---
title: "CVE-2026-55559: Yamcs vulnerable to Remote Code Execution via instance-template argument YAML injection (createInstance)"
url: https://erroneo.us/security/cve/CVE-2026-55559/
description: "CVE-2026-55559 (org.yamcs:yamcs-core (Maven)): Yamcs is a mission control framework. Prior to 5.12.8 and 5.13.2, Yamcs inserts templateArgs from POST…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-08-28
date_modified: 2026-08-28
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-55559"
kev: false
cvss: 9.8
epss: 0.00784
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-55559: Yamcs vulnerable to Remote Code Execution via instance-template argument YAML injection (createInstance)

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.8% , percentile 56: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 56% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes org.yamcs:yamcs-core (Maven) >= 5.13.0, <= 5.13.1, fixed in 5.13.2 ; org.yamcs:yamcs-core (Maven) <= 5.12.7, fixed in 5.12.8 . Check your lockfile, including transitive dependencies.

## What it is

Yamcs is a mission control framework. Prior to 5.12.8 and 5.13.2, Yamcs inserts templateArgs from POST /api/instances and PATCH /api/instances/{instance} into YAML through VarStatement.append in yamcs-core/src/main/java/org/yamcs/templating/VarStatement.java without YAML-context escaping. The rendered configuration is parsed by YamcsServer.createInstance and loaded by YamcsServerInstance, allowing an attacker to inject a services entry for org.yamcs.ProcessRunner. Deployments without security.yaml expose the operation through the guest superuser, while secured deployments require SystemPrivilege.CreateInstances. Successful exploitation executes commands as the Yamcs service account. This issue is fixed in versions 5.12.8 and 5.13.2.

## Affected packages

- Maven `org.yamcs:yamcs-core` >= 5.13.0, <= 5.13.1 → fixed: 5.13.2
- Maven `org.yamcs:yamcs-core` <= 5.12.7 → fixed: 5.12.8

## Affected products (CVE record)

- yamcs yamcs: < 5.12.8, >= 5.13.0, < 5.13.2

## Severity

- GitHub_M CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
- GitHub CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.78% probability of exploitation activity in the next 30 days; percentile 56.

## Sources

- [GitHub Advisory GHSA-73mf-m39p-wpm9](https://github.com/advisories/GHSA-73mf-m39p-wpm9)
- [CVE.org record CVE-2026-55559](https://www.cve.org/CVERecord?id=CVE-2026-55559)
- [NVD entry CVE-2026-55559](https://nvd.nist.gov/vuln/detail/CVE-2026-55559)
- [github.com](https://github.com/yamcs/yamcs/security/advisories/GHSA-73mf-m39p-wpm9)
- [github.com](https://github.com/yamcs/yamcs/commit/549f295cf8c5496a5e799d6bec2432ef976c82aa)
- [github.com](https://github.com/yamcs/yamcs/commit/7192da1c49bdf5ab1d72e579a47766a7c43e87c8)
- [github.com](https://github.com/yamcs/yamcs/releases/tag/yamcs-5.12.8)
- [github.com](https://github.com/yamcs/yamcs/releases/tag/yamcs-5.13.2)

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Source page: https://erroneo.us/security/cve/CVE-2026-55559/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt