The Network Time Protocol, or NTP, syncs time between machines on the network, and runs over port 123 UDP. It's typically configured once by network administrators and often is not updated, according to Symantec, which discovered a major jump in attacks via the protocol over the past few weeks.
"NTP is one of those set-it-and-forget-it protocols that is configured once, and most network administrators don't worry about it after that. Unfortunately, that means it is also not a service that is upgraded often, leaving it vulnerable to these reflection attacks," says Allan Liska, a Symantec researcher in blog post last week.
Attackers appear to be employing NTP for DDoSing similar to the way DNS is being abused in such attacks. They transmit small spoofed packets requesting a large amount of data sent to the DDoS target's IP address. According to Symantec, it's all about abusing the so-called "monlist" command in an older version of NTP. Monlist returns a list of the last 600 hosts that have connected to the server. "For attackers the monlist query is a great reconnaissance tool. For a localized NTP server it can help to build a network profile. However, as a DDoS tool, it is even better because a small query can redirect megabytes worth of traffic," Liska explains in the post.
Monlist modules can be found in NMAP as well as in Metasploit, for example. Metasploit includes monlist DDoS exploit module.
The spike in NTP reflection attacks occurred mainly in mid-December, with close to 15,000 IPs affected, and dropped off significantly after Dec. 23, according to Symantec's data.
Symantec recommends that organizations update their NTP implementations to version 4.2.7, which does not use the monlist command. Another option is to disable access to monlist in older versions of NTP. "By disabling monlist, or upgrading so the command is no longer there, not only are you protecting your network from unwanted reconnaissance, but you are also protecting your network from inadvertently being used in a DDoS attack," Liska says.
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