# CVE-2026-4266: WatchGuard Firebox Insecure Deserialization in Fireware Access Portal

> An attacker with local write access can execute arbitrary code via insecure deserialization in the Access Portal feature of WatchGuard Fireware OS.

- **Severity:** High 8.4 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 30 Mar 2026
- **Updated:** 27 Aug 2026
- **Also known as:** GHSA-mmhr-mfjp-vv8j

## What is CVE-2026-4266?

CVE-2026-4266 is a high-severity unsafe deserialization vulnerability in Watchguard Fireware, published on 30 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

An Insecure Deserialization vulnerability in WatchGuard Fireware OS allows an attacker that has obtained write access to the local filesystem through another vulnerability to execute arbitrary code in the context of the portald user.

Note, this vulnerability does not affect Firebox platforms that do not support the Access Portal feature, including the T15 and T35.

Weakness: CWE-502 (Deserialization of Untrusted Data)

## Which versions are affected by CVE-2026-4266?

It affects 1 product.


Products: Watchguard Fireware.

## How do I fix CVE-2026-4266?

No fix has been published for CVE-2026-4266 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-4266 being exploited?

There is no known exploitation of CVE-2026-4266 in the wild and no public exploit code.

## Other vulnerabilities in Watchguard Fireware

- [CVE-2022-26318](https://insights.sbomify.com/CVE-2022-26318/): WatchGuard Firebox and XTM Appliances Arbitrary Code Execution (critical 9.8, exploited in the wild)
- [CVE-2025-14733](https://insights.sbomify.com/CVE-2025-14733/): WatchGuard Firebox Out of Bounds Write (critical 9.3, exploited in the wild)
- [CVE-2025-9242](https://insights.sbomify.com/CVE-2025-9242/): WatchGuard Firebox Out-of-Bounds Write (critical 9.3, exploited in the wild)
- [CVE-2022-23176](https://insights.sbomify.com/CVE-2022-23176/): WatchGuard Firebox and XTM Privilege Escalation (high 8.8, exploited in the wild)
- [CVE-2022-31789](https://insights.sbomify.com/CVE-2022-31789/): Integer overflow in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-4266 and fix it if it is affected.

What it is: CVE-2026-4266 is a high-severity unsafe deserialization vulnerability in Watchguard Fireware, published on 30 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-4266 in the wild and no public exploit code.

Affected product: Watchguard Fireware. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-4266/ (Markdown: https://insights.sbomify.com/CVE-2026-4266/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-4266
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-4266
- OSV: https://osv.dev/vulnerability/CVE-2026-4266

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-4266/
Summaries are generated and may contain mistakes; the official description above is the record.