Do you need to prevent Blocking Memcached Exploit in Zimbra ?

This guide is for you.

Memcached is an open source distributed memory caching system.

Here at Ibmi Media, as part of our Server Management Services, we regularly help our Customers to perform Zimbra related configurations.

In this context, we shall look into blocking Memcached exploit.

Nature of Zimbra blocking Memcached exploit ?

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

If Zimbra Memcache servers are behind the firewall, we recommend blocking ingress and egress traffic on port 11211 from the Internet to Zimbra Memcache servers.

You can apply the following methods if the Zimbra Memcache server is not behind a firewall and accessible over the Internet.

For Zimbra Single Server Installation

Configure Memcached to listen on 127.0.0.1 in order to avoid this attack.

We use the below commands:

su – zimbra
/opt/zimbra/bin/zmprov ms `zmhostname` zimbraMemcachedBindAddress 127.0.0.1
/opt/zimbra/bin/zmprov ms `zmhostname` zimbraMemcachedClientServerList 127.0.0.1

Then restart Memcached:

zmmemcachedctl restart

For Zimbra Multi-Server Installation

The workaround is to block traffic on port 11211 from the Internet and allow only from Zimbra proxy servers. First, we need to enable/start iptables or ufw on the server.

Subsequently, make sure Zimbra’s other ports are not block in the firewall.

We run the below commands in the given sequence on ALL Memcached servers.

Iptables rules for Redhat based servers

Drop all connections to port 11211 by running the commands:

iptables -I INPUT -p udp –dport 11211 -j DROP
iptables -I INPUT -p tcp –dport 11211 -j DROP

Now, we accept connections from localhost:

iptables -I INPUT -p udp -s 127.0.0.1 –dport 11211 -j ACCEPT
iptables -I INPUT -p tcp -s 127.0.0.1 –dport 11211 -j ACCEPT

Then we accept connections from other proxy servers. 

Run below two commands for each proxy server IP in the Zimbra setup:

iptables -I INPUT -p udp -s <Proxy IP> –dport 11211 -j ACCEPT
iptables -I INPUT -p tcp -s <Proxy IP> –dport 11211 -j ACCEPT

UFW rules for Ubuntu servers

First, drop all connections to port 11211. To do this, execute:

ufw deny 11211

Then, accept connections from localhost:

ufw allow from 127.0.0.1 to any port 11211

On the other hand, we accept connections from other proxy servers.

Then we run below two commands for each proxy server IP in the Zimbra setup:

ufw allow from <Proxy1 IP> to any port 11211

Testing:

UDP

The below command should give an empty response when executed from any host except proxy servers allowed. If we see the non-empty response that contains PID etc details, then the server is vulnerable.

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

The above command may fail on RedHat servers because nc does not recognize “-q” option. We can use “-w 1 ” instead.

TCP

With the below command, we won’t be able to connect from any host except proxy servers allowed above. If we are able to connect, the server is vulnerable.

telnet <IP of zimbra memcache server> 11211

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