Who: Developers whose server sends requests to URLs that users or customers supply (webhooks, callbacks).
What: Reject non-http(s) URLs and loopback, private, link-local, cloud-metadata and special-use hosts before any request is made.
Use when: When saving or sending to a user-supplied URL; keep allowPrivate for local development only.
Avoid when: As your only defense. It does not resolve DNS, so DNS rebinding is not covered; also pin the resolved address, or send through an egress proxy.