Make a PUT request from the client to upload the file to S3.Generate a pre-signed upload URL on the server and return it to the client.Send the file metadata to the server to validate the request.Implement a file input handler in client to receive a file from the user.Here are the basic steps to building a secure, direct client upload with a pre-signed URL: Right: a direct client upload using a server signed URL. The image below shows the difference in flow between a server-proxied upload and a direct client upload. Instead, we'll make a server request to validate the upload metadata (checking user authorization, file type, size, etc), generate a signed URL, and return it to the client. Of course, generating the pre-signed URL requires service provided credentials with sufficient credentials, so the signing cannot be done on the client. Our approach to direct client uploads will leverage pre-signed URLs to let the client upload directly to our storage provider. This special URL allows us to write to the referenced object and only the referenced object. This is a URL referencing our S3 object, with a signature parameter appended. As it turns out, S3 (and other storage providers) allow us to generate something like this in the form of a pre-signed URL. The only secure solution is to let a client upload only the content that we authorize.Įssential what we want is a one-time key that allows a direct upload of a file that we allow. Even if we create separate keys with limited access for the client, an attacker could still upload arbitrary content allowing them to fill up storage, perform DOS attacks, etc. If we were to try to use the same credentials we use on the backend, it means they pass through the client and can be intercepted by an attacker. The challenge is giving the client permission to upload the file in a secure fashion. But implementing direct client uploads must be done with caution: if implemented incorrectly, it can lead to security issues like incorrect permissions or leaking of credentials. Security Considerations of Direct Client Uploadsĭirect client uploads offer a way to improve on the user experience and resource use problems associated with server-proxied uploads. This sounds simple, but is a bit tricky in practice! To do a direct upload in a secure fashion, there are a couple of additional considerations. This can be increased, but may be out of your control as an application developer, and even a larger setting may be too small for your application.ĭirect uploads, in contrast, upload your content directly to the storage provider, by passing the need to upload the content to your own server. Just as an example, the popular nginx server, used to "front" many web services, has a default upload limit of 1MB. This means your upload is slow, and it consumes unnecessary server resources.įurthermore, while the configuration of course varies by server software and hosting provider, you'll often find your app bumps up against file-size limits when you try to upload large files. Instead of delivering the file upload to its ultimate destination, you're transferring the file twice: first to your server, and then to your storage provider. This approach works okay, but it's a bit inefficient. The most straightforward approach to file uploads is to build out a backend API to process and deliver the file content to the storage provider. Sever-Proxied Uploads vs Direct Client Uploads Though the code examples are presented in JavaScript both the server and client calls, this approach can work with any server and client technologies, including native mobile client apps. In this post, we'll learn how to build secure, direct file uploads from web and mobile clients to AWS S3, skipping the step of sending the data to the server. While the most straightforward solution is to upload files first to your backend server and then to the storage provider, this approach can cause performance issues and other problems. Typically these uploads are stored in a CDN or other storage provider such as Amazon S3. If you're developing an app which allows users to upload user-generated content such as photos, video or any other kind of document, you'll need to spend some time building out some infrastructure to handle file uploads.
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