diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 0a8f4a3..4adaf7d 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -5148,6 +5148,18 @@
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
+    /* If we're in the middle of a fragmented TLS handshake message,
+     * we don't accept any other message type. For TLS 1.3, the spec forbids
+     * interleaving other message types between handshake fragments. For TLS
+     * 1.2, the spec does not forbid it but we do. */
+    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_STREAM &&
+        ssl->badmac_seen_or_in_hsfraglen != 0 &&
+        ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE) {
+        MBEDTLS_SSL_DEBUG_MSG(1, ("non-handshake message in the middle"
+                                  " of a fragmented handshake message"));
+        return MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
+    }
+
     /*
      * Handle particular types of records
      */