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fix(transfer-sol): require payer signature + checked lamport math #532

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fix(transfer-sol): require payer signature + checked lamport math #532

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34 changes: 30 additions & 4 deletions basics/transfer-sol/anchor/programs/transfer-sol/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,12 +27,36 @@ pub mod transfer_sol {
ctx: Context<TransferSolWithProgram>,
amount: u64,
) -> Result<()> {
**ctx.accounts.payer.try_borrow_mut_lamports()? -= amount;
**ctx.accounts.recipient.try_borrow_mut_lamports()? += amount;
// Security invariants:
// - The source account must authorize the transfer (is_signer)
// - The source account must be owned by this program (direct lamports mutation)
// - Prevent under/overflow on lamport arithmetic

let payer_lamports = ctx.accounts.payer.to_account_info().lamports();
require!(payer_lamports >= amount, TransferSolError::InsufficientFunds);

**ctx.accounts.payer.try_borrow_mut_lamports()? = payer_lamports
.checked_sub(amount)
.ok_or(TransferSolError::LamportArithmeticOverflow)?;
Comment on lines +35 to +40
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Copilot AI Feb 15, 2026

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New failure modes are introduced here (missing payer signature, insufficient funds, arithmetic error), but the test suite only covers the success path. Add negative tests that (1) omit .signers([payerAccount]) and assert the instruction fails due to missing signature, and (2) attempt to transfer more than the payer balance and assert InsufficientFunds is returned (and balances remain unchanged).

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let recipient_lamports = ctx.accounts.recipient.to_account_info().lamports();
**ctx.accounts.recipient.try_borrow_mut_lamports()? = recipient_lamports
.checked_add(amount)
.ok_or(TransferSolError::LamportArithmeticOverflow)?;

Ok(())
}
}

#[error_code]
pub enum TransferSolError {
#[msg("Insufficient funds in payer account")]
InsufficientFunds,

#[msg("Lamport arithmetic overflow/underflow")]
LamportArithmeticOverflow,
}

#[derive(Accounts)]
pub struct TransferSolWithCpi<'info> {
#[account(mut)]
Expand All @@ -44,12 +68,14 @@ pub struct TransferSolWithCpi<'info> {

#[derive(Accounts)]
pub struct TransferSolWithProgram<'info> {
/// CHECK: Use owner constraint to check account is owned by our program
// NOTE: This account must sign the transaction, otherwise *anyone* could drain lamports
// from program-owned accounts passed into this instruction.
#[account(
mut,
owner = id() // value of declare_id!()
)]
payer: UncheckedAccount<'info>,
payer: Signer<'info>,

#[account(mut)]
recipient: SystemAccount<'info>,
}
1 change: 1 addition & 0 deletions basics/transfer-sol/anchor/tests/test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,7 @@ describe("Anchor: Transfer SOL", () => {
payer: payerAccount.publicKey,
recipient: recipientAccount.publicKey,
})
.signers([payerAccount])
.rpc();

const recipientBalance = await provider.connection.getBalance(
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