Impact

When a GCS service is configured with the cache_control option, ActiveStorage::Service::GCSService#url_for_direct_upload signs direct-upload URLs with V4, and the V4 signature covers only the Cache-Control and Host headers. A direct-upload client can therefore send a Content-Type of its choosing, and can omit the declared Content-MD5, without invalidating the URL. GCS stores the supplied content type as object metadata, which takes precedence over the response-content-type that Active Storage sets on download URLs, so an uploaded object could be served as inline HTML under the application's bucket path on storage.googleapis.com. Script in such an object runs on the storage origin rather than the application's, but could register a Service Worker scoped to the bucket path and read or replace a victim's later responses for signed URLs in that bucket.

Applications that use the GCS service with cache_control configured and accept direct uploads are affected. GCS services without cache_control sign with V2 and are not affected.

Details

The fixed releases sign every GCS direct-upload URL with V2, which binds an empty Content-Type and the declared Content-MD5. The configured Cache-Control is still returned by headers_for_direct_upload and sent by the client, but it is no longer part of the signature, and direct-upload URLs for these services are no longer limited to V4's seven-day maximum expiry. See the Active Storage CHANGELOG.

Workarounds

Removing the cache_control option from the GCS service configuration makes Active Storage sign direct uploads with V2.

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/8d93f6a872e9550a91bf0e4fce50083a235ef3b1.patch
• 8.0: https://github.com/rails/rails/commit/ddc08ba61a8001e7402864c62e466cf732fc7614.patch

Credit

This issue was responsibly reported by HackerOne researcher thepo1es.