External attestation and certification audit reports are considered the best method to demonstrate assurance in cloud services to multiple customers because they provide an independent verification of the cloud service provider's controls and practices. These reports are conducted by third-party auditors and offer a level of transparency and trust that cannot be achieved through self-assessments or internal documents. They help ensure that the cloud provider meets industry standards and regulatory requirements, which is crucial for customers to assess the risk and compliance posture of their cloud service providers.
References = The importance of external attestation and certification audit reports is supported by the Cloud Security Alliance (CSA) and ISACA, which state that the CCAK credential prepares IT and security professionals to ensure that the right controls are in place and to mitigate the risks and costs of audit management and penalties for non-compliance1.