Duplicate Mutable Accounts

Summary #

When an instruction requires two mutable accounts of the same type, an attacker can pass in the same account twice, leading to unintended mutations.
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.

Checking for Duplicate Mutable Accounts in Rust #

In Rust, you can simply compare the public keys of the accounts and return an error if they are identical:

if ctx.accounts.account_one.key() == ctx.accounts.account_two.key() {
  return Err(ProgramError::InvalidArgument)
}

Using Constraints in Anchor #

In Anchor, you can add an explicit constraint to an account, ensuring it is not the same as another account.

Lesson #

Duplicate Mutable Accounts occur when an instruction requires two mutable accounts of the same type. If the same account is passed twice, it can be mutated in unintended ways, potentially causing security vulnerabilities.

No check #

Consider a program that updates a data field for user_a and user_b in a single instruction. If the same account is passed for both user_a and user_b, the program will overwrite the data field with the second value, potentially leading to unintended side effects.

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<()> {
        ctx.accounts.user_a.data = a;
        ctx.accounts.user_b.data = b;
        Ok(())
    }
}
 
#[derive(Accounts)]
pub struct Update<'info> {
    #[account(mut)]
    pub user_a: Account<'info, User>,
    #[account(mut)]
    pub user_b: Account<'info, User>,
}
 
#[account]
#[derive(Default)]
pub struct User {
    pub data: u64,
}

Adding a check in Rust #

To avoid this, add a check in the instruction logic to ensure the accounts are different:

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)
        }
        ctx.accounts.user_a.data = a;
        ctx.accounts.user_b.data = b;
        Ok(())
    }
}
 
#[derive(Accounts)]
pub struct Update<'info> {
    #[account(mut)]
    pub user_a: Account<'info, User>,
    #[account(mut)]
    pub user_b: Account<'info, User>,
}
 
#[account]
#[derive(Default)]
pub struct User {
    pub data: u64,
}

Using Anchor Constraint #

An even better solution in Anchor is to use the constraint keyword in the account validation struct.

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.

This ensures the check is performed automatically during account validation:

use anchor_lang::prelude::*;
 
declare_id!("AjBhRphs24vC1V8zZM25PTuLJhJJXFnYbimsZF8jpJAS");
 
#[program]
pub mod duplicate_mutable_accounts_recommended {
    use super::*;
 
    pub fn update(ctx: Context<Update>, a: u64, b: u64) -> Result<()> {
        ctx.accounts.user_a.data = a;
        ctx.accounts.user_b.data = b;
        Ok(())
    }
}
 
#[derive(Accounts)]
pub struct Update<'info> {
    #[account(
        mut,
        constraint = user_a.key() != user_b.key())]
    pub user_a: Account<'info, User>,
    #[account(mut)]
    pub user_b: Account<'info, User>,
}
 
#[account]
#[derive(Default)]
pub struct User {
    pub 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 and 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

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.

constants.rs

pub const DISCRIMINATOR_SIZE: usize = 8;

lib.rs

use anchor_lang::prelude::*;
 
mod constants;
use constants::DISCRIMINATOR_SIZE;
 
declare_id!("Lo5sj2wWy4BHbe8kCSUvgdhzFbv9c6CEERfgAXusBj9");
 
#[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 = DISCRIMINATOR_SIZE + PlayerState::INIT_SPACE
    )]
    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]
#[derive(Default, InitSpace)]
pub struct PlayerState {
    pub player: Pubkey,
    pub choice: Option<RockPaperScissors>,
}
 
#[derive(AnchorSerialize, AnchorDeserialize, Clone, Copy, PartialEq, Eq, InitSpace)]
pub enum RockPaperScissors {
    Rock,
    Paper,
    Scissors,
}

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("Invokes 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 are completed 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 do not make a whole lot of sense for the game, but 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 be compared 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.

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>,
}

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("Invokes 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 Lo5sj2wWy4BHbe8kCSUvgdhzFbv9c6CEERfgAXusBj9 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 Lo5sj2wWy4BHbe8kCSUvgdhzFbv9c6CEERfgAXusBj9 consumed 3414 of 200000 compute units',
'Program Lo5sj2wWy4BHbe8kCSUvgdhzFbv9c6CEERfgAXusBj9 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.

Completed the lab?

Push your code to GitHub and tell us what you thought of this lesson!