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MEDIUM severity

CVE-2026-126085 — activestorage

MEDIUMFix: rails/rails@7fe85a7

CVE-2026-126085 is a medium-severity (CVSS 5.3) CWE-602 vulnerability in activestorage. A fix is available for activestorage — see the affected versions and patch details below.

Active Storage: Possible circumvention of file size limits in Active Storage direct uploads to Google Cloud Storage

Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Real-World Exposure

2 pkgs affected
💎activestorage💎activestorage

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

Impact

ActiveStorage::Service::GCSService#url_for_direct_upload does not bind the declared content length into the signed direct-upload URL, and #headers_for_direct_upload returns no length constraint, so the URL authorizes an upload body of any size. A direct-upload client could store a file larger than the byte_size it declared, and the resulting blob keeps the declared size, possibly bypassing application file size validations and quotas. This is the weakness that CVE-2020-8162 fixed in the S3 service.

Applications that store files in Google Cloud Storage and accept direct uploads are affected.

Details

The fixed releases sign an x-goog-content-length-range header that matches the blob's declared byte_size and return it from headers_for_direct_upload, so Cloud Storage rejects an upload body of any other size. Browsers send this header with the direct upload, so a bucket's CORS configuration must list x-goog-content-length-range in responseHeader, or browser direct uploads fail after the upgrade. The Active Storage CHANGELOG and the CORS example in the Active Storage guide show the change.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

Credit

This issue was responsibly reported by HackerOne researcher f4cts.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
💎RubyGemsactivestorage≥ 8.1.0.beta1&&< 8.1.4.18.1.4.1bundle update activestorage --conservative
💎RubyGemsactivestorageall versions8.0.5.2bundle update activestorage --conservative

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for activestorage, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update activestorage to 8.1.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-126085 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This vulnerability is rated as Moderate severity because it is limited to bypassing application-level file size constraints or storage quotas without enabling arbitrary code execution or data disclosure. Impact is confined to environments utilizing Active Storage specifically configured to handle direct client uploads…

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Source: Red Hat security advisory for CVE-2026-126085 (CC BY 4.0)

Frequently Asked Questions

### Impact `ActiveStorage::Service::GCSService#url_for_direct_upload` does not bind the declared content length into the signed direct-upload URL, and `#headers_for_direct_upload` returns no length constraint, so the URL authorizes an upload body of any size. A direct-upload client could store a file larger than the `byte_size` it declared, and the resulting blob keeps the declared size, possibly bypassing application file size validations and quotas. This is the weakness that CVE-2020-8162 fixed in the S3 service. Applications that store files in Google Cloud Storage and accept direct upload
O3 Security · Impact-Aware SCA

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