# What is CVE-2026-66066?

> CVE-2026-66066 is a critical-severity flaw (CVSS 9.5) in the software it affects, published on 30 July 2026. It is not on the list of flaws known to be exploited today. The latest forecast puts its chance of use in the next 30 days at 28%. If you run an affected version, plan in the vendor’s fix.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2026-66066

## At a glance

- **Verdict:** Fix this soon. Not confirmed as exploited yet, but serious or likely enough that it should not wait for a quiet month.
- **Precursor Intelligence risk score:** 56 out of 100 (Medium); action: Plan remediation. Kept up to date as the evidence changes.
- **Severity:** Critical (CVSS 9.5 out of 10)
- **Known to be exploited:** No, not on the list of flaws known to be exploited today
- **Chance of use in the next 30 days:** 28% (latest exploit-likelihood forecast)
- **Published:** 30 July 2026
- **Record last updated:** 10 September 2026
- **Affected products:** 0

## What is CVE-2026-66066? In plain terms

It can be attacked from anywhere on the internet without logging in, and nobody has to click anything.

### The official description

Action Pack is a framework for handling and responding to web requests. In versions prior to 7.2.3.2, 8.0.5.1 and 8.1.3.1, Active Storage does not disable libvips operations marked unsafe for untrusted content, allowing a crafted upload to invoke such an operation. Consuming applications are affected when configured to use libvips and accept image uploads from untrusted users. An unauthenticated attacker may exploit this behavior to read arbitrary files accessible to the Rails process, including environment variables and application secrets. Exposure of credentials such as secret_key_base or external-service tokens may enable remote code execution or lateral movement. This issue has been fixed in versions 7.2.3.2, 8.0.5.1 and 8.1.3.1.

### What kind of flaw is it?

- CWE-1188: Initialization of a Resource with an Insecure Default

### How an attack on it works

- Where an attacker can be: Anywhere on the internet
- How hard it is to pull off: Easy: no special conditions
- Login needed first: No login needed
- Does someone have to click: No, nobody has to do anything

Severity vector: `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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X`

## Is CVE-2026-66066 being exploited? Not that we know of

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 28%. That can change at any time, which is why we keep watching.

Exploit likelihood over the last 7 days: 28%, 28%, 28%, 28%, 28%, 28%, 28%.

### Timeline

- 30 July 2026: Published
- 25 August 2026: Exploit likelihood jumped
- 10 September 2026: Record last updated
- 25 September 2026: Exploit likelihood fell

### How Precursor Intelligence ranks it

CVE-2026-66066 scores **56 out of 100 (Medium)**, with the action "Plan remediation".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2026-66066?

No affected products have been listed for this flaw yet.

## How do I fix CVE-2026-66066?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the latest update from the vendor. The references list where the vendor has published details.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Keep an eye on it.** Track the product and we tell you as soon as we spot this flaw on the list of exploited flaws, or a jump in its exploit likelihood.

### References

- [github.com](https://github.com/rails/rails/commit/1c01bb587206ee6eb0e1179c2cef96a6a47acb1e)
- [github.com](https://github.com/rails/rails/commit/349e7a5d5b4b715af1e416db824f3c078a7d59e5)
- [github.com](https://github.com/rails/rails/commit/d79b7f4aa17dec8ce4960fef05733c8c0c7ef49a)
- [github.com](https://github.com/rails/rails/releases/tag/v7.2.3.2)
- [github.com](https://github.com/rails/rails/releases/tag/v8.0.5.1)
- [github.com](https://github.com/rails/rails/releases/tag/v8.1.3.1)
- [github.com](https://github.com/rails/rails/security/advisories/GHSA-xr9x-r78c-5hrm)
- [github.com](https://github.com/rubysec/ruby-advisory-db/blob/master/gems/activestorage/CVE-2026-66066.yml)
- [thehackernews.com](https://thehackernews.com/2026/07/critical-rails-flaw-could-let.html)
- [openwall.com](http://www.openwall.com/lists/oss-security/2026/07/29/9)
- [openwall.com](http://www.openwall.com/lists/oss-security/2026/08/01/6)
- [ethiack.com](https://ethiack.com/info-hub/research/kindarails2shell-rails-rce-cve-2026-66066)
- [github.com](https://github.com/rails/rails-forensics-CVE-2026-66066)

## Questions

### What is CVE-2026-66066?

CVE-2026-66066 is a critical-severity flaw in the software it affects, published on 30 July 2026. It can be attacked from anywhere on the internet without logging in, and nobody has to click anything.

### How serious is CVE-2026-66066?

It scores 9.5 out of 10 on the CVSS severity scale, which is critical. Our risk score for CVE-2026-66066 is 56 out of 100, which is Medium. Its action is "Plan remediation". Not confirmed as exploited yet, but serious or likely enough that it should not wait for a quiet month.

### Is CVE-2026-66066 being exploited?

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 28%. That can change at any time, which is why we keep watching.

### How do I fix CVE-2026-66066?

Install the update the vendor names in its advisory. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet.

## Does CVE-2026-66066 affect you?

Precursor Intelligence keeps checking for you:

- **Tell us what you run.** Pick your software from a list, or let us work it out from your website.
- **We match it to every flaw.** Over 400,000 published flaws, checked against your list.
- **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it.
- **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools.

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