A system for managing custom code within a data computing platform determines that a request for one or more uniform resource identifiers external to the platform is being made by custom code executing in the platform. In response to the determination, the system checks a whitelist of allowable external URIs against the requested one or more URIs and allows access to the requested one or more URIs if a match is detected with the whitelist, otherwise access by the custom code to the requested one or more URIs is denied. In addition, or alternatively, the system checks a blacklist of disallowed external URIs against the requested one or more URIs and denies access to the requested one or more URIs if a match is detected with the blacklist, otherwise access by the custom code to the requested one or more URIs is allowed. The blacklist can override the whitelist.