From a963c5fc472318457da8dec1c27e818e17f4ada6 Mon Sep 17 00:00:00 2001
From: Ruby Martin <ruby@wolfssl.com>
Date: Wed, 25 Mar 2026 16:35:34 -0600
Subject: [PATCH] add check to prevent integer underflow

---
 src/internal.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/internal.c b/src/internal.c
index 3555a96eb5..4280153719 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -21240,7 +21240,8 @@ static int SanityCheckCipherText(WOLFSSL* ssl, word32 encryptSz)
     if (ssl->specs.cipher_type == block) {
 #ifdef HAVE_ENCRYPT_THEN_MAC
         if (ssl->options.startedETMRead) {
-            if ((encryptSz - MacSize(ssl)) % ssl->specs.block_size) {
+            if (encryptSz < minLength ||
+                (encryptSz - MacSize(ssl)) % ssl->specs.block_size) {
                 WOLFSSL_MSG("Block ciphertext not block size");
                 WOLFSSL_ERROR_VERBOSE(SANITY_CIPHER_E);
                 return SANITY_CIPHER_E;