From e34ccaf4817613563ce616fdb20d060bf6d47507 Mon Sep 17 00:00:00 2001
From: Juliusz Sosinowicz <juliusz@wolfssl.com>
Date: Wed, 9 Sep 2020 23:23:32 +0200
Subject: [PATCH] Fix window check

If `curLT` then diff needs to be decremented. For example: `diff` = 1 represents last packet so it would be the `window[idx] & (1 << 0)` bit of the window variable.
---
 src/internal.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/internal.c b/src/internal.c
index 17c6c0cf7..72b9dd2f1 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -12906,8 +12906,15 @@ static WC_INLINE int DtlsCheckWindow(WOLFSSL* ssl)
     }
 #endif
     else if (curLT) {
-        word32 idx = diff / DTLS_WORD_BITS;
-        word32 newDiff = diff % DTLS_WORD_BITS;
+        word32 idx;
+        word32 newDiff;
+        if (diff == 0) {
+            WOLFSSL_MSG("DTLS sanity check failed");
+            return 0;
+        }
+        diff--;
+        idx = diff / DTLS_WORD_BITS;
+        newDiff = diff % DTLS_WORD_BITS;
 
         /* verify idx is valid for window array */
         if (idx >= WOLFSSL_DTLS_WINDOW_WORDS) {