A security audit often employs many of the similar techniques followed during security assessments but are required to be performed by independent auditors. An organization's internal security staff perform routine security testing and assessments. However, security audits differ from this approach. Security assessments and testing are internal to the organization and are intended to find potential security gaps.

Audits are similar to assessments but are conducted with the intent of demonstrating the effectiveness of security controls to a relevant third party. Audits ensure that there's no conflict of interest in testing the control effectiveness. Hence, audits tend to provide a completely unbiased view of the security posture.

The security assessment reports and the audit reports might look similar; however, they are both meant for different audiences. The audience for the audit report mainly includes higher management, the board of directors, government authorities, and any other relevant stakeholders.

There are two main types of audits:

• Internal audit: The organization's internal audit team performs the internal audit. The internal audit reports are intended for the organization's internal audience. It is ensured that the internal audit team has a completely independent reporting line to avoid conflicts of interest with the business processes they assess.
• External audit: An external audit is conducted by a trusted external auditing firm. External audits carry a higher degree of external validity since the external auditors virtually don't have any conflict of interest with the organization under assessment. There are many firms that perform external audits, but most people place the highest credibility with the so-called big four audit firms:
  • Ernst & Young
  • Deloitte & Touche
  • PricewaterhouseCoopers
  • KPMG

Audits performed by these firms are generally considered acceptable by most investors and governing bodies and regulators.