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CVE-2026-55559 – Remote Code Execution – Yamcs Mission Control Framework < 5.12.8 and 5.13.0–5.13.1

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Summary

CVE-2026-55559 is a critical remote code execution vulnerability in the Yamcs mission control framework caused by improper YAML escaping of user-supplied template arguments in the instance creation API. Attackers can inject a malicious service entry into the rendered instance configuration to execute arbitrary commands as the Yamcs service account. The flaw carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: templateArgs submitted via POST /api/instances and PATCH /api/instances/{instance} are inserted into a YAML document through VarStatement.append without YAML-context escaping; the existing EscapeFilter only performs HTML escaping and leaves YAML-significant characters (newlines, colons, indentation) untouched.
  • Trigger: an attacker submits a crafted template argument that injects a services entry for org.yamcs.ProcessRunner into the instance configuration.
  • The rendered configuration is parsed by YamcsServer.createInstance and loaded by YamcsServerInstance, causing the injected process runner service to execute attacker-controlled commands.
  • Attack vector: network, no user interaction required. Exploitation normally requires the CreateInstances privilege, but in unsecured deployments lacking a security.yaml file, the default guest account has superuser rights, making the endpoint reachable without authentication.
  • Impact: arbitrary command execution as the Yamcs service account, exposing cryptographic secrets (secret keys, LDAP/OIDC credentials, TLS keys), telemetry and command history manipulation, and full host compromise in multi-instance deployments.

Affected software

  • Yamcs (yamcs-core) versions prior to 5.12.8
  • Yamcs (yamcs-core) versions 5.13.0 up to (but not including) 5.13.2

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: upgrade to Yamcs 5.12.8 (for 5.12.x deployments) or 5.13.2 (for 5.13.x deployments).
  • If immediate patching is not possible: ensure a security.yaml file is configured so the guest account does not default to superuser, and restrict the CreateInstances privilege to trusted administrators only.
  • Restrict network exposure of the Yamcs HTTP API (/api/instances) to trusted internal networks until patched.

References

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