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

What is CVE-2026-126084?

CVE-2026-126084 is a medium-severity vulnerability in rails, published on 10 Nov 2026.

Official description

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 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. This issue is fixed in versions 8.1.4.1 and 8.0.5.2.

Weakness: CWE-345

Which versions are affected by CVE-2026-126084?

2 open source packages ship an affected version, including activestorage.

How do I fix CVE-2026-126084?

Upgrade activestorage to a fixed version for your release line: 8.0.5.2 or 8.1.4.1.

Is CVE-2026-126084 being exploited?

There is no known exploitation of CVE-2026-126084 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2026-126084, GHSA-3w93-6hmw-whh4, UBUNTU-CVE-2026-126084.