Blocking Memcached Attack

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Detailed in this blog post, Zimbra memcached may face "memcrashd" attack on port 11211. By-default memcached listens on server IP address which is accessible in the network and on Internet if there is no firewall.

If your Zimbra memcache servers are behind firewall, we recommend blocking access on port 11211 from Internet to zimbra memcache servers. Once done, you do not need to follow further steps on this wiki.

If your zimbra memcache server is NOT behind a firewall and accessible over Internet, then proceed.

For Zimbra Single Server Installation

Configure memcached to listen on only to avoid this attack. Use below commands.

 su - zimbra
 /opt/zimbra/bin/zmprov ms `zmhostname` zimbraMemcachedBindAddress 
 /opt/zimbra/bin/zmprov ms `zmhostname` zimbraMemcachedClientServerList

Restart memcached:

 zmmemcachedctl restart

For Zimbra Multi Server Installation

On zimbra multi server setup, workaround is to block UDP access on port 11211. First you need to enable/start iptables or ufw on the server. Make sure zimbra's other ports are not blocked in the firewall. Run below commands in the given sequence on ALL memcache servers.

Iptables rules for Redhat based servers

 iptables -I INPUT -p udp -s --dport 11211 -j ACCEPT
 iptables -I INPUT -p udp --dport 11211 -j DROP

UFW rules for Ubuntu based servers

 ufw allow from proto udp to any port 11211 
 ufw deny from any proto udp to any port 11211 


Below command should give an empty response when executed from any host except localhost.

 echo -en "\x00\x00\x00\x00\x00\x01\x00\x00stats\r\n" | nc -q1 -u <IP of zimbra memcache server> 11211

If you see non-empty response that contains PID etc details, then your server is vulnerable.

Above command may fail on redhat servers because nc does not have "-q" option. You can use "-w 1 " instead of it.

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