Jwt Misconfiguration in Chi with Dynamodb

JWT misconfigurations in a Chi application that uses DynamoDB as a session or token store can expose authentication bypass or token tampering risks. Chi is a minimalistic Clojure web framework that relies on explicit middleware to handle authentication, and DynamoDB is often used to store session data or to look up user identities. When JWT validation is not strictly enforced—such as skipping signature verification, accepting unsigned tokens, or failing to validate the iss (issuer) and aud (audience) claims—an attacker can forge tokens that the application mistakenly trusts.

In this stack, misconfigured token handling can intersect with DynamoDB access patterns. For example, if your application uses a JWT subject (sub) to directly query a DynamoDB table without enforcing strict input validation, an attacker can manipulate the sub claim to perform IDOR or BOLA (Insecure Direct Object References). A typical vulnerable pattern is retrieving user data via a raw token claim without verifying that the requesting user is authorized to access that record. Because DynamoDB access patterns are often keyed by user identifiers, a predictable or unvalidated sub enables attackers to read or modify other users' data.

Another risk specific to Chi and DynamoDB is the lack of proper token expiration and replay handling. Chi applications may rely on middleware that decodes JWTs but does not enforce exp checks or maintain a robust revocation mechanism stored in DynamoDB. Without server-side state or a denylist stored in DynamoDB, stolen tokens remain valid until they expire. Additionally, if your JWT library does not enforce strict algorithms—such as accepting none or asymmetric keys when only symmetric signing is expected—an attacker can switch the algorithm to HS256 and sign the token with a known secret, leading to full authentication bypass.

Middleware configuration plays a crucial role. In Chi, authentication is typically implemented as a route-specific handler or a wrap-around middleware. If the JWT verification middleware is applied inconsistently—such as being omitted for administrative endpoints or applied after data retrieval from DynamoDB—privilege escalation becomes feasible. For instance, an endpoint that fetches user details from DynamoDB using a :user-id path parameter may rely on a JWT claim for filtering. If the JWT is not validated before the DynamoDB query, an attacker can modify the path parameter to access other users' data, and insufficient authorization checks on the server side will allow the request to succeed.

Real-world attack patterns include token substitution, where an attacker replaces a valid JWT with another user's token, and algorithm confusion, where the server accepts an unsigned token. These vulnerabilities are amplified when DynamoDB is used as the source of truth for user roles or permissions without additional context checks. A misconfigured Chi application can therefore expose sensitive data or allow unauthorized operations, even when DynamoDB permissions appear restrictive, because the application layer does not enforce proper authorization on every request.

To secure JWT handling in Chi when using DynamoDB, you must enforce strict token validation, validate claims, and ensure that DynamoDB queries are scoped to the authenticated subject. Below are concrete code examples that demonstrate a robust approach using the cheshire library for JSON parsing, clj-http or an HTTP client for DynamoDB calls, and a JWT library such as clj-jwt.

1. Validate JWT signature and claims before querying DynamoDB

Always verify the token signature, issuer, audience, and expiration before using any claims in a DynamoDB request. Never trust path parameters or token payloads directly.

(ns myapp.auth
  (:require [clj-jwt.core :as jwt]
            [clj-time.core :as t]
            [clj-time.coerce :as c]))

(defn verify-jwt [token public-key]
  (let [decoded (jwt/unsign token public-key)]
    (when (and decoded
               (not (jwt/expired? decoded))
               (= "my-app" (jwt/claim decoded :iss))
               (= "my-api" (jwt/claim decoded :aud)))
      decoded)))

2. Scope DynamoDB queries by authenticated subject

Use the validated sub claim to construct key expressions that only return the caller’s own data. Avoid using path parameters directly as DynamoDB keys without cross-checking against the authenticated subject.

(ns myapp.dynamodb
  (:require [clj-http.client :as http]))

(defn get-user-record [token public-key user-table]
  (let [payload (verify-jwt token public-key)
        sub (:sub payload)
        response (http/get (str "https://dynamodb.region.amazonaws.com/")
                           {:query-params {"TableName" user-table
                                           "KeyConditionExpression" "user_id = :uid"
                                           ":uid" sub}})])
    (if (and (= 200 (:status response))
             (seq (:body response)))
      (-> (:body response) first)
      (throw (Exception. "Unauthorized or not found")))))

3. Enforce algorithm and reject unsigned tokens

Configure your JWT library to reject tokens with unexpected algorithms. Do not accept none or allow asymmetric keys when you expect symmetric signing.

(defmethod jwt/verify "HS256" [token key]
  (jwt/unsign token key))

(defn strict-verify [token public-key]
  (jwt/unsign token public-key {:alg "HS256"}))

4. Use short-lived tokens and DynamoDB-based revocation checks

Store a token identifier (jti) or a denylist in DynamoDB for immediate revocation. Check this store on each authenticated request to mitigate the impact of stolen tokens.

(defn token-revoked? [token-id table]
  (let [response (http/get (str "https://dynamodb.region.amazonaws.com/")
                           {:query-params {"TableName" table
                                           "KeyConditionExpression" "jti = :tid"
                                           ":tid" token-id}})])
  (boolean (seq (:body response)))))

5. Apply consistent middleware in Chi

Ensure your Chi routes apply JWT verification uniformly. Define a reusable authentication handler that validates the token and attaches the claims to the request map before any DynamoDB interaction.

(defn authenticated-routes [handler]
  (fn [request]
    (let [token (some-> request :headers (get "authorization") (re-find #"Bearer (\S+)"))
          claims (verify-jwt token public-key)]
      (if claims
        (handler (assoc request :claims claims))
        {:status 401 :body "Unauthorized"}})))

;; Usage in a Chi route
(GET "/users/*user-id" [request] ; BAD: path parameter used without validation
  ...)

(GET "/profile" [request] ; GOOD: claims used to scope DynamoDB query
  (let [sub (-> request :claims :sub)
        profile (fetch-user-from-dynamodb sub "users-table")]
    {:status 200 :body profile}))

By combining strict JWT validation with disciplined DynamoDB query scoping, you reduce the risk of IDOR and privilege escalation in Chi applications. Always validate input on both the API gateway and the data layer, and treat the JWT payload as untrusted until proven otherwise.