From 19821995fbf57e26f9fb009724e847c7f4d07d97 Mon Sep 17 00:00:00 2001
From: Reuven Podmazo <reuven@enigma.co>
Date: Thu, 27 May 2021 16:30:01 +0300
Subject: [PATCH] check allowance expiration before increasing or decreasing it

---
 src/contract.rs | 28 +++++++++++++++++++++++-----
 src/state.rs    |  9 +++++++++
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/src/contract.rs b/src/contract.rs
index 6a8efcd..ac84031 100644
--- a/src/contract.rs
+++ b/src/contract.rs
@@ -993,9 +993,7 @@ fn use_allowance<S: Storage>(
 ) -> StdResult<()> {
     let mut allowance = read_allowance(storage, owner, spender)?;
 
-    if allowance.expiration.map(|ex| ex < env.block.time) == Some(true) && allowance.amount != 0 {
-        allowance.amount = 0;
-        write_allowance(storage, owner, spender, allowance)?;
+    if allowance.is_expired_at(&env.block) {
         return Err(insufficient_allowance(0, amount));
     }
     if let Some(new_allowance) = allowance.amount.checked_sub(amount) {
@@ -1338,7 +1336,17 @@ fn try_increase_allowance<S: Storage, A: Api, Q: Querier>(
     let spender_address = deps.api.canonical_address(&spender)?;
 
     let mut allowance = read_allowance(&deps.storage, &owner_address, &spender_address)?;
-    allowance.amount = allowance.amount.saturating_add(amount.u128());
+
+    // If the previous allowance has expired, reset the allowance.
+    // Without this users can take advantage of an expired allowance given to
+    // them long ago.
+    if allowance.is_expired_at(&env.block) {
+        allowance.amount = amount.u128();
+        allowance.expiration = None;
+    } else {
+        allowance.amount = allowance.amount.saturating_add(amount.u128());
+    }
+
     if expiration.is_some() {
         allowance.expiration = expiration;
     }
@@ -1373,7 +1381,17 @@ fn try_decrease_allowance<S: Storage, A: Api, Q: Querier>(
     let spender_address = deps.api.canonical_address(&spender)?;
 
     let mut allowance = read_allowance(&deps.storage, &owner_address, &spender_address)?;
-    allowance.amount = allowance.amount.saturating_sub(amount.u128());
+
+    // If the previous allowance has expired, reset the allowance.
+    // Without this users can take advantage of an expired allowance given to
+    // them long ago.
+    if allowance.is_expired_at(&env.block) {
+        allowance.amount = 0;
+        allowance.expiration = None;
+    } else {
+        allowance.amount = allowance.amount.saturating_sub(amount.u128());
+    }
+
     if expiration.is_some() {
         allowance.expiration = expiration;
     }
diff --git a/src/state.rs b/src/state.rs
index 058c4e5..a9d9a6a 100644
--- a/src/state.rs
+++ b/src/state.rs
@@ -303,6 +303,15 @@ pub struct Allowance {
     pub expiration: Option<u64>,
 }
 
+impl Allowance {
+    pub fn is_expired_at(&self, block: &cosmwasm_std::BlockInfo) -> bool {
+        match self.expiration {
+            Some(time) => block.time >= time,
+            None => false, // allowance has no expiration
+        }
+    }
+}
+
 pub fn read_allowance<S: Storage>(
     store: &S,
     owner: &CanonicalAddr,