Using fail2ban to block unauthorized calls to CloudStack API

Apache CloudStack does not have a build-in mechanism to rate-limit failed authentication attemps on the API. This potentially allows an attacker to brute-force credentials and gain access.

The api.allowed.source.cidr.list configuration option in CloudStack can be used to globally or on an account level limit the source IPs where the API allows requests from. This is always good to do (if possible), but it does not cover every use-case.

Sometimes you just want to keep malicious traffic outside the door and fail2ban can help there.

Nginx proxy in front of CloudStack

A common use-case is that the Management server of Apache CloudStack is not directly connected to the network, but placed behind a reverse proxy like Nginx or something similar.

This proxy can then also handle SSL termination.

In this example we’re using Nginx as a proxy.

fail2ban

Using fail2ban we can scan the access logs of Nginx and block IP addresses who are abusing our API. In this case we filter on two HTTP status codes:

  • 401
  • 531

This results in that we create the following files:

  • /etc/fail2ban/jail.d/nginx-401.conf
  • /etc/fail2ban/jail.d/nginx-531.conf
  • /etc/fail2ban/filter.d/nginx-401.conf
  • /etc/fail2ban/filter.d/nginx-531.conf

jail.d/nginx-401.conf

[nginx-401]
enabled = true
port = http,https
filter = nginx-401
action = iptables-allports
logpath = %(nginx_access_log)s
bantime = 3600
findtime = 600
maxretry = 25
ignoreip = 127.0.0.1/8

filter.d/nginx-401.conf

[Definition]
failregex = ^ -."(GET|POST|HEAD).HTTP.*" 401
ignoreregex =

Change 401 to 531 where needed to also block HTTP codes 531.

iptables

The action taken by fail2bain is iptables-allports which causes iptables to block all traffic from the particular source IP when it is being banned.

Published by

Wido den Hollander

I am the CTO at PCextreme B.V. and i will keep this blog just to post some interesting information about my daily work on Linux (and other) systems.