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.

A YAML injection vulnerability exists in Yamcs mission control framework prior to versions 5.12.8 and 5.13.2. The flaw resides in VarStatement.append, which inserts templateArgs from POST/PATCH API endpoints into YAML configuration without proper escaping. An attacker can inject a malicious services entry for org.yamcs.ProcessRunner into the rendered configuration, leading to arbitrary command execution as the Yamcs service account. Unsecured deployments lacking security.yaml are exposed via the guest superuser, making exploitation trivially accessible. Secured deployments require the SystemPrivilege.CreateInstances privilege to exploit. The vulnerability affects the instance management API endpoints and the underlying YAML templating engine. Fixes are available in Yamcs versions 5.12.8 and 5.13.2. Organizations running Yamcs in mission-critical or space/aerospace environments should prioritize patching immediately.