diff --git a/src/internal.c b/src/internal.c
index fd82be8738..c009452d62 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -19088,6 +19088,8 @@ static int DoHandShakeMsg(WOLFSSL* ssl, byte* input, word32* inOutIdx,
 
     /* curSize has already been reduced to content-only (padSz subtracted)
      * in ProcessReply, so curStartIdx + curSize bounds the content. */
+    if (*inOutIdx > (word32)ssl->curStartIdx + ssl->curSize)
+        return BUFFER_ERROR;
     inputLength = ssl->curStartIdx + ssl->curSize - *inOutIdx;
 
     /* If there is a pending fragmented handshake message,
diff --git a/src/tls13.c b/src/tls13.c
index 5137ad8133..a3ad1eacf5 100644
--- a/src/tls13.c
+++ b/src/tls13.c
@@ -13936,6 +13936,8 @@ int DoTls13HandShakeMsg(WOLFSSL* ssl, byte* input, word32* inOutIdx,
 
     /* curSize has already been reduced to content-only (padSz subtracted)
      * in ProcessReply, so curStartIdx + curSize bounds the content. */
+    if (*inOutIdx > (word32)ssl->curStartIdx + ssl->curSize)
+        return BUFFER_ERROR;
     inputLength = ssl->curStartIdx + ssl->curSize - *inOutIdx;
 
     /* If there is a pending fragmented handshake message,