CVE-2026-126074 is a medium-severity (CVSS 4.3) CWE-248 vulnerability in activestorage. A fix is available for activestorage — see the affected versions and patch details below.
Active Storage: Possible DoS vulnerability in Active Storage filename handling
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
activestorage💎activestorageReal-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::Filename#base and #extension_with_delimiter, and therefore #extension, pass the stored filename unmodified to File.basename and File.extname, which raise ArgumentError when the name contains a NUL byte. Active Storage stores the client-supplied filename verbatim, so a user who is permitted to upload an attachment could persist such a filename on databases that accept NUL bytes in string columns, such as SQLite and MySQL/MariaDB. Each subsequent request that builds a variant or preview of the blob, opens it, or renders it as an Action Text attachment then raises, possibly resulting in a persistent DoS vulnerability for the pages that display the blob until it is purged or its filename is corrected. Applications that use PostgreSQL are not affected, because PostgreSQL rejects the row.
Details
Filename#base and #extension_with_delimiter now derive from Filename#sanitized, the form that Filename#to_s and the Content-Disposition header already use. For a filename that #sanitized changes, such as one containing a NUL byte, an invalid byte sequence, a path separator or another replaced character, the base and extension now agree with #to_s; the variant format inferred from such a name may change, and an existing variant may be regenerated once. Stored filenames are not modified, and no data migration is required.
Releases
The fixed releases are available at the normal locations.
Patches for those who cannot upgrade at once:
- 8.1: https://github.com/rails/rails/commit/60f22af88b71eae251f112d8c0237a08a8832692.patch
- 8.0: https://github.com/rails/rails/commit/9b0d4bef2211a2c08cab2e0ea17fa1d0efd5d47a.patch
Credit
This issue was found by the Anthropic OSS Scanner (ANT-2026-UQM5Z4YL).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | activestorage | ≥ 8.1.0.beta1&&< 8.1.4.1 | 8.1.4.1bundle update activestorage --conservative |
| 💎RubyGems | activestorage | all versions | 8.0.5.2bundle update activestorage --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
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.
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-2026-126074 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
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.
This vulnerability is rated as having a Moderate severity because exploitation requires an authenticated or authorized user permitted to upload file attachments, and impact is confined to a persistent denial of service on specific application views that process the malicious asset. Deployments backed by PostgreSQL are…
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-2026-126074 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-126074 in your dependencies?
Find it across RubyGems, including transitive dependencies.