# Active Storage has possible arbitrary file read and remote code execution in Active Storage variant processing

Advisory record · `GHSA-xr9x-r78c-5hrm`

- **Identifiers:** `GHSA-xr9x-r78c-5hrm` · `CVE-2026-66066`
- **State:** Confirmed
- **Severity:** CVSS 9.5 Critical
- **Critical:** Yes: CVSS 9+, CVSS 7+ and EPSS
- **Weakness (CWE):** [CWE-1188](https://cwe.mitre.org/data/definitions/1188.html)
- **Tags:** [CWE-1188](https://agentprime.io/advisories/tag/cwe-1188.md)
- **Published:** 30 Jul 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 30 Jul 2026, 18:23 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Critical

Critical, because:

- CVSS score 9.5: 9.0 or more.
- CVSS score 9.5 (7.0 or more) and EPSS 0.30 (0.1 or more): a high score and a real chance of exploitation in the next 30 days.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected packages

- `activestorage` (RubyGems): affected `<7.2.3.2 || >=8.0.0.beta1, <8.0.5.1 || >=8.1.0.beta1, <8.1.3.1`; fixed in `7.2.3.2`, `8.0.5.1`, `8.1.3.1`

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 9.5 Critical, CVSS 4.0: `CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.30, the 98th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 15:16 UTC: First seen by agentprime
- 31 Oct 2026, 15:16 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-xr9x-r78c-5hrm)

## Sources

- [GitHub Advisory Database: GHSA-xr9x-r78c-5hrm](https://github.com/advisories/GHSA-xr9x-r78c-5hrm)
- [OSV: GHSA-xr9x-r78c-5hrm](https://osv.dev/vulnerability/GHSA-xr9x-r78c-5hrm)
- <https://github.com/rails/rails/security/advisories/GHSA-xr9x-r78c-5hrm>
- <https://github.com/rails/rails/commit/1c01bb587206ee6eb0e1179c2cef96a6a47acb1e>
- <https://github.com/rails/rails/commit/349e7a5d5b4b715af1e416db824f3c078a7d59e5>
- <https://github.com/rails/rails/commit/d79b7f4aa17dec8ce4960fef05733c8c0c7ef49a>
- <https://github.com/rails/rails>
- <https://github.com/rails/rails/releases/tag/v7.2.3.2>
- <https://github.com/rails/rails/releases/tag/v8.0.5.1>
- <https://github.com/rails/rails/releases/tag/v8.1.3.1>
- <https://github.com/rubysec/ruby-advisory-db/blob/master/gems/activestorage/CVE-2026-66066.yml>
- <https://thehackernews.com/2026/07/critical-rails-flaw-could-let.html>
- <https://www.cve.org/CVERecord/SearchResults?query=CVE-2026-66066>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-xr9x-r78c-5hrm>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-xr9x-r78c-5hrm/>. Everything for agents: <https://agentprime.io/llms.txt>.