What is a Presigned URL?
A presigned URL grants time-limited authorization for a specific storage operation without revealing long-lived credentials. Its signature constrains permitted parameters, resources, and validity.
How Presigned URLs work
A presigning service computes an authentication signature over a storage request and places the resulting authorization material in a URL or its query parameters. The storage endpoint independently checks the signature, request method, target, signed fields, and expiry, so application credentials never need to reach the browser or downloader. In delivery architecture this creates a direct, temporary path between a client and object storage while the application retains control over issuance.
Key facts
- 1A presigned URL is a bearer capability: anyone who obtains the complete URL can generally perform its authorized request until it expires or an underlying permission is withdrawn.
- 2The HTTP method and selected headers may be part of the signature. Changing a signed content type, checksum, path, or query value can make an otherwise unexpired upload fail validation.
- 3Object-storage authorization and browser CORS are separate checks. A valid signature can still fail in browser JavaScript when the storage origin does not permit that origin, method, or header.
When Presigned URLs matter
Issue one for direct browser uploads or private downloads when traffic should bypass the application server. Excessive validity, permissive operations, or URL leakage can expose the referenced object until expiration.
Common use cases for delivery
These examples cover delivery broadly, not specifically Presigned URLs.
- Serving image, audio, video, and document derivatives to a geographically distributed audience.
- Protecting private assets worldwide with expiring or signed requests.
- Reducing repeated processing and origin traffic by caching deterministic results.
Working with delivery
This guidance covers delivery broadly, not just Presigned URLs.
A client requests an asset using a URL or playback manifest. A delivery layer evaluates authorization and cache state, serves a cached response when possible, or retrieves the asset from its origin before forwarding and optionally caching it.
Delivery choices determine more than download speed. Cache keys, origin behavior, authorization, geographic routing, invalidation, and egress cost decide whether an asset is fast, current, and available to the right audience.
What you gain
- Edge caching places frequently requested assets closer to viewers.
- Explicit cache and authorization rules reduce avoidable origin work.
- Multiple delivery variants let clients request an asset suited to their context.
What it costs
- Long cache lifetimes improve hit ratio but make replacement and invalidation more difficult.
- Signed access protects private media but adds key management, clock, and cache-partitioning concerns.
- More variants improve client fit while increasing storage, cache fragmentation, and operational complexity.
Before production
- 1Define cache keys, cache lifetime, invalidation, and authorization behavior explicitly.
- 2Measure time to first byte, cache-hit ratio, egress, and behavior after an origin failure.
- 3Test signed and unsigned requests at the CDN edge, not only against the origin.