CVE-2026-126084: Ruby on Rails Active Storage GCS signature bypass

CVE-2026-126084 · Severity: medium · CVSS 4.7 · Published 2026-11-10

Rails Active Storage, which manages file uploads to cloud storage services, fails to properly validate file uploads to Google Cloud Storage when caching is enabled. An attacker can upload arbitrary file types (including HTML) and have them served from Google's storage domain, potentially allowing malicious scripts to run and steal or modify other users' uploaded files.

Technologies: Ruby on Rails Active Storage, activestorage (RubyGems). Vendors: Ruby on Rails, RubyGems.

Quick answers

Is CVE-2026-126084 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-126084 being exploited in the wild.
How severe is CVE-2026-126084?
Medium severity, with a CVSS score of 4.7, published 10 November 2026.
Which products and versions are affected?
Ruby on Rails Active Storage before 8.1.4.1 and 8.0.5.2; activestorage (RubyGems) >= 8.1.0.beta1, < 8.1.4.1.
Is there a fix for CVE-2026-126084?
Yes: fixed in 8.1.4.1, 8.0.5.2. Until then: Disable the cache_control option on GCS services if direct uploads are not required, as GCS services without cache_control use V2 signing and are not affected. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-126084 attacked?
It is attacked over the network.
Am I affected by CVE-2026-126084?
Check whether you run Ruby on Rails Active Storage or activestorage (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • Fixed in: 8.1.4.1, 8.0.5.2

Disable the cache_control option on GCS services if direct uploads are not required, as GCS services without cache_control use V2 signing and are not affected.

Technical details

When Google Cloud Storage (GCS) service is configured with cache_control, Active Storage signs direct-upload URLs with V4 signatures that do not cover Content-Type or Content-MD5 headers. An attacker can exploit this to upload a file with a Content-Type of their choice (e.g., text/html) and omit the declared Content-MD5 without invalidating the URL. The uploaded object is then served with the attacker-supplied content type, allowing inline HTML execution and Service Worker registration on the storage origin.

Affected products

  • Ruby on Rails Active Storage before 8.1.4.1 and 8.0.5.2

Identifiers

  • CVE-2026-126084
  • GHSA-3w93-6hmw-whh4

Timeline

  • 2026-11-10: disclosed

References

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