From 50448ef7c696540a8de89f80cbac088619993837 Mon Sep 17 00:00:00 2001
From: Ruby Martin <ruby@wolfssl.com>
Date: Wed, 25 Mar 2026 10:22:10 -0600
Subject: [PATCH] add guard for integer underflow in DecryptTls13

---
 src/tls13.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/tls13.c b/src/tls13.c
index e63e824d79..870c8f94a5 100644
--- a/src/tls13.c
+++ b/src/tls13.c
@@ -2966,11 +2966,15 @@ int DecryptTls13(WOLFSSL* ssl, byte* output, const byte* input, word16 sz,
                  const byte* aad, word16 aadSz)
 {
     int    ret    = 0;
-    word16 dataSz = sz - ssl->specs.aead_mac_size;
+    word16 dataSz;
     word16 macSz  = ssl->specs.aead_mac_size;
     word32 nonceSz = 0;
 
     WOLFSSL_ENTER("DecryptTls13");
+    if (sz < ssl->specs.aead_mac_size) {
+        return BAD_FUNC_ARG;
+    }
+    dataSz = sz - ssl->specs.aead_mac_size;
 
 #if defined(WOLFSSL_RENESAS_TSIP_TLS)
     ret = tsip_Tls13AesDecrypt(ssl, output, input, sz);