Updating the contract

Without overflow checks enabled, if val is currently 127, the next time it is incremented it will overflow, wrapping around to be -128. Likewise, it can underflow if currently -128 and you decrement.

This would obviously be very bad news if you were building your own smart-contract-based currency!

Let's see if we can make val overflow. And then let's see how to prevent this.

Method 1: mash increment 128 times

With the existing contract, you could use the RAEN Admin panel and mash the Submit button on the increment form 128 times.

This could take a while, though! Let's see if we can improve the code to make it easier.

Method 2: new default

If you're following along with your own contract, you can change the default value of val to start at 127 rather than 0. This is a good way to learn about where the default of 0 currently comes from.

If you get stuck while following along, you can see this method and the following all implemented on this branch.

Above pub struct Counter, change the derive macro by removing the Default trait:

-#[derive(Default, BorshDeserialize, BorshSerialize)]
+#[derive(BorshDeserialize, BorshSerialize)]

Now, below the closing bracket of pub struct Counter, you can implement default manually:

fn main() {
pub struct Counter {
    val: i8,

impl Default for Counter {
    fn default() -> Self {
        Self { val: 127 }

If you're following along

Build and re-deploy to see the new default:

near dev-deploy $(raen build --release -q)

Refresh your browser page to load in the updated contract. Check get_num.

But oh no, if you've already incremented or decremented the number, you'll see that this made no change. Which is what you'd expect! It only changes the default. That is, the value that is shown if you didn't actually change it and store the new value.

To see this default in action, you can delete the neardev folder and run the dev-deploy command again. This will create a brand new NEAR testnet account before deploying your contract. You'll need to copy the new dev-1234-1234 address it gives you and paste it into RAEN Admin.

Now when you check get_num, you'll see the number you set as a default! Call increment (you'll need to sign in again, since this is a new contract) and watch it overflow.

If you're not

The contract deployed at counter.raendev.testnet already has a value for val saved to contract storage. It will not make use of a new default.

Let's try a different way.

Method 3: new set_num

See if you can write your own set_num method based on the code for increment and decrement in the contract. If you need a hint, here's what it looks like.

If you're following along

Save your file with new set_num, then build and re-deploy:

near dev-deploy $(raen build --release -q)

Unlike Method 2, you don't need to create a new testnet account. Just refresh the RAEN Admin panel and see the set_num method appear under Change Methods. Go ahead and try it out!

If you're not

The reference code has been deployed to v2.counter.raendev.testnet for you to play with.

Note that this code already fixed the overflow issue. If you want to see the overflow in action, you'll have to follow along with your own code.

Fixing that overflow

Now you can call increment and see that the number goes from 127 to -128. Oops! How do we fix it?

Open up the Cargo.toml file. Cargo.toml is to Rust what package.json is to NodeJS. You'll see sections for dependencies and dev-dependencies and more generic information about this package/contract/crate.

See those lines with overflow-checks = false? Change them to overflow-checks = true. Now rebuild and redeploy:

near dev-deploy $(raen build --release -q)

You'll see that this takes longer to build than when you make changes to your contract itself. Rust needs to recompile all of your dependencies using the new setting.

Refresh your admin panel, set the number back to 127, and call increment. You'll see an error message. This means the contract panicked rather than allowing the integer overflow. It worked!

Digging Deeper: wait, aren't blockchains immutable?

You may have heard that blockchains are immutable, that they provide permanent ledgers of all on-chain activity.

How does this fit with the fact that you just upgraded a smart contract, and it was actually super easy? It's even called a contract—doesn't that imply that it should be hard to change? If it's so easy for a contract owner to alter the contract, how can anyone using it trust they won't be scammed?

Great questions!

On other blockchains, it is in fact very difficult to upgrade a contract. They work around this with proxy contracts and other patterns.

But NEAR has that handy account system, where each account can have multiple keys. We already learned about Function Call Access Keys last unit; that's what makes it safe to have a web-based wallet, with web apps that store a restricted private key in a browser's local storage.

The other kind of key is a Full Access key. When you run dev-deploy, you get a Full Access key to a brand new testnet account. If you have a Full Access key, you can do anything with that account. Deploy or remove a contract, call other contracts, send tokens, create sub-accounts, even delete the account.

(Note that even all of these actions are stored on the permanent blockchain ledger! If you're running or know the address of an archival node, you can query the state of a given account at a given block/second in time, even if the contract on that account has changed or the account has been completely deleted. If you're querying a regular non-archival node, it will probably only keep about two days of history.)

In the early days of launching and testing a smart contract, it's nice to be able to leave the contract in "trusted mode," keeping Full Access keys around so you can iterate quickly. But if you want people to trust your contract with large investments, you'll want to get a security review from a reputable company, and those companies will want to make sure you don't have any Full Access keys still on the contract (you can get a list of all keys on any NEAR account with near keys in your command line), and that you didn't build in any sneaky ways to add new Full Access keys given only a Function Call key.

(Note that there still patterns that allow you to upgrade such a contract, such as requiring a review board to vote on proposed upgrades.)

If you're using a smart contract, especially if you're considering trusting it with significant assets, you should check that it had a security review first!


In this unit we learned about integer overflows, and played with them by updating our code to change the default value and add a new change method. We saw that we only need to refresh the RAEN Admin page to see the latest changes. We fixed the integer overflow bug by changing Cargo.toml settings. And we also learned about blockchain immutability and Full Access keys.

Next, let's learn how to deploy to a nicely-named account like counter.raendev.testnet.