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

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• 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.

From the GitHub advisory

Summary

templateArgs sent to POST /api/instances (and PATCH /api/instances/{instance}) are written into the rendered instance config as raw text, then parsed as YAML and loaded. Yamcs instantiates each services: entry by its class:, so injecting YAML through a template arg lets you add a services: entry for org.yamcs.ProcessRunner and run a command on the host. The args aren't escaped for YAML or validated server-side.

Needs the CreateInstances privilege. With no security.yaml the guest user is superuser=true and the API is unauthenticated, so it's reachable without auth, same default exposure as CVE-2026-46562. The 5.12.7 algorithm-edit fix doesn't touch this path.

Details

VarStatement appends arg values with no escaping:

Weakness type: CWE-94, CWE-470, CWE-1336.

Product: org.yamcs:yamcs-core (Maven) · Assigned by: GitHub_M · Published: 2026-08-28

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• 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.

Timeline

• 2026-08-28 — CVE record last updated
• 2026-08-28 — CVE record published by GitHub_M
• 2026-08-28 — GitHub-reviewed advisory GHSA-73mf-m39p-wpm9 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-73mf-m39p-wpm9 github.com
2. CVE.org record CVE-2026-55559 cve.org
3. NVD entry CVE-2026-55559 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com

Critical Maven advisories · Today’s Security Digest