Server-side request forgery (SSRF) is the vulnerability that the tester exploited by querying the provider's metadata and getting the credentials used by the instance to authenticate itself. SSRF is a type of attack that abuses a web application to make requests to other resources or services on behalf of the web server. This can allow an attacker to access internal or external resources that are otherwise inaccessible or protected. In this case, the tester was able to access the metadata service of the cloud provider, which contains sensitive information about the instance, such as credentials, IP addresses, roles, etc.