Shared access signature

How it works

It is the blob-storage equivalent of a presigned URL. Read, write, and delete can be granted separately. A signature that allows delete is not a download link. Store the signing key away from the app server if you can.

A SAS allows read on one blob until Friday. A mis-issued SAS allows write on the container. An upload from a stranger lands. They reissue read-only and rotate the account key that signed the bad one.

How it differs

SAS is not a Microsoft account login. It is a signed string. Treat it like a password that happens to be a URL.

Logging frameworks that record full URLs store the signature.

Redact query strings.

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 shared access signature, 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 shared access signature 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.