Bug 2571278 (CVE-2026-126084)

Summary: CVE-2026-126084 activestorage: activestorage: Arbitrary script execution via direct upload URL signature manipulation in GCS service
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akostadi, amasferr, dmayorov, eshamard, jlledo, jvasik, kaycoth, pantinor, rblanco, tsedmik
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Active Storage. When configured to use Google Cloud Storage (GCS) with cache control enabled, direct upload URL signatures do not properly protect object content headers. A remote attacker can exploit this vulnerability by uploading files with an arbitrary Content-Type header, causing the cloud storage service to serve malicious files as inline HTML. Consequently, an attacker can achieve arbitrary script execution within the storage domain, allowing them to intercept or tamper with subsequent user requests and responses in that storage bucket.
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Description OSIDB Bzimport 2026-11-10 21:21:37 UTC 
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.