# CVE-2026-20340: Cisco Secure Firewall Management Center Software Deserialization Arbitrary Root Command Execution Vulnerability

> Authenticated attackers can execute arbitrary root commands via unsafe deserialization in Cisco Secure Firewall Management Center.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Sep 2026
- **Updated:** 18 Sep 2026
- **Also known as:** GHSA-xpr9-grjf-6fm6

## What is CVE-2026-20340?

CVE-2026-20340 is a high-severity unsafe deserialization vulnerability in Cisco Secure Firewall Management Center (FMC), published on 16 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A vulnerability in Cisco Secure FMC Software could allow an authenticated, remote attacker to execute arbitrary commands at the root privilege level.

This vulnerability is due to unsecured deserialization of web-management user-controlled data. An attacker could exploit this vulnerability by authenticating to the device and sending a crafted HTTP payload. A successful exploit could allow the attacker to save the crafted payload and then execute it on the underlying operating system as root.
To exploit this vulnerability, the attacker must have valid credentials for a user account with at least the role of Security Analyst (read-only).

Weakness: CWE-502 (Deserialization of Untrusted Data)

## Which versions are affected by CVE-2026-20340?


## How do I fix CVE-2026-20340?

No fix has been published for CVE-2026-20340 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-20340 being exploited?

There is no known exploitation of CVE-2026-20340 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-20340 and fix it if it is affected.

What it is: CVE-2026-20340 is a high-severity unsafe deserialization vulnerability in Cisco Secure Firewall Management Center (FMC), published on 16 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-20340 in the wild and no public exploit code.

Affected product: Cisco Secure Firewall Management Center (FMC). 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-2026-20340/ (Markdown: https://insights.sbomify.com/CVE-2026-20340/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-20340
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20340
- OSV: https://osv.dev/vulnerability/CVE-2026-20340

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-20340/
Summaries are generated and may contain mistakes; the official description above is the record.