feat(validation): add dust transaction rejection guard

src/fees.rs

@@ -216,6 +216,8 @@ pub fn add_corridor_fees(
     variable_fee: u64,
 ) -> Result<CorridorFeePool, ContractError> {
     admin.require_auth();
+    // Reject dust deposits that fall below the minimum transfer threshold.
+    crate::validation::dust::check_min_transfer(collected)?;
     let data = TimeLockedUpgradeContract::get_data(env.clone())?;
     if data.admin != admin {
         return Err(ContractError::NotAdmin);

src/lib.rs

@@ -210,6 +210,9 @@ pub enum ContractError {
     InsufficientBondForPenalty = 46,
     /// The final swap output is below the caller's minimum acceptable amount.
     SlippageExceeded = 47,
+    /// Incoming deposit or swap amount is below the minimum transfer threshold.
+    /// Rejects dust / micro-denomination spam to preserve ledger throughput.
+    AmountTooLow = 48,
     NullifierAlreadyUsed = 48,
     InvalidProof = 49,
 }

src/router/multihop.rs

@@ -262,6 +262,9 @@ fn execute_single_hop(
         return Err(ContractError::ZeroSwapAmount);
     }
 
+    // Reject dust swap inputs below the minimum transfer threshold.
+    crate::validation::dust::check_min_transfer(amount_in)?;
+
     // Resolve corridor fee pool for the input asset.
     let mut pool = fees::get_corridor_fee_pool(env.clone(), step.asset_in);
     if pool.asset != step.asset_in {

src/validation.rs

@@ -18,6 +18,12 @@
 //! from sufficiently liquid markets that cannot be easily manipulated through
 //! temporary capital injection attacks.
 
+// ── Submodules ────────────────────────────────────────────────────────────────
+
+/// Anti-spam dust transaction guard.
+pub mod dust;
+pub use dust::{check_min_transfer, MIN_TRANSFER_AMOUNT};
+
 use soroban_sdk::{contracttype, symbol_short, Address, Env, Map, Symbol, Vec};
 
 use crate::{AssetId, ContractError, CONSENSUS_CACHE_KEY, STAKE_REGISTRY_KEY};

src/validation/dust.rs

@@ -0,0 +1,119 @@
+//! Anti-spam dust transaction guard for StellarFlow payment corridors.
+//!
+//! Rejects micro-denominated deposits and swap amounts that fall below the
+//! minimum economic threshold. Dust transactions waste ledger throughput and
+//! are commonly used in spam attacks across payment corridors.
+//!
+//! # Usage
+//!
+//! Call [`check_min_transfer`] at the top of any deposit or swap handler:
+//!
+//! ```rust,ignore
+//! use crate::validation::dust::check_min_transfer;
+//!
+//! pub fn deposit(env: Env, amount: u64) -> Result<(), ContractError> {
+//!     check_min_transfer(amount)?;
+//!     // … rest of deposit logic
+//!     Ok(())
+//! }
+//! ```
+
+use crate::ContractError;
+
+/// Minimum transfer amount (in stroops) accepted by deposit and swap entry points.
+///
+/// Transactions carrying an amount below this threshold are rejected with
+/// [`ContractError::AmountTooLow`] to preserve ledger throughput and block
+/// spam across payment corridors.
+///
+/// Set to 10,000 stroops (0.001 XLM) — enough to cover base reserve costs
+/// while being negligibly small for any legitimate cross-border remittance.
+pub const MIN_TRANSFER_AMOUNT: u64 = 10_000;
+
+/// Reject a transfer whose `amount` is below [`MIN_TRANSFER_AMOUNT`].
+///
+/// # Errors
+///
+/// Returns [`ContractError::AmountTooLow`] when `amount < MIN_TRANSFER_AMOUNT`.
+///
+/// # Examples
+///
+/// ```rust,ignore
+/// assert!(check_min_transfer(10_000).is_ok());
+/// assert_eq!(check_min_transfer(9_999), Err(ContractError::AmountTooLow));
+/// assert_eq!(check_min_transfer(0),     Err(ContractError::AmountTooLow));
+/// ```
+pub fn check_min_transfer(amount: u64) -> Result<(), ContractError> {
+    if amount < MIN_TRANSFER_AMOUNT {
+        return Err(ContractError::AmountTooLow);
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // ── Rejection cases ──────────────────────────────────────────────────────
+
+    #[test]
+    fn rejects_zero_amount() {
+        assert_eq!(check_min_transfer(0), Err(ContractError::AmountTooLow));
+    }
+
+    #[test]
+    fn rejects_one_stroop() {
+        assert_eq!(check_min_transfer(1), Err(ContractError::AmountTooLow));
+    }
+
+    #[test]
+    fn rejects_one_below_threshold() {
+        assert_eq!(
+            check_min_transfer(MIN_TRANSFER_AMOUNT - 1),
+            Err(ContractError::AmountTooLow)
+        );
+    }
+
+    #[test]
+    fn rejects_typical_dust_amount_100_stroops() {
+        assert_eq!(check_min_transfer(100), Err(ContractError::AmountTooLow));
+    }
+
+    #[test]
+    fn rejects_half_threshold() {
+        assert_eq!(
+            check_min_transfer(MIN_TRANSFER_AMOUNT / 2),
+            Err(ContractError::AmountTooLow)
+        );
+    }
+
+    // ── Acceptance cases ─────────────────────────────────────────────────────
+
+    #[test]
+    fn accepts_exactly_at_threshold() {
+        assert!(check_min_transfer(MIN_TRANSFER_AMOUNT).is_ok());
+    }
+
+    #[test]
+    fn accepts_one_above_threshold() {
+        assert!(check_min_transfer(MIN_TRANSFER_AMOUNT + 1).is_ok());
+    }
+
+    #[test]
+    fn accepts_typical_remittance_one_xlm() {
+        // 1 XLM = 10_000_000 stroops — well above threshold.
+        assert!(check_min_transfer(10_000_000).is_ok());
+    }
+
+    #[test]
+    fn accepts_large_transfer() {
+        assert!(check_min_transfer(u64::MAX).is_ok());
+    }
+
+    // ── Constant sanity ──────────────────────────────────────────────────────
+
+    #[test]
+    fn min_transfer_constant_is_ten_thousand() {
+        assert_eq!(MIN_TRANSFER_AMOUNT, 10_000);
+    }
+}