Sigvex
SolanaRemediation3 min read

Lamport Drain Remediation

How to prevent unauthorized SOL transfers by validating signer authorization and stored authority before any lamport modification.

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Lamport Drain Remediation

Overview

Lamport drain vulnerabilities arise when a Solana program transfers SOL (lamports) without verifying that the source account’s authority has signed the transaction. The remediation requires checking is_signer on the authority account and verifying that the signer’s key matches the stored authority in the vault account data before any lamport modification.

Related Detector: Lamport Drain

Before (Vulnerable)

use solana_program::{account_info::AccountInfo, entrypoint::ProgramResult};

pub fn withdraw(accounts: &[AccountInfo], amount: u64) -> ProgramResult {
    let vault = &accounts[0];
    let recipient = &accounts[1];

    // VULNERABLE: no authorization check — anyone can drain any account
    **vault.lamports.borrow_mut() -= amount;
    **recipient.lamports.borrow_mut() += amount;
    Ok(())
}

After (Fixed)

use solana_program::{
    account_info::AccountInfo,
    entrypoint::ProgramResult,
    program_error::ProgramError,
    pubkey::Pubkey,
};

pub fn withdraw(accounts: &[AccountInfo], amount: u64) -> ProgramResult {
    let authority = &accounts[0];
    let vault = &accounts[1];
    let recipient = &accounts[2];

    // Check 1: authority must have signed the transaction
    if !authority.is_signer {
        return Err(ProgramError::MissingRequiredSignature);
    }

    // Check 2: authority must match the stored authority in vault data
    let vault_data = vault.data.borrow();
    let stored_authority = Pubkey::try_from(&vault_data[0..32])
        .map_err(|_| ProgramError::InvalidAccountData)?;
    if authority.key != &stored_authority {
        return Err(ProgramError::InvalidAccountData);
    }

    // Safe to transfer
    **vault.lamports.borrow_mut() -= amount;
    **recipient.lamports.borrow_mut() += amount;
    Ok(())
}

Alternative Mitigations

Anchor has_one and close constraints — an account-validation framework handles authorization and lamport transfer declaratively:

use anchor_lang::prelude::*;

#[account]
pub struct Vault {
    pub authority: Pubkey,
    pub balance: u64,
}

#[derive(Accounts)]
pub struct CloseVault<'info> {
    pub authority: Signer<'info>,            // Enforces is_signer
    #[account(
        mut,
        has_one = authority,                  // Enforces vault.authority == authority.key()
        close = recipient                     // Transfers all lamports and closes the account
    )]
    pub vault: Account<'info, Vault>,
    #[account(mut)]
    pub recipient: SystemAccount<'info>,
}

pub fn close_vault(ctx: Context<CloseVault>) -> Result<()> {
    // Anchor validates authority via Signer<'info> + has_one constraint
    // Lamport transfer is handled by close = recipient
    Ok(())
}

PDA-signed transfers — for program-owned accounts, use invoke_signed with the PDA’s seeds instead of checking is_signer:

use solana_program::program::invoke_signed;
use solana_program::system_instruction;

pub fn transfer_from_pda(
    accounts: &[AccountInfo],
    amount: u64,
    pda_bump: u8,
) -> ProgramResult {
    let pda_account = &accounts[0];
    let recipient = &accounts[1];

    // PDA signs via invoke_signed — no is_signer check needed
    invoke_signed(
        &system_instruction::transfer(pda_account.key, recipient.key, amount),
        &[pda_account.clone(), recipient.clone()],
        &[&[b"vault", &[pda_bump]]],
    )
}

Common Mistakes

Checking is_signer but not verifying key — if any signer can authorize a transfer, an attacker can create their own account, sign with it, and drain any vault whose authority is not verified.

Checking key but not is_signer — an attacker can pass an account whose key matches the stored authority but that has not signed the transaction.

Forgetting to zero lamports on close — when closing an account, set both lamports = 0 and the data to zeros; otherwise the runtime may not fully close the account.

References