How to Securely Upload Files to AWS S3 from Flutter Web and Mobile

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A developer guide published on DEV Community outlines a secure method for uploading files to AWS S3 from Flutter apps on both web and mobile platforms. The approach uses presigned URLs generated by a backend server, ensuring AWS credentials are never exposed in the client app. The author warns against the common but insecure practice of embedding AWS access keys directly in Flutter code, which can be easily extracted by decompiling the app or inspecting browser dev tools. The implementation requires only the http package in Flutter and works consistently across Android, iOS, and web, with an additional CORS configuration needed on S3 for web builds. The method was developed from a real-world delivery app project, where it also reduced data costs by enabling direct client-to-S3 uploads instead of routing files through a backend proxy.

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