Cloud computing is considered one of the most dominant paradigms in the Information Technology (IT) industry nowadays. It supports multi-tenancy to fulfil future increasing demands for accessing and using resources provisioned over the Internet. Multi-tenancy enables to share computing physical resources among cloud computing tenants and offers cost-effective, on-demand scaling. However, multi-tenancy in cloud computing has unique vulnerabilities such as clients' co-residence and virtual machine physical co-residency. Physical co-residency of virtual machines can facilitate attackers with an ability to interfere with another virtual machine running on the same physical machine due to an insufficient logical isolation. In the worst scenario, attackers can exfiltrate sensitive information of victims on the same physical machine by using hardware side-channels. Side-channel attacks are an implementation level attack on cryptographic systems. They exploit the correlation between the higher level functionality of the software and the underlying hardware phenomena. There are various types of side-channels attacks, which are classified according to hardware medium they target and exploit, for instance, cache side-channel attacks. CPU caches are one of the most hardware devices targeted by adversaries because it has high-rate interactions and sharing between processes. Furthermore, full encryption keys of well-known algorithms (i.e. RSA and AES) have been broken using simple spying processes to spy and collect information about cache lines, which have been accessed. This information is analysed and linked to the current virtual machine, which occupies the processor. The target of this paper is to explore potential security issues related to side-channel attacks, particularly cache side-channel attacks in cloud computing. It highlights research directions, investigates various real attack scenarios and gaps in the existing approaches that proposed to prevent and defend against cache side-channel attacks in the cloud computing.