InformationThe /var directory is used by daemons and other system services to temporarily store dynamic data. Some directories created by these processes may be world-writable.
The reasoning for mounting /var on a separate partition is as follow.
Protection from resource exhaustion
The default installation only creates a single / partition. Since the /var directory may contain world-writable files and directories, there is a risk of resource exhaustion. It will essentially have the whole disk available to fill up and impact the system as a whole. In addition, other operations on the system could fill up the disk unrelated to /var and cause unintended behavior across the system as the disk is full. See man auditd.conf for details.
Fine grained control over the mount
Configuring /var as its own file system allows an administrator to set additional mount options such as noexec/nosuid/nodev. These options limits an attackers ability to create exploits on the system. Other options allow for specific behaviour. See man mount for exact details regarding filesystem-independent and filesystem-specific options.
Protection from exploitation
An example of exploiting /var may be an attacker establishing a hard-link to a system setuid program and wait for it to be updated. Once the program was updated, the hard-link would be broken and the attacker would have his own copy of the program. If the program happened to have a security vulnerability, the attacker could continue to exploit the known flaw.
Resizing filesystems is a common activity in cloud-hosted servers. Separate filesystem partitions may prevent successful resizing, or may require the installation of additional tools solely for the purpose of resizing operations. The use of these additional tools may introduce their own security considerations.
SolutionFor new installations, during installation create a custom partition setup and specify a separate partition for /var.
For systems that were previously installed, create a new partition and configure /etc/fstab as appropriate.