInt321213init() {1415self.playID = TopShot.nextPlayID1617// Increment the ID so that it isn't used again18TopShot.nextPlayID = TopShot.nextPlayID + 11920emit PlayCreated(id: self.playID, metadata: metadata)21}22}23}
This is a risk because anyone can create the Play
struct as many times as they want,
which could increment the nextPlayID
field to the max UInt32
value,
effectively preventing new plays from being created. It also would emit bogus events.
This bug was fixed by instead updating the contract state in the admin function that creates the plays.
1// Update contract state in admin resource functions2// GOOD3//4pub contract TopShot {56// The Record that is used to track every unique play ID7pub var nextPlayID: UInt3289pub struct Play {1011pub let playID: UInt321213init() {14self.playID = TopShot.nextPlayID15}16}1718pub resource Admin {1920// Protected within the private admin resource21pub fun createPlay() {22// Create the new Play23var newPlay = Play()2425// Increment the ID so that it isn't used again26TopShot.nextPlayID = TopShot.nextPlayID + UInt32(1)2728emit PlayCreated(id: newPlay.playID, metadata: metadata)2930// Store it in the contract storage31TopShot.playDatas[newPlay.playID] = newPlay32}33}34}