Active Storage: Possible circumvention of file size limits in Active Storage direct uploads to Google Cloud Storage

Impact

ActiveStorage::Service::GCSService#url_for_direct_upload does not bind the declared content length into the signed direct-upload URL, and #headers_for_direct_upload returns no length constraint, so the URL authorizes an upload body of any size. A direct-upload client could store a file larger than the byte_size it declared, and the resulting blob keeps the declared size, possibly bypassing application file size validations and quotas. This is the weakness that CVE-2020-8162 fixed in the S3 service.

Applications that store files in Google Cloud Storage and accept direct uploads are affected.

Details

The fixed releases sign an x-goog-content-length-range header that matches the blob's declared byte_size and return it from headers_for_direct_upload, so Cloud Storage rejects an upload body of any other size. Browsers send this header with the direct upload, so a bucket's CORS configuration must list x-goog-content-length-range in responseHeader, or browser direct uploads fail after the upgrade. The Active Storage CHANGELOG and the CORS example in the Active Storage guide show the change.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/e19916cea31cbfb0030b165ae853b741f8dd57a7.patch
• 8.0: https://github.com/rails/rails/commit/7fe85a72a789eae5ee070ce3ee1dad10a8a5f70c.patch

Credit

This issue was responsibly reported by HackerOne researcher f4cts.