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


- **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 10 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** DEBIAN-CVE-2026-126084, GHSA-3w93-6hmw-whh4, UBUNTU-CVE-2026-126084

## 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 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-126084?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `activestorage` | RubyGems | All versions | 8.0.5.2 |
| `activestorage` | RubyGems | From 8.1.0.beta1 | 8.1.4.1 |

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126084 and fix it if it is affected.

What it is: CVE-2026-126084 is a medium-severity vulnerability in rails, published on 10 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-126084 in the wild and no public exploit code.

Affected packages and fixed versions:
- activestorage (RubyGems): affected all versions; fixed in 8.0.5.2
- activestorage (RubyGems): affected from 8.1.0.beta1; fixed in 8.1.4.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-126084/ (Markdown: https://insights.sbomify.com/CVE-2026-126084/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126084
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126084
- OSV: https://osv.dev/vulnerability/CVE-2026-126084

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126084/
Summaries are generated and may contain mistakes; the official description above is the record.