The best way to provide assurance of the effectiveness of vendor security controls is to require independent control assessments. Independent control assessments are evaluations of the vendor's security controls by a third-party auditor or assessor, such as an external auditor, a certification body, or a testing laboratory.
Independent control assessments provide an objective and unbiased opinion on the adequacy and performance of the vendor's security controls, as well as the compliance with relevant standards and regulations.
Independent control assessments can also provide evidence and assurance to the customers of the vendor's security posture and capabilities. Reviewing vendor control self-assessments (CSA), vendor service level agreement (SLA) metrics, or vendor references from existing customers are not as reliable or credible as independent control assessments, because they may be biased, incomplete, or outdated.