Upload a file to Filestack and lock down subsequent access with security policies and HMAC signatures
domain: filestack.com · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Upload a file (creating a Filelink) either through a Filestack client SDK/picker or directly against the File API's store endpoint, providing your API key and either multipart file data or a public source URL
Enable security on the application in the developer portal, then construct a JSON security policy object containing an `expiry` (Unix timestamp) and a `call` list restricting which actions (e.g. read, store, convert, remove) are permitted
Sign the base64url-encoded policy using HMAC-SHA256 with the app's secret key, generated and kept only on the backend, to produce the request's signature
Attach the resulting `policy` and `signature` as query parameters (or via HTTP Basic auth with `app` as the username and the secret as the password) on requests that require security, such as overwrite or delete
Set short expiries and the minimum necessary `call` scope per policy so a leaked policy/signature pair has limited blast radius
Known gotchas
The application secret key used to sign policies must never be exposed client-side — policies should be generated and signed on the backend, then handed to the client only as the already-signed policy/signature pair
Download and overwrite/delete operations have different security requirements: plain downloads generally don't need a policy unless security is enabled account-wide, while overwrite/delete always require one
Because every policy embeds its own expiry, a stale cached policy/signature pair will start failing requests once that timestamp passes, even if the underlying Filelink itself is unaffected
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