From 150dc7a2b483b8338a3e185c478b4b23ee884e71 Mon Sep 17 00:00:00 2001 From: Miroslav Lichvar Date: Mon, 1 Aug 2016 20:24:24 +0200 Subject: [PATCH] ntpd: respond only to client and symmetric active packets The busybox NTP implementation doesn't check the NTP mode of packets received on the server port and responds to any packet with the right size. This includes responses from another NTP server. An attacker can send a packet with a spoofed source address in order to create an infinite loop of responses between two busybox NTP servers. Adding more packets to the loop increases the traffic between the servers until one of them has a fully loaded CPU and/or network. Signed-off-by: Miroslav Lichvar Signed-off-by: Denys Vlasenko --- networking/ntpd.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/networking/ntpd.c b/networking/ntpd.c index 130cef0af..8ca62cf1b 100644 --- a/networking/ntpd.c +++ b/networking/ntpd.c @@ -2051,6 +2051,13 @@ recv_and_process_client_pkt(void /*int fd*/) goto bail; } + /* Respond only to client and symmetric active packets */ + if ((msg.m_status & MODE_MASK) != MODE_CLIENT + && (msg.m_status & MODE_MASK) != MODE_SYM_ACT + ) { + goto bail; + } + query_status = msg.m_status; query_xmttime = msg.m_xmttime; -- 2.25.1