Memcached DDoS – There’s Still Time to Save Your Mind

In case you haven’t heard, there’s a new vector for Distributed Denial of Service (DDoS) attacks out there right now and it’s pretty massive. The first mention I saw this week was from Cloudflare, where they details that they were seeing a huge influx of traffic from UDP port 11211. That’s the port used by memcached, a database caching system.

Surprisingly, or not, there were thousands of companies that had left UDP/11211 open to the entire Internet. And, by design, memcached responds to anyone that queries that port. Also, carefully crafted packets can be amplified to have massive responses. In Cloudflare’s testing they were able to send a 15 byte packet and get a 134KB response. Given that this protocol is UDP and capable of responding to forged packets in such a way as to make life miserable for Cloudflare and, now, Github, which got blasted with the largest DDoS attack on record.

How can you fix this problem in your network? There are many steps you can take, whether you are a system admin or a network admin:

  • Go to Shodan and see if you’re affected. Just plug in your company’s IP address ranges and have it search for UDP 11211. If you pop up, you need to find out why memcached is exposed to the internet.
  • If memcached isn’t supposed to be publicly available, you need to block it at the edge. Don’t let anyone connect to UDP port 11211 on any device inside your network from outside of it. That sounds like a no-brainer, but you’d be surprised how many firewall rules aren’t carefully crafted in that way.
  • If you have to have memcached exposed, make sure you talk to that team and find out what their bandwidth requirements are for the application. If it’s something small-ish, create a policer or QoS policy that rate limits the memcached traffic so there’s no way it can exceed that amount. And if that amount is more than 100Mbit of traffic, you need to have an entirely different discussion with your developers.
  • From Cloudflare’s blog, you can disable UDP on memcached on startup by adding the -U 0 flag. Make sure you check with the team that uses it before you disable it though before you break something.

Tom’s Take

Exposing unnecessary services to the Internet is asking for trouble. Given an infinite amount of time, a thousand monkeys on typewriters will create a Shakespearean play that details how to exploit that service for a massive DDoS attack. The nature of protocols to want to help make things easier doesn’t make our jobs easier. They respond to what they hear and deliver what they’re asked. We have to prevent bad actors from getting away with things in the network and at the system level because application developers rarely ask “may I” before turning on every feature to make users happy.

Make sure you check your memcached settings today and immunize yourself from this problem. If Github got blasted with 1.3Tbps of traffic this week there’s no telling who’s going to get hit next.

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