Recently Websense discovered malware that caused quite a lot of media attention, because it used ICMP to send personal data out of the victims pc or network. In many networks, outgoing ICMP is unrestricted because it can aid in solving connectivity and routing problems. While (partially) blocking outgoing ICMP would certainly solve this problem, not everone will be willing to do so. Note however that the idea of using ICMP as a covert channel is hardly new.
Yesterday I wrote about creating custom Sancp queries and adding them to Sguil as canned reports. Today i tried to create a query that can be used to detect anomolies in ICMP traffic leaving your network. Like yesterday, i did this in the form of a ‘canned report’. One limitation is that there seems to be no way to get any information on exact ICMP codes and types.
I tried to create a query that would do the following. I assumed that there are at least two ways the traffic can stand out. Either by it’s volume, or by non-default packet sizes. Normally ping uses a fixed packet size of 64 bytes on my Linux system, Windows 2000 defaults to 32 bytes. If the packet size is not what you would expect, something fishy might be going on. So the query tries to find the execptional cases in this respect.
Volume is another factor. To tranfser substantial amounts of data through ICMP, the volume of the packets will need to be quite high as well. Note however that Linux’ ping utility for example pings until you tell it to stop, and it happened to me before that i accidently left it running an entire night, so a large volume doesn’t have to mean there is something wrong. The target IP is also very interesting. If it is IP belonging to Google there is a big chance that it is just a test. In the example below most volume is to 126.96.36.199 which is the server in my ISP’s network that can be reached under ‘ping.xs4all.nl’.
In this report you can see that 192.168.1.2 pinged 188.8.131.52 with an average packet size of 1480 bytes, which is certainly not normal. It transfered 0.08MB to this IP. Another amount of 2.24MB was transfered to the IP 184.108.40.206, with a average packet size of 64.33 bytes. This is weird because it is not exactly 64 bytes, what I would expect. This can be explained by me running a default ping for a long time and after that doing a few tests with larger sizes.
Here is the query:
TOPTENICMP||Top ICMP users in the LAN talking to the WAN, with source and destination, volume and average packet size, sorted by packet size (fields: source ip, dest ip, from source MB, from source packet size, from dest MB, from dest packet size)||query||select INET_NTOA(src_ip), INET_NTOA(dst_ip), sum(src_bytes)/1048576 as from_src_bytes, sum(src_bytes)/sum(src_pkts) as from_src_avg_pktsize, sum(dst_bytes)/1048576 as from_dst_bytes, sum(dst_bytes)/sum(dst_pkts) as from_dst_avg_pktsize from sancp INNER JOIN sensor ON sancp.sid=sensor.sid WHERE start_time > %%STARTTIME%% AND end_time < %%ENDTIME%% AND %%SENSORS%% AND ip_proto = 1 AND ((src_ip between INET_ATON("192.168.0.0") and INET_ATON("192.168.255.255")) or (src_ip between INET_ATON("10.0.0.0") and INET_ATON("10.255.255.255")) or (src_ip between INET_ATON("172.16.0.0") and INET_ATON("172.31.255.255"))) and ((dst_ip not between INET_ATON("192.168.0.0") and INET_ATON("192.168.255.255")) and (dst_ip not between INET_ATON("10.0.0.0") and INET_ATON("10.255.255.255")) and (dst_ip not between INET_ATON("172.16.0.0") and INET_ATON("172.31.255.255"))) GROUP BY src_ip,dst_ip ORDER BY from_src_avg_pktsize DESC,from_src_bytes DESC LIMIT 10||6||
A large portion of the query is dedicated to making sure the source IP is in a private IP range and the destination IP isn’t. I have highlighted the part where the average packet size is calculated. The query is first sorted by the average packet size flowing from the source IP to the WAN, then by the volume in that same direction. Since I focus on data leaving the network, I sorted on the LAN to WAN direction.
To use this query, copy-paste it into /etc/sguild/sguild.reports and restart sguild. It should appear in the Sensor Reports.