Cross Site Request Forgery in Chi with Mongodb

Cross Site Request Forgery (CSRF) in a Chi application that uses MongoDB can occur when an authenticated user’s browser is tricked into making unintended requests to the backend. Because Chi is a lightweight, functional web framework for Clojure, routes typically call handlers that perform operations such as updates or inserts against a MongoDB collection without additional per-request origin verification. When anti-CSRF controls such as synchronizer tokens or custom request checks are absent, an attacker can craft a malicious site that triggers state-changing HTTP methods (POST, PUT, PATCH, DELETE) against your Chi endpoints. If the user is logged in, the request will carry the session cookie or JWT, and MongoDB operations will execute with the user’s privileges.

Chi’s composability can inadvertently encourage patterns where handlers assume requests are legitimate once authentication middleware has run. For example, a handler that directly uses the raw request parameters to build a MongoDB update map can be exploited if the request is forged. An attacker might cause a transfer by submitting a form or image tag pointing to /api/transfer, and if the Chi handler performs {"$set"} updates based on unchecked parameters, the attacker’s desired update may be applied in the context of the victim’s session. MongoDB itself does not enforce CSRF protections; it executes whatever operations the application sends. Therefore, the combination of Chi’s minimal routing and MongoDB’s direct operation execution amplifies the impact of missing anti-CSRF safeguards, leading to unauthorized actions such as changing email addresses, modifying roles, or initiating transactions.

Moreover, if the Chi application exposes unauthenticated endpoints that still perform MongoDB writes based on user-controlled input, CSRF risk increases. Consider an endpoint that updates user preferences via a POST with JSON parsed from the request body. Without verifying the request origin or requiring a token, an attacker can lure a victim to a page that issues a forged POST with crafted JSON, modifying the victim’s data in MongoDB. Because the scan checks such unauthenticated attack surfaces, it can identify missing CSRF protections and flag related findings under Authentication and BOLA/IDOR checks. Remediation focuses on ensuring every state-changing operation validates the request source and binds actions to the authenticated identity explicitly rather than relying solely on session presence.

To defend against CSRF in Chi applications using MongoDB, apply explicit origin and anti-CSRF token validation before executing any database operation. For state-changing routes, require a same-site cookie attribute and implement double-submit cookie patterns or encrypted synchronizer tokens. Validate the Origin or Referer header on the server side, and ensure that each modifying request includes a token that cannot be read by third-party sites. Combine this with tightly scoped MongoDB updates that reference the authenticated user’s ID from the verified session, rather than from request parameters.

Below are concrete Chi handler examples that demonstrate secure handling with MongoDB. The first example shows a protected transfer route that validates an anti-CSRF token and uses a parameterized update that binds the user identity from the session to prevent tampering:

(ns secure-handler
  (:require [cheshire.core :as json]
            [clj-http.client :as http]
            [monger.core :as mg]
            [monger.collection :as mc]))

(defn validate-csrf-token [request]
  (let [session-token (get-in request [:session :csrf-token])
        param-token (get-in request [:params :csrf-token])]
    (= session-token param-token)))

(defn transfer-handler [request]
  (if (validate-csrf-token request)
    (let [user-id (:user-id (:session request))
          amount (Integer/parseInt (get-in request [:params :amount]))
          target (get-in request [:params :target])]
      (mc/update-by-id "accounts"
                       target
                       {"$inc" {"balance" amount}})
      (mc/update-by-id "accounts"
                       user-id
                       {"$inc" {"balance" (- amount)}})
      {:status 200 :body (json/generate-string {:ok true})})
    {:status 403 :body (json/generate-string {:error "invalid csrf"})}))

In this pattern, the CSRF token is stored in the session and verified on each POST. The MongoDB updates use the authenticated user-id from the session, not from the request parameters, preventing attackers from changing the target account. Additionally, prefer using $inc with numeric values rather than replacing entire documents to minimize injection surface.

Another approach is to set SameSite and Secure flags on session cookies and to enforce CORS policies for API endpoints. For less traditional Chi routes that directly accept JSON bodies, ensure that the origin is checked and that per-request tokens are required:

(defn api-update-handler [request]
  (let [origin (get-in request [:headers "origin"])
        referer (get-in request [:headers "referer"])
        allowed-origin? (some #(= % origin) ["https://app.example.com" "https://api.example.com"])]
    (if (and allowed-origin?
             (= (get-in request [:session :user-id]) (get-in request [:params :user-id])))
      (let [update-map (json/parse-string (slurp (:body request)) true)
            safe-update (update update-map "$set" assoc :last-updated (java.util.Date.))]
        (mc/update-by-id "userdata"
                         (get-in request [:params :user-id])
                         safe-update)
        {:status 200 :body (json/generate-string {:status "updated"})})
      {:status 403 :body (json/generate-string {:error "forbidden"})})))

These remediation patterns ensure that each MongoDB write is explicitly bound to the authenticated identity and protected by CSRF tokens or strict origin checks. The scanner can then verify that endpoints either require tokens or validate origins, reducing the likelihood of successful CSRF attacks against your Chi and MongoDB stack.