CORS

How it works

The browser sends a preflight. The storage host must answer with the allowed origin and methods. A missing header fails the browser and succeeds in curl. The file never left the page.

A portal on app.example uploads to storage.example. Without CORS allowing PUT from app.example, the browser blocks a 2 GB upload before it starts. Adding the origin and PUT makes the same URL work.

How it differs

CORS is not access control. It is a browser courtesy check. A server that relies on it alone is open to non-browser clients.

Wildcard origins with credentials are refused by browsers.

List the real origin.

On the ticket

• The practical close is a log line: time, actor, byte count, result.
• Without that line the transfer is a story.
• With it, the next person can see whether this door did what the ticket claimed.
• If the path is shared, say so in the partner profile so a later change does not silently pick a different limit, key, or region.
• On a real ticket, write down the door, the byte count, and the clock.
• For cors, that means naming the host or bucket, the expected size, and the time the other side must have a complete file.
• A progress bar is not that record.
• A 200 response that arrives before the complete call is not that record.
• If a retry is allowed, say how many and whether it resumes.
• If a person must approve the send, name the person.
• Partners who receive cors files should match on hash or size before they import.
• A same-length corrupt file passes a size check and fails a hash.
• Keep the published hash off the only channel an attacker can edit, or treat it as a corruption check rather than a substitution check.
• When the path changes, new key, new region, new cap, update the profile the same day so the next run does not use a stale limit.