This Cloudflare security feature enforces a policy that prevents CNAME records from being configured to point a domain to a resource hosted under a different user's account. It is designed for Cloudflare administrators and account owners who need to maintain strict control over their domain and resource associations.

• Cross-Account CNAME Blocking: The system automatically detects and blocks DNS configurations where a CNAME record attempts to resolve a domain to a hostname (like an R2 bucket or other Cloudflare service) that belongs to another, separate Cloudflare account.
• Enforces Account Isolation: This policy is a fundamental security control that ensures resources and domains cannot be inadvertently or maliciously linked across account boundaries, protecting the integrity of each user's setup.
• Prevents Resource Hijacking: By stopping unauthorized CNAME mappings, it eliminates a potential vector where an external party could point a domain they control to your Cloudflare-hosted assets without permission.
• Clear Error Messaging: When a violation is detected, the service returns a specific error (1014) to the end-user, informing them that the requested host is configured as a disallowed CNAME across accounts.
• Guides Correct Configuration: For legitimate use cases, such as linking a custom domain to an R2 bucket, the system directs users to employ the proper method via a custom domain setup within their own account, ensuring the mapping is authorized and secure.