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Fencer Vulns
critical 9.8

CVE-2026-55559 json

Yamcs: Remote Code Execution via instance-template argument YAML injection (createInstance)

published 2026-08-28 2 mo ago modified 2026-09-08 2 mo ago also EUVD-2026-67843 , GHSA-73mf-m39p-wpm9

cvss
9.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.8%
EPSS 0.8% chance of exploitation in 30 days, percentile 56%
public exploits
0
none known

Summary

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.

Patch / fix

  • maven org.yamcs:yamcs-core 5.12.8, 5.13.2
  • org.yamcs yamcs-core 5.12.8, 5.13.2
  • spaceapplications yamcs 5.12.8, 5.13.2

CISA SSVC

Exploitation
Proof of concept is public
Automatable
yes
Technical impact
total
Assessed
2 mo ago

Exploit prediction

0.78% chance of exploitation in the next 30 days, higher than 56% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/yamcs/yamcs/security/advisories/GHSA-73mf-m39p-wpm9 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
GitHub 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected packages

ecosystem package affected fixed in
maven org.yamcs:yamcs-core
>=5.13.0, <5.13.2
<5.12.8
5.13.2
5.12.8

Affected products

  • yamcs yamcs

Classification

Weakness
  • CWE-94 Improper Control of Generation of Code
  • CWE-470 Use of Externally-Controlled Input to Select Classes or Code
  • CWE-1336 Improper Neutralization of Special Elements Used in a Template Engine

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.