CVE-2026-126084: Active Storage: Unsigned Content-Type in Active Storage GCS direct uploads with cache_control

Active Storage makes it simple to upload and reference files in cloud services and attach those files to Active Record objects. Prior to 8.1.4.1 and 8.0.5.2, when a GCS service is configured with the cachecontrol option, ActiveStorage::Service::GCSService#urlfordirectupload 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 cachecontrol configured and accept direct uploads are affected. GCS services without cachecontrol sign with V2 and are not affected. This issue is fixed in versions 8.1.4.1 and 8.0.5.2.