# HG changeset patch
# User Vladislav Grishenko <themiron@users.noreply.github.com>
# Date 1592227338 -18000
# Node ID 3974f087d9c0c0b526736776b4b8684f1cc74469
# Parent  c0f12eaf95c9f46a5a69560c5468dd38e8cfff97
Disallow leading lines before the ident for server (#102)

Per RFC4253 4.2 clients must be able to process other lines of data
before the version string, server behavior is not defined neither
with MUST/SHOULD nor with MAY.
If server process up to 50 lines too - it may cause too long hanging
session with invalid/evil client that consume host resources and
potentially may lead to DDoS on poor embedded boxes.
Let's require first line from client to be version string and fail
early if it's not - matches both RFC and real OpenSSH behavior.

diff -r c0f12eaf95c9 -r 3974f087d9c0 common-session.c
--- a/common-session.c	Thu Jun 11 00:09:15 2020 +0800
+++ b/common-session.c	Mon Jun 15 18:22:18 2020 +0500
@@ -370,8 +370,11 @@
 	int len = 0;
 	char done = 0;
 	int i;
-	/* If they send more than 50 lines, something is wrong */
-	for (i = 0; i < 50; i++) {
+
+	/* Servers may send other lines of data before sending the
+	 * version string, client must be able to process such lines.
+	 * If they send more than 50 lines, something is wrong */
+	for (i = IS_DROPBEAR_CLIENT ? 50 : 1; i > 0; i--) {
 		len = ident_readln(ses.sock_in, linebuf, sizeof(linebuf));
 
 		if (len < 0 && errno != EINTR) {