Brute Force Attack in Chi with Jwt Tokens

A brute force attack against JWT tokens in a Chi application typically targets the token verification or derivation process. In Chi, routes are composed as middleware, and if an endpoint that validates JWTs does not enforce strict rate limits or account lockout, an attacker can submit many token guesses to discover a valid signature or payload. JWTs are often protected by a symmetric secret (HMAC) or an asymmetric private key (RSA/ECDSA). If the secret or key is weak, or if the token does not include sufficient entropy in its claims, brute-forcing becomes more feasible.

Chi routes do not inherently protect against credential or token brute forcing; you must add explicit guards. Without per-user or per-ip rate limiting and without binding token usage to a specific scope or nonce, an attacker can repeatedly call a protected Chi endpoint with modified tokens and observe differences in response timing or error messages. Some JWT libraries may also leak information through timing differences when validating signatures, which can aid an offline brute-force effort. Additionally, if your Chi service accepts unauthenticated requests to endpoints that issue or refresh tokens, an attacker can generate many token candidates and validate them against endpoints that do not enforce strong input validation or strict audience/issuer checks.

Another vector involves token replay across user scopes. If your Chi handlers trust the JWT payload without additional property-level authorization checks, a brute-forced token from one user may be reused to access another user’s data if the handler does not enforce resource-level ownership. This compounds the risk: an attacker need not crack the signature if they can simply iterate through valid token structures or predictable subject identifiers. Because Chi applications often compose multiple middlewares, a missing rate limiter or authorization guard at one route can expose the entire composition to token brute forcing.

Real-world attack patterns mirror the OWASP API Top 10 2023 category Broken Object Level Authorization (BOLA) and can be discovered by scanning Chi endpoints with middleware that validates JWTs but lacks strict access controls. For example, an endpoint like /api/users/:id that decodes a JWT to extract a user ID is vulnerable if it does not ensure that the requesting subject matches the ID in the token, and if it does not enforce rate limits on token validation attempts. Tools like middleBrick can detect these gaps by correlating unauthenticated scans and JWT validation behaviors with missing authorization checks.

To summarize, the combination of Chi routing, JWT validation, and missing operational guards such as rate limiting and property-level authorization creates conditions where brute forcing tokens is plausible. The scanner checks for these gaps by testing the unauthenticated attack surface and comparing runtime findings against the OpenAPI spec, highlighting where JWT usage intersects with weak access controls.

Remediation focuses on tightening token validation, enforcing strict authorization, and adding operational protections. Always verify the token signature with the correct key, validate standard claims (iss, aud, exp, nbf), and bind tokens to the intended scope and session. In Chi, implement explicit middleware that performs these checks before reaching sensitive handlers.

Example: Secure JWT validation in Chi

(ns myapp.handler
  (:require [cheshire.core :as json]
            [buddy.sign.jwt :as jwt]
            [ring.util.response :as resp]))

(def secret "your-256-bit-secret-here-store-in-env")

(defn verify-jwt [request]
  (let [auth-header (get-in request [:headers "authorization"])
        token (when auth-header (re-find #"Bearer\s+(\S+)" auth-header))
        token (and token (second token))]
    (cond
      (not token)
      {:error :unauthorized :message "Missing token"}

      (not (jwt/verify token secret))
      {:error :unauthorized :message "Invalid signature"}

      :else
      (let [payload (jwt/unsign token secret)
            {exp :exp iss :iss aud :aud} payload]
        (cond
          (not (<= (System/currentTimeMillis) exp))
          {:error :unauthorized :message "Token expired"}
          (not= iss "my-issuer")
          {:error :unauthorized :message "Invalid issuer"}
          (not (some #{aud} ["my-api" "mobile-app"]))
          {:error :unauthorized :message "Invalid audience"}
          :else
          {:ok payload})))))

(defn authenticated-route [request]
  (let [verification (verify-jwt request)]
    (if (:error verification)
      (resp/response {:error (:message verification)})
      (-> request
          (assoc :identity (:payload verification))
          (handler/ok-response)))))

Add property-level authorization and rate limiting

Even with valid JWTs, enforce that users can only access their own resources. Use Chi middleware to compare claims with route parameters and apply per-ip or per-user rate limits to deter brute forcing.

(ns myapp.middleware
  (:require [ring.util.response :as resp]))

(defn rate-limit [handler]
  (let (state (atom {})))
    (fn [request]
      (let [ip (:remote-addr request)
            now (System/currentTimeMillis)
            entries (get @state ip)]
        (if (and entries (> (- now (first entries)) 60000))
          (if (> (count entries) 100)
            (resp/response "Too many requests")
            (do (swap! state update ip conj now)
                (handler request)))
          (do (swap! state assoc ip [now])
              (handler request))))))

(defn ensure-owner [request]
  (let [user-id (:sub (:identity request))
        resource-id (Long/parseLong (get-in request [:params :id]))]
    (if (= user-id resource-id)
      (handler/ok-response)
      (resp/response {:error "Forbidden"}))))

Use strong keys and rotate secrets

Generate secrets with sufficient entropy and rotate them periodically. For asymmetric tokens, use RS256 and keep the private key protected. When rotating keys, support key IDs (kid) and maintain a short overlap to avoid service disruption.

(ns myapp.config
  (:require [buddy.core.codecs :as codecs]))

(def public-key (codecs/hex->bytes "your-public-key-hex"))
(def private-key (codecs/hex->bytes "your-private-key-hex"))

By combining correct JWT verification, property-level authorization, and rate limiting, you mitigate brute force risks for Chi applications that use JWT tokens.