This post covers a system-level DoS vector on Solana: if your program initializes an account via the System Program’s create_account, a third party may be able to pre-fund the destination address and cause the initialization to fail permanently.

TL;DR#

• create_account rejects the destination if it already has non-zero lamports (AccountAlreadyInUse).
• Attackers can often precompute addresses you will initialize (especially PDAs) and pre-fund them with the minimum lamports to keep the account rent-exempt, making the DoS persistent.
• Fix: don’t rely on create_account when the destination might be pre-funded; use the safer transfer + allocate + assign flow (or let Anchor handle it).

Account Creation#

In Solana, account creation is commonly performed by invoking the System Program’s create_account instruction from within a program.

This pattern shows up most often when initializing program-derived addresses (PDAs) with invoke_signed.

Below is a simplified example:

rust
fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    _instruction_data: &[u8],
) -> ProgramResult {
    let accounts_iter = &mut accounts.iter();
    let payer = next_account_info(accounts_iter)?;
    let new_account = next_account_info(accounts_iter)?;
    let system_program = next_account_info(accounts_iter)?;

    // Typical vulnerable pattern: initializing a PDA using create_account.
    // If `new_account` was pre-funded (lamports > 0), this will fail with AccountAlreadyInUse.
    let space: u64 = 8 + 8;
    let lamports: u64 = Rent::get()?.minimum_balance(space as usize);
    let seeds: &[&[u8]] = &[b"data", payer.key.as_ref() /*, bump */];

    invoke_signed(
        &solana_program::system_instruction::create_account(
            payer.key,
            new_account.key,
            lamports,
            space,
            program_id,
        ),
        &[payer.clone(), new_account.clone(), system_program.clone()],
        &[seeds],
    )?;

    msg!("Account created successfully.");
    Ok(())
}

This code assumes that new_account has never been created before.

How is it DoSed#

At first glance, calling create_account appears safe. However, to understand the risk, we need to look at how the instruction behaves internally.

Why pre-funding is possible#

On Solana, it’s possible to transfer lamports to an address even if it has never been explicitly “created” by your program. In practice, the attacker can include the target pubkey as a writable account meta and execute a System Program transfer. The result is a (system-owned) account with:

• lamports > 0
• data_len == 0
• owner == System Program

That’s enough to trip the create_account guard.

According to the documentation in system_instruction.rs:

rust
//! Account creation typically involves three steps: [`allocate`] space,
//! [`transfer`] lamports for rent, [`assign`] to its owning program. The
//! [`create_account`] function does all three at once. All new accounts must
//! contain enough lamports to be [rent exempt], or else the creation
//! instruction will fail.

Conceptually, create_account is a convenience wrapper around:

1. allocate
2. transfer
3. assign

Runtime Behavior#

The actual runtime logic lives in system_processor.rs

rust
#[allow(clippy::too_many_arguments)]
fn create_account(
    from_account_index: IndexOfAccount,
    to_account_index: IndexOfAccount,
    to_address: &Address,
    lamports: u64,
    space: u64,
    owner: &Pubkey,
    signers: &HashSet<Pubkey>,
    invoke_context: &InvokeContext,
    transaction_context: &TransactionContext,
    instruction_context: &InstructionContext,
) -> Result<(), InstructionError> {
    // if it looks like the `to` account is already in use, bail
    {
        let mut to = instruction_context
            .try_borrow_instruction_account(transaction_context, to_account_index)?;
        if to.get_lamports() > 0 {
            ic_msg!(
                invoke_context,
                "Create Account: account {:?} already in use",
                to_address
            );
            return Err(SystemError::AccountAlreadyInUse.into());
        }

        allocate_and_assign(&mut to, to_address, space, owner, signers, invoke_context)?;
    }
    transfer(
        from_account_index,
        to_account_index,
        lamports,
        invoke_context,
        transaction_context,
        instruction_context,
    )
}

The key observation is the following check:

rust
        if to.get_lamports() > 0 {
            ic_msg!(
                invoke_context,
                "Create Account: account {:?} already in use",
                to_address
            );
            return Err(SystemError::AccountAlreadyInUse.into());
        }

Any account with a non-zero lamport balance is considered “already in use.”

Note: although the docs describe “allocate → transfer → assign” conceptually, the runtime implementation performs the “already in use” check first, then allocate_and_assign, and finally transfer. The DoS is specifically about that early lamports > 0 guard.

Pre-funding DoS Vector#

If the destination account address can be precomputed (for example, a PDA derived from predictable seeds), an attacker can:

1. Precompute the target address
2. Transfer the minimum lamports needed to keep it rent-exempt (commonly the minimum balance for a system-owned, 0-data account)
3. Cause create_account to revert with AccountAlreadyInUse, blocking the entire logic.

This results in a persistent DoS, as subsequent attempts to initialize the account will fail. While brute-forcing arbitrary addresses is impractical, this attack becomes realistic when PDA seeds are simple or predictable (e.g., user_pubkey, mint, or static identifiers).

When are you vulnerable?#

You’re in the danger zone if all of these are true:

1. Your program (or client) derives a destination address that an attacker can predict (commonly a PDA).
2. Your initialization path uses System Program create_account.
3. You don’t have a fallback path when lamports > 0.

Mitigations#

There are two commonly used mitigation strategies:

1. Drain pre-funded lamports
   • If you can sign for the address (e.g., it’s your PDA, so you can invoke_signed), you can transfer the lamports out first so the balance is zero, then proceed.
   • This is not always available (e.g., if the destination is not a PDA you control).
2. Avoid create_account entirely
   • Manually split the process into:
     • transfer
     • allocate
     • assign
   • This avoids the AccountAlreadyInUse check.

Practical mitigation pattern (manual)#

If you control the destination (most commonly because it’s a PDA), you can branch on the current lamports and safely recover even when the address is pre-funded.

The key is: allocate and assign are allowed on a system-owned, zero-data account, but they require the account itself to sign (so for PDAs you must use invoke_signed).

rust
let current_lamports = new_account.lamports();

if current_lamports == 0 {
    // Safe to use create_account
    invoke_signed(
        &system_instruction::create_account(
            payer.key,
            new_account.key,
            rent.minimum_balance(space as usize),
            space,
            program_id,
        ),
        &[payer.clone(), new_account.clone(), system_program.clone()],
        &[seeds],
    )?;
} else {
    // 1) top up to rent-exempt if needed
    let required = rent
        .minimum_balance(space as usize)
        .saturating_sub(current_lamports);
    if required > 0 {
        invoke(
            &system_instruction::transfer(payer.key, new_account.key, required),
            &[payer.clone(), new_account.clone(), system_program.clone()],
        )?;
    }

    // 2) allocate space (requires signature)
    invoke_signed(
        &system_instruction::allocate(new_account.key, space),
        &[new_account.clone(), system_program.clone()],
        &[seeds],
    )?;

    // 3) assign owner (requires signature)
    invoke_signed(
        &system_instruction::assign(new_account.key, program_id),
        &[new_account.clone(), system_program.clone()],
        &[seeds],
    )?;
}

How Anchor Handles It#

Anchor’s #[account(init)] and #[account(init_if_needed)] constraints explicitly address this issue by generating code that:

1. Checks field.lamports() at runtime
2. Uses create_account only when lamports == 0
3. Otherwise falls back to the safer transfer + allocate + assign flow

Example user code:

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

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

In generate_constraint_init_group, the owner is checked and the generate_create_account is being called.

rust
            // Define the owner of the account being created. If not specified,
            // default to the currently executing program.
            let (owner, owner_optional_check) = match owner {
                None => (
                    quote! {
                        __program_id
                    },
                    quote! {},
                ),

                Some(o) => {
                    // We clone the `check_scope` here to avoid collisions with the
                    // `payer_optional_check`, which is in a separate scope
                    let owner_optional_check = check_scope.clone().generate_check(o);
                    (
                        quote! {
                            &#o
                        },
                        owner_optional_check,
                    )
                }
            };

rust
            // CPI to the system program to create the account.
            let create_account = generate_create_account(
                field,
                quote! {space},
                owner.clone(),
                quote! {#payer},
                seeds_with_bump,
            );

Internally, anchor-generated code checks whether the account already holds lamports. If so, it avoids create_account and falls back to a safer flow.

rust
fn generate_create_account(
    field: &Ident,
    space: proc_macro2::TokenStream,
    owner: proc_macro2::TokenStream,
    payer: proc_macro2::TokenStream,
    seeds_with_nonce: proc_macro2::TokenStream,
) -> proc_macro2::TokenStream {
    // Field, payer, and system program are already validated to not be an Option at this point
    quote! {
        // If the account being initialized already has lamports, then
        // return them all back to the payer so that the account has
        // zero lamports when the system program's create instruction
        // is eventually called.
        let __current_lamports = #field.lamports();
        if __current_lamports == 0 {
            // Create the token account with right amount of lamports and space, and the correct owner.
            let space = #space;
            let lamports = __anchor_rent.minimum_balance(space);
            let cpi_accounts = anchor_lang::system_program::CreateAccount {
                from: #payer.to_account_info(),
                to: #field.to_account_info()
            };
            let cpi_context = anchor_lang::context::CpiContext::new(system_program.key(), cpi_accounts);
            anchor_lang::system_program::create_account(cpi_context.with_signer(&[#seeds_with_nonce]), lamports, space as u64, #owner)?;
        } else {
            require_keys_neq!(#payer.key(), #field.key(), anchor_lang::error::ErrorCode::TryingToInitPayerAsProgramAccount);
            // Fund the account for rent exemption.
            let required_lamports = __anchor_rent
                .minimum_balance(#space)
                .max(1)
                .saturating_sub(__current_lamports);
            if required_lamports > 0 {
                let cpi_accounts = anchor_lang::system_program::Transfer {
                    from: #payer.to_account_info(),
                    to: #field.to_account_info(),
                };
                let cpi_context = anchor_lang::context::CpiContext::new(system_program.key(), cpi_accounts);
                anchor_lang::system_program::transfer(cpi_context, required_lamports)?;
            }
            // Allocate space.
            let cpi_accounts = anchor_lang::system_program::Allocate {
                account_to_allocate: #field.to_account_info()
            };
            let cpi_context = anchor_lang::context::CpiContext::new(system_program.key(), cpi_accounts);
            anchor_lang::system_program::allocate(cpi_context.with_signer(&[#seeds_with_nonce]), #space as u64)?;
            // Assign to the spl token program.
            let cpi_accounts = anchor_lang::system_program::Assign {
                account_to_assign: #field.to_account_info()
            };
            let cpi_context = anchor_lang::context::CpiContext::new(system_program.key(), cpi_accounts);
            anchor_lang::system_program::assign(cpi_context.with_signer(&[#seeds_with_nonce]), #owner)?;
        }
    }
}

Important clarifications when reading this generated code:

• The comment “return them all back to the payer … so that the account has zero lamports” is misleading in this excerpt: the mitigation here is primarily avoiding create_account when __current_lamports > 0.
• The comment “Assign to the spl token program” is also misleading: the generated code assigns the account to #owner (the owner passed into the constraint), which is not necessarily the SPL Token program.
• with_signer(&[#seeds_with_nonce]) is what makes this work for PDAs: it supplies the PDA signature required by allocate and assign.

Key logic (simplified):

rust
let current_lamports = field.lamports();
if current_lamports == 0 {
    // create_account
} else {
    // transfer (rent top-up)
    // allocate
    // assign (to the intended program owner)
}

By doing so, Anchor effectively mitigates the pre-funding DoS vector.

References#

• https://github.com/solana-labs/solana/blob/master/programs/system/src/system_processor.rs
• https://github.com/solana-foundation/anchor/blob/347c0599b8310d84af4086cfe5c975733a9e17cd/lang/syn/src/codegen/accounts/constraints.rs#L1675-L1728