Chargen (UDP port 19) – Reflected ddos

Lately I’m seeing chargen service being abused a lot to execute distributed denial of service attacks.
It’s not just “standard ddos”… it’s a reflected ddos with a massive amplification rate!!!
(Amplification rate can be as high as 512x… that means with that just a 100mbit pipe a malicius attacker could easely accomplish a 10gbit+ ddos!)

What is chargen?

From wikipedia:
In the UDP implementation of the protocol, the server sends a UDP datagram containing a random number (between 0 and 512) of characters every time it receives a datagram from the connecting host.

Apparently there’s absolutely no handshake at all with chargen… only the TCP version (obviously) requires handshake…

How are hosts running chargen (UDP) used as botnets?

To execute the attack people are sending spoofed UDP packets with a forged source IP address to hundreds of hosts running chargen (and there are many of them!).
These hosts just reply to the apparent source of such packet as they are intendend to do… the problem is that they are replying to the forged IP address… that host has never requested something to them!

Is my machine vulnerable?

To test if your machine could be exploited just run:

echo t | nc -u X.X.X.X 19

replace X.X.X.X with an IP running chargen… If you got a reply you just found a host that can be used as part of a ddos botnet…

How can I make my machine secure?

disable chargen service:
(please be aware of another weak point of chargen: looks like it can also be used to let machines running chargen attack each-other… guess what happens when you have two chargen sending packets each other with to&from port 19 udp… we got a loop! :D)

if you are running chargen on one of your hosts: CLOSE THAT PORT (IN UDP 19)!
if you are a carrier/ISP that allows spoofed traffic to leave your network: HOPE YOU GET BANKRUPT AND CLOSE YOUR DOORS FOR GOOD!


read more for additional details and logs

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