diff --git a/contracts/oracle-verifier/src/lib.rs b/contracts/oracle-verifier/src/lib.rs
index 467d29b..7e1dd10 100644
--- a/contracts/oracle-verifier/src/lib.rs
+++ b/contracts/oracle-verifier/src/lib.rs
@@ -1277,6 +1277,11 @@ impl OracleVerifier {
     /// consume storage/instruction budget (griefing).
     pub fn set_min_submit_interval(env: Env, admin: Address, seconds: u64) {
         Self::require_admin(&env, &admin);
+        // Issue #624: reject zero -- a zero interval removes the per-oracle
+        // rate-limit guard and allows unlimited oracle submissions per block.
+        if seconds == 0 {
+            panic_with_error!(&env, Error::InvalidInput);
+        }
         env.storage()
             .instance()
             .set(&StorageKey::MinSubmitInterval, &seconds);
diff --git a/contracts/policy-engine/src/lib.rs b/contracts/policy-engine/src/lib.rs
index cdde810..c8b1c32 100644
--- a/contracts/policy-engine/src/lib.rs
+++ b/contracts/policy-engine/src/lib.rs
@@ -647,6 +647,11 @@ impl PolicyEngine {
         if duration_days == 0 {
             panic_with_error!(env, Error::InvalidDurationRange);
         }
+        // Issue #623: validate duration_days against the product ceiling
+        // explicitly.  The computed (end_time - start_time) must never
+        // exceed max_duration_days * 86_400 seconds.  Checking the
+        // parameter here (before any arithmetic) ensures this holds even
+        // for scheduled-start policies where start_time != now.
         if duration_days > product.max_duration_days {
             panic_with_error!(env, Error::DurationTooLong);
         }
@@ -683,6 +688,15 @@ impl PolicyEngine {
         if end_time <= start_time {
             panic_with_error!(env, Error::InvalidDurationRange);
         }
+        // Issue #623: defense-in-depth -- confirm the computed span does
+        // not exceed max_duration_days * 86_400 seconds.  This catches any
+        // future code path that might compute duration_secs independently.
+        let max_duration_secs = (product.max_duration_days as u64)
+            .checked_mul(86_400)
+            .unwrap_or(u64::MAX);
+        if end_time.saturating_sub(start_time) > max_duration_secs {
+            panic_with_error!(env, Error::DurationTooLong);
+        }
         let policy_id = Self::next_policy_id(env);
 
         let policy = Policy {
diff --git a/contracts/risk-pool/src/lib.rs b/contracts/risk-pool/src/lib.rs
index dfbc61b..38c91e6 100644
--- a/contracts/risk-pool/src/lib.rs
+++ b/contracts/risk-pool/src/lib.rs
@@ -390,6 +390,8 @@ impl RiskPool {
             }
             minted - burn_shares
         } else {
+            // Issue #621: integer division truncates toward zero; the
+            // MIN_SHARES guard below catches the resulting 0 case.
             amount.checked_mul(total_shares)
                 .and_then(|v| v.checked_div(total_deposited))
                 .unwrap_or_else(|| panic_with_error!(&env, Error::Overflow))
@@ -539,6 +541,11 @@ impl RiskPool {
 
         let available_liquidity = total_deposited.saturating_sub(total_locked);
         if available_liquidity <= 0 { panic_with_error!(&env, Error::Undercollateralized); }
+        // Issue #621: shares * total_deposited / total_shares truncates
+        // toward zero; if the pool has absorbed losses total_deposited
+        // may be tiny relative to total_shares and a small redemption
+        // rounds to 0.  The ZeroAmount guard below rejects such cases so
+        // no shares are ever burned without returning capital.
         let amount = shares.checked_mul(total_deposited)
             .and_then(|v| v.checked_div(total_shares))
             .unwrap_or_else(|| panic_with_error!(&env, Error::Overflow));
@@ -692,9 +699,14 @@ impl RiskPool {
             panic_with_error!(&env, Error::InsufficientShares);
         }
 
+        // Issue #621: share-to-amount calculation can underflow to 0 when
+        // from_pos.deposited has been eroded by losses while shares stayed
+        // constant.  A zero-amount transfer burns the shares without moving
+        // any capital, so reject it explicitly.
         let amount = shares.checked_mul(from_pos.deposited)
             .and_then(|v| v.checked_div(from_pos.shares))
             .unwrap_or_else(|| panic_with_error!(&env, Error::Overflow));
+        if amount == 0 { panic_with_error!(&env, Error::ZeroAmount); }
 
         let pending_yield_from = Self::settle_yield(&env, &mut from_pos);
         from_pos.deposited = from_pos.deposited.saturating_sub(amount);