ServerBuddies Support Blog

How to Prevent Fork Bombing attack

Linux has provided various resource limits for each user, such as user limit (ulimit) for CPU time, number of processes (nproc), memory usage etc. ulimit is the solution to prevent fork bombing attack. It defines maximum number of processes allowed (ulimit –u) for each user.

There are two types of ulimit. One is HARD limit and the other is SOFT Limit. Non root
user cannot change the hard limit by executing commands like ulimit –u value.

Root user on remote server or Host needs to set the ulimit to prevent fork bombing attack. ulimit are set in /etc/security/limits.conf file. To set ulimit for number of processes, option provided is nproc. Hard and soft limit need to be set for that.

For example:

Prevent subdomains and domain aliases in the DNS zones belonging to other users in Plesk.

By default, users can create new subdomains and domain aliases in the DNS zones belonging to other users. This means that they can set up Web sites and e-mail accounts which could be used for spamming, fishing or identity theft.

To prevent users from setting up domains and domain aliases in the DNS zones belonging to other users:

1. Go to Settings > Restrict Creation of Subzones (in the DNS group).
2. Select the Do not let users create DNS subzones in other users’ DNS superzones check box.
3. Click OK.