Index: server/protocol.c
===================================================================
--- server/protocol.c	(revision 1888902)
+++ server/protocol.c	(working copy)
@@ -1060,6 +1060,14 @@
         return 0;
     }
 
+    if ((r->proto_num != HTTP_VERSION(0, 9)) && (r->proto_num != HTTP_VERSION(1, 0)) &&
+        (r->proto_num != HTTP_VERSION(1, 1)) && (r->proto_num != HTTP_VERSION(2, 0))) {
+        ap_log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r, APLOGNO()
+                      "client sent unsupported HTTP version request: %s", r->uri);
+        r->status = HTTP_VERSION_NOT_SUPPORTED;
+        return 0;
+    }
+
     if ((!r->hostname && (r->proto_num >= HTTP_VERSION(1, 1)))
         || ((r->proto_num == HTTP_VERSION(1, 1))
             && !apr_table_get(r->headers_in, "Host"))) {