Broken Access Control in Chi with Mongodb

Broken Access Control is a common API security risk where authorization checks are missing or inconsistent, allowing one user to access or modify another user's data. When building services in Chi (a Clojure web framework) with MongoDB as the backend, this risk can emerge from a mismatch between route/endpoint permissions and the data-access layer. In Chi, routes are typically defined as middleware or handlers that process HTTP requests, and MongoDB queries are executed via a library such as Monger or the official MongoDB driver. If these components are not tightly coupled with per-request identity checks, endpoints may unintentionally expose data or allow privilege escalation.

For example, consider an endpoint that retrieves user profiles by ID. If the handler in Chi directly uses an ObjectId from user input without verifying that the requesting user has permission to view that specific document, the API becomes vulnerable to Insecure Direct Object References (IDOR), a subset of Broken Access Control. A malicious actor could iterate through known IDs and access other users' profiles. MongoDB does not enforce application-level permissions; it only responds to queries. Therefore, it is the responsibility of the Chi application to enforce proper ownership and role checks before constructing and executing queries. Without these checks, even a well-structured MongoDB collection can leak data.

Another scenario involves role-based access control (RBAC) implemented inconsistently. Chi handlers may rely on a shared middleware that attaches user claims to the request map, but if certain routes skip this middleware or if MongoDB queries do not filter by role or tenant, users might gain access to administrative functions or sensitive datasets. For instance, an endpoint that returns all payment records should first verify that the user has the admin role; otherwise, it should restrict results to the user's own records. In MongoDB, this is typically enforced by adding a filter such as {userId: userId} or {tenantId: tenantId} to the query. If the Chi handler omits this filter because of a logic error or copy-paste pattern, the API effectively bypasses authorization at the data layer.

Additionally, method-level access control can be overlooked. Chi applications often use different HTTP methods on the same route (e.g., GET vs DELETE). If authorization is only enforced for read operations but not for deletions, a user with read-only permissions could potentially send a DELETE request and remove resources they should not touch. MongoDB operations like delete-one or delete-many will execute as long as the query syntax is valid, making it essential for Chi handlers to validate permissions before invoking these operations. Using a unified data-access function that always applies user-based filters and role checks can reduce this risk.

Because MongoDB returns data based solely on the query provided, any missing authorization logic in Chi handlers directly translates to missing enforcement in the API response. This is why Broken Access Control in Chi with MongoDB requires careful attention to both framework-level routing and database query construction. Regular audits of handler functions and corresponding MongoDB filters are necessary to ensure that every endpoint enforces identity and role checks aligned with the principle of least privilege.

To remediate Broken Access Control in Chi when using MongoDB, you must enforce user ownership and role-based checks directly in your handlers before any database operation. The following examples demonstrate concrete patterns using Monger and the official MongoDB driver for Clojure, ensuring that each query includes appropriate filters tied to the authenticated user.

First, define a helper that extracts the current user ID and role from the request map, typically populated by an authentication middleware:

(defn current-user [request]
  (get-in request [:session :user]))

(defn user-id [request]
  (:user-id (current-user request)))

(defn user-role [request]
  (:role (current-user request)))

Next, create a function that builds a base query enforcing user ownership for tenant-based data:

(defn build-owner-query [collection user-id]
  (merge collection {:user-id user-id}))

Use this function in a Chi GET handler to ensure users only retrieve their own documents:

((GET "/api/profile/:id") request
  (let [profile-id (ObjectId. (:id request))
        user-id (user-id request)
        query {:_id profile-id :user-id user-id}
        profile (monger/find-one-as-map db "profiles" query)]
    (if profile
      (json-response profile)
      (response/not-found "Profile not found"))))

For endpoints that list multiple records, apply the same ownership filter to prevent users from enumerating others' data:

((GET "/api/notes") request
  (let [user-id (user-id request)
        notes (monger/find-maps db "notes" (build-owner-query {:user-id user-id} user-id))]
    (json-response notes)))

For administrative endpoints, enforce role-based filtering in addition to ownership or skip ownership checks entirely for elevated roles:

((GET "/api/admin/users") request
  (if (= (user-role request) "admin")
    (let [users (monger/find-maps db "users" {})]
      (json-response users))
    (response/status (response/response "Forbidden") 403)))

When modifying data, always re-validate ownership before performing updates or deletions:

((DELETE "/api/notes/:id") request
  (let [note-id (ObjectId. (:id request))
        user-id (user-id request)
        result (monger/delete-by-id db "notes" note-id {:multi false})]
    (if (and result (pos? (:n result)))
      (response/json-response {:status "deleted"})
      (response/status (response/response "Not found or unauthorized") 404))))

These patterns ensure that MongoDB queries in Chi are always scoped by the requesting user’s identity or role, effectively mitigating Broken Access Control. By consistently applying filters in handlers, you align the API’s authorization logic with the data-access layer.