diff options
authorMiroslav Lichvar <>2016-08-01 18:24:24 (GMT)
committerDenys Vlasenko <>2016-08-01 18:25:06 (GMT)
commit150dc7a2b483b8338a3e185c478b4b23ee884e71 (patch)
parent6b5abc95969caf270d269ae640bb64e6bf8a7996 (diff)
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 <>
1 files changed, 7 insertions, 0 deletions
diff --git a/networking/ntpd.c b/networking/ntpd.c
index 130cef0..8ca62cf 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;