​
Summary

  • When an instruction requires two mutable accounts of the same type, an attacker can pass in the same account twice, causing the account to be mutated in unintended ways.
  • To check for duplicate mutable accounts in Rust, simply compare the public keys of the two accounts and throw an error if they are the same. 
    if ctx.accounts.account_one.key() == ctx.accounts.account_two.key() {
    return Err(ProgramError::InvalidArgument)
}
  • In Anchor, you can use constraint to add an explicit constraint to an account checking that it is not the same as another account.

​
Lesson

Duplicate Mutable Accounts refers to an instruction that requires two mutable accounts of the same type. When this occurs, you should validate that two accounts are different to prevent the same account from being passed into the instruction twice. Since the program treats each account as separate, passing in the same account twice could result in the second account being mutated in unintended ways. This could result in very minor issues, or catastrophic ones - it really depends on what data the code changes and how these accounts are used. Regardless, this is a vulnerability all developers should be aware of.

​
No check

For example, imagine a program that updates a data field for user_a and user_b in a single instruction. The value that the instruction sets for user_a is different from user_b. Without verifying that user_a and user_b are different, the program would update the data field on the user_a account, then update the data field a second time with a different value under the assumption that user_b is a separate account. You can see this example in the code below.Tthere is no check to verify that user_a and user_b are not the same account. Passing in the same account for user_a and user_b will result in the data field for the account being set to b even though the intent is to set both values a and b on separate accounts. Depending on what data represents, this could be a minor unintended side-effect, or it could mean a severe security risk. allowing user_a and user_b to be the same account could result in 
use anchor_lang::prelude::*;

declare_id!("Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS");

#[program]
pub mod duplicate_mutable_accounts_insecure {
    use super::*;

    pub fn update(ctx: Context<Update>, a: u64, b: u64) -> Result<()> {
        let user_a = &mut ctx.accounts.user_a;
        let user_b = &mut ctx.accounts.user_b;

        user_a.data = a;
        user_b.data = b;
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Update<'info> {
    user_a: Account<'info, User>,
    user_b: Account<'info, User>,
}

#[account]
pub struct User {
    data: u64,
}

​
Add check in instruction

To fix this problem with plan Rust, simply add a check in the instruction logic to verify that the public key of user_a isn’t the same as the public key of user_b, returning an error if they are the same. 
if ctx.accounts.user_a.key() == ctx.accounts.user_b.key() {
    return Err(ProgramError::InvalidArgument)
}
This check ensures that user_a and user_b are not the same account. 
use anchor_lang::prelude::*;

declare_id!("Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS");

#[program]
pub mod duplicate_mutable_accounts_secure {
    use super::*;

    pub fn update(ctx: Context<Update>, a: u64, b: u64) -> Result<()> {
        if ctx.accounts.user_a.key() == ctx.accounts.user_b.key() {
            return Err(ProgramError::InvalidArgument.into())
        }
        let user_a = &mut ctx.accounts.user_a;
        let user_b = &mut ctx.accounts.user_b;

        user_a.data = a;
        user_b.data = b;
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Update<'info> {
    user_a: Account<'info, User>,
    user_b: Account<'info, User>,
}

#[account]
pub struct User {
    data: u64,
}

​
Use Anchor constraint

An even better solution if you’re using Anchor is to add the check to the account validation struct instead of the instruction logic. You can use the #[account(..)] attribute macro and the constraint keyword to add a manual constraint to an account. The constraint keyword will check whether the expression that follows evaluates to true or false, returning an error if the expression evaluates to false. The example below moves the check from the instruction logic to the account validation struct by adding a constraint to the #[account(..)] attribute. 
use anchor_lang::prelude::*;

declare_id!("Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS");

#[program]
pub mod duplicate_mutable_accounts_recommended {
    use super::*;

    pub fn update(ctx: Context<Update>, a: u64, b: u64) -> Result<()> {
        let user_a = &mut ctx.accounts.user_a;
        let user_b = &mut ctx.accounts.user_b;

        user_a.data = a;
        user_b.data = b;
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Update<'info> {
    #[account(constraint = user_a.key() != user_b.key())]
    user_a: Account<'info, User>,
    user_b: Account<'info, User>,
}

#[account]
pub struct User {
    data: u64,
}

​
Lab

Let’s practice by creating a simple Rock Paper Scissors program to demonstrate how failing to check for duplicate mutable accounts can cause undefined behavior within your program. This program will initialize “player” accounts and have a separate instruction that requires two player accounts to represent starting a game of rock paper scissors.
  • An initialize instruction to initialize a PlayerState account
  • A rock_paper_scissors_shoot_insecure instruction that requires two PlayerState accounts, but does not check that the accounts passed into the instruction are different
  • A rock_paper_scissors_shoot_secure instruction that is the same as the rock_paper_scissors_shoot_insecure instruction but adds a constraint that ensures the two player accounts are different

​
1. Starter

To get started, download the starter code on the starter branch of this repository. The starter code includes a program with two instructions and the boilerplate setup for the test file. The initialize instruction initializes a new PlayerState account that stores the public key of a player and a choice field that is set to None. The rock_paper_scissors_shoot_insecure instruction requires two PlayerState accounts and requires a choice from the RockPaperScissors enum for each player, but does not check that the accounts passed into the instruction are different. This means a single account can be used for both PlayerState accounts in the instruction. 
use anchor_lang::prelude::*;
use borsh::{BorshDeserialize, BorshSerialize};

declare_id!("Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS");

#[program]
pub mod duplicate_mutable_accounts {
    use super::*;

    pub fn initialize(ctx: Context<Initialize>) -> Result<()> {
        ctx.accounts.new_player.player = ctx.accounts.payer.key();
        ctx.accounts.new_player.choice = None;
        Ok(())
    }

    pub fn rock_paper_scissors_shoot_insecure(
        ctx: Context<RockPaperScissorsInsecure>,
        player_one_choice: RockPaperScissors,
        player_two_choice: RockPaperScissors,
    ) -> Result<()> {
        ctx.accounts.player_one.choice = Some(player_one_choice);

        ctx.accounts.player_two.choice = Some(player_two_choice);
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Initialize<'info> {
    #[account(
        init,
        payer = payer,
        space = 8 + 32 + 8
    )]
    pub new_player: Account<'info, PlayerState>,
    #[account(mut)]
    pub payer: Signer<'info>,
    pub system_program: Program<'info, System>,
}

#[derive(Accounts)]
pub struct RockPaperScissorsInsecure<'info> {
    #[account(mut)]
    pub player_one: Account<'info, PlayerState>,
    #[account(mut)]
    pub player_two: Account<'info, PlayerState>,
}

#[account]
pub struct PlayerState {
    player: Pubkey,
    choice: Option<RockPaperScissors>,
}

#[derive(Clone, Copy, BorshDeserialize, BorshSerialize)]
pub enum RockPaperScissors {
    Rock,
    Paper,
    Scissors,
}

​
2. Test rock_paper_scissors_shoot_insecure instruction

The test file includes the code to invoke the initialize instruction twice to create two player accounts. Add a test to invoke the rock_paper_scissors_shoot_insecure instruction by passing in the playerOne.publicKey for as both playerOne and playerTwo. 
describe("duplicate-mutable-accounts", () => {
	...
	it("Invoke insecure instruction", async () => {
        await program.methods
        .rockPaperScissorsShootInsecure({ rock: {} }, { scissors: {} })
        .accounts({
            playerOne: playerOne.publicKey,
            playerTwo: playerOne.publicKey,
        })
        .rpc()

        const p1 = await program.account.playerState.fetch(playerOne.publicKey)
        assert.equal(JSON.stringify(p1.choice), JSON.stringify({ scissors: {} }))
        assert.notEqual(JSON.stringify(p1.choice), JSON.stringify({ rock: {} }))
    })
})
Run anchor test to see that the transactions completes successfully, even though the same account is used as two accounts in the instruction. Since the playerOne account is used as both players in the instruction, note the choice stored on the playerOne account is also overridden and set incorrectly as scissors. 
duplicate-mutable-accounts
  ✔ Initialized Player One (461ms)
  ✔ Initialized Player Two (404ms)
  ✔ Invoke insecure instruction (406ms)
Not only does allowing duplicate accounts not make a whole lot of sense for the game, it also causes undefined behavior. If we were to build out this program further, the program only has one chosen option and therefore can’t compare against a second option. The game would end in a draw every time. It’s also unclear to a human whether playerOne’s choice should be rock or scissors, so the program behavior is strange.

​
3. Add rock_paper_scissors_shoot_secure instruction

Next, return to lib.rs and add a rock_paper_scissors_shoot_secure instruction that uses the #[account(...)] macro to add an additional constraint to check that player_one and player_two are different accounts. 
#[program]
pub mod duplicate_mutable_accounts {
    use super::*;
		...
        pub fn rock_paper_scissors_shoot_secure(
            ctx: Context<RockPaperScissorsSecure>,
            player_one_choice: RockPaperScissors,
            player_two_choice: RockPaperScissors,
        ) -> Result<()> {
            ctx.accounts.player_one.choice = Some(player_one_choice);

            ctx.accounts.player_two.choice = Some(player_two_choice);
            Ok(())
        }
}

#[derive(Accounts)]
pub struct RockPaperScissorsSecure<'info> {
    #[account(
        mut,
        constraint = player_one.key() != player_two.key()
    )]
    pub player_one: Account<'info, PlayerState>,
    #[account(mut)]
    pub player_two: Account<'info, PlayerState>,
}

​
7. Test rock_paper_scissors_shoot_secure instruction

To test the rock_paper_scissors_shoot_secure instruction, we’ll invoke the instruction twice. First, we’ll invoke the instruction using two different player accounts to check that the instruction works as intended. Then, we’ll invoke the instruction using the playerOne.publicKey as both player accounts, which we expect to fail. 
describe("duplicate-mutable-accounts", () => {
	...
    it("Invoke secure instruction", async () => {
        await program.methods
        .rockPaperScissorsShootSecure({ rock: {} }, { scissors: {} })
        .accounts({
            playerOne: playerOne.publicKey,
            playerTwo: playerTwo.publicKey,
        })
        .rpc()

        const p1 = await program.account.playerState.fetch(playerOne.publicKey)
        const p2 = await program.account.playerState.fetch(playerTwo.publicKey)
        assert.equal(JSON.stringify(p1.choice), JSON.stringify({ rock: {} }))
        assert.equal(JSON.stringify(p2.choice), JSON.stringify({ scissors: {} }))
    })

    it("Invoke secure instruction - expect error", async () => {
        try {
        await program.methods
            .rockPaperScissorsShootSecure({ rock: {} }, { scissors: {} })
            .accounts({
                playerOne: playerOne.publicKey,
                playerTwo: playerOne.publicKey,
            })
            .rpc()
        } catch (err) {
            expect(err)
            console.log(err)
        }
    })
})
Run anchor test to see that the instruction works as intended and using the playerOne account twice returns the expected error. 
'Program Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS invoke [1]',
'Program log: Instruction: RockPaperScissorsShootSecure',
'Program log: AnchorError caused by account: player_one. Error Code: ConstraintRaw. Error Number: 2003. Error Message: A raw constraint was violated.',
'Program Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS consumed 5104 of 200000 compute units',
'Program Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS failed: custom program error: 0x7d3'
The simple constraint is all it takes to close this loophole. While somewhat contrived, this example illustrates the odd behavior that can occur if you write your program under the assumption that two same-typed accounts will be different instances of an account but don’t explicitly write that constraint into your program. Always think about the behavior you’re expecting from the program and whether that is explicit. If you want to take a look at the final solution code you can find it on the solution branch of the repository.

​
Challenge

Just as with other lessons in this unit, your opportunity to practice avoiding this security exploit lies in auditing your own or other programs. Take some time to review at least one program and ensure that any instructions with two same-typed mutable accounts are properly constrained to avoid duplicates. Remember, if you find a bug or exploit in somebody else’s program, please alert them! If you find one in your own program, be sure to patch it right away.