Http Request Smuggling in Chi with Dynamodb

HTTP request smuggling occurs when an application processes HTTP requests differently in transit versus after they reach the backend, enabling attackers to smuggle requests across security boundaries. In a Chi application that uses Amazon DynamoDB as a persistence layer, this typically arises from mismatched parsing of requests by the router and by any intermediate proxies or load balancers, combined with how the application builds and forwards requests to DynamoDB.

Chi is a minimalistic Clojure routing library. When you compose routes and handlers, each request is transformed into a Ring request map that your handlers consume. If your Chi routes or middleware modify or forward the request body (for example to pass a JSON payload to a DynamoDB PutItem call) without canonicalizing the message, and a proxy normalizes the message differently, an attacker can craft requests that are interpreted in conflicting ways. Common patterns that expose smuggling risks include:

• Using chunked transfer encoding while a proxy expects Content-Length–based framing.
• Forwarding the raw HTTP body to downstream services or building DynamoDB requests by concatenating headers and body without strict validation.
• Accepting ambiguous request targets that Chi may route differently than a fronting proxy, causing one path to hit your application while another reaches an unintended internal endpoint that also talks to DynamoDB.

An attacker can exploit this to perform request smuggling attacks such as CL.TE or TE.CL, potentially smuggling a request that bypasses authentication checks or reaches an administrative DynamoDB operation. Because DynamoDB operations are often authorized based on the caller identity extracted from the application layer, smuggling can lead to privilege escalation or unauthorized data access. For example, a smuggled request might invoke a DynamoDB UpdateItem with an unexpected key, modifying records the attacker should not touch.

To detect these issues with middleBrick, you can scan your Chi endpoint without credentials. The scanner runs 12 parallel checks including Input Validation, Property Authorization, and Unsafe Consumption, and it maps findings to frameworks such as OWASP API Top 10. This helps identify whether your request handling and DynamoDB integration expose an inconsistent parsing surface.

Remediation focuses on ensuring that request parsing is consistent between the edge and your application, and that every interaction with DynamoDB is explicit and validated. Below are concrete patterns for a Chi application using the AWS SDK for Java (v2) to interact with DynamoDB.

1. Canonicalize incoming requests before forwarding

Do not forward raw HTTP messages. Parse the body once, validate it, and then construct a new request for DynamoDB. For JSON payloads, use a strict schema and avoid concatenating raw strings.

(ns myapp.handler
  (:require [cheshire.core :as json]
            [aws.sdk.dynamodb :as ddb]))

(defn parse-body [request]
  (try
    (json/parse-string (:body request) true)
    (catch Exception _
      {:error :invalid-json})))

(defn put-item-handler [request]
  (let [payload (parse-body request)]
    (if (:error payload)
      {:status 400 :body (str "Invalid JSON")}
      (let [client (ddb/client {:region :us-east-1})
            result (dddb/put-item client
                                  {:table-name "MyTable"
                                   :item      payload})]
        {:status 200 :body (str "OK")}))))

2. Enforce strict Content-Length and avoid ambiguous transfer encodings

Configure Chi to reject requests that mix chunked and content-length semantics in a way that could confuse proxies. Use middleware that validates the presence and correctness of Content-Length when a body is expected, and reject requests with both Transfer-Encoding and Content-Length.

(ns myapp.middleware
  (:require [ring.util.request :as req]))

(defn validate-content-length [handler]
  (fn [request]
    (let [content-length (req/content-length request)
          transfer-encoding (get-in request [:headers "transfer-encoding"])]
      (if (and content-length transfer-encoding)
        {:status 400 :body "Conflicting Transfer-Encoding and Content-Length"}
        (handler request)))))

3. Use strongly-typed DynamoDB expressions and avoid string concatenation

When constructing requests for DynamoDB, use expression builders rather than interpolating user input into JSON or SQL-like strings. This prevents injection and ensures that keys and values are correctly typed.

(ns myapp.dynamodb
  (:require [aws.sdk.dynamodb :as ddb]))

(defn safe-update [table key-id new-value]
  (let [client (ddb/client {:region :us-east-1})
        result (ddb/update-item client
                                {:table-name   table
                                 :key          {"id" {:s key-id}}
                                 :update-exp   "SET #val = :v"
                                 :expression-attr-names  {"#val" "value"}
                                 :expression-attr-values {":v"   {:s new-value}}})]
    result))

4. Validate and normalize routing targets

Ensure that Chi routes map to explicit handler functions and that no route can be interpreted differently based on header-sensitive parsing. Avoid catch‑all routes that may inadvertently forward malformed requests to internal administrative handlers that also use DynamoDB.

(ns myapp.routes
  (:require [ring.util.response :as resp]
            [compojure.core :refer [defroutes POST GET]]))

(defroutes api-routes
  (POST "/items" [] myapp.handler/put-item-handler)
  (GET  "/items/:id" [] myapp.handler/get-item-handler)
  (route/not-found "Not Found"))

5. Apply middleware for input validation and schema enforcement

Validate headers, method, and body against an expected schema before constructing any DynamoDB request. This reduces the risk that a smuggled request reaches DynamoDB with unexpected keys or values.

(ns myapp.validation
  (:require [malli.core :as m]
            [malli.validate :as mv]))

(def item-schema
  [:map
   [:id :string]
   [:value :string]])

(defn validate-item [body]
  (mv/validate item-schema body))

By combining these practices—canonical parsing, strict input validation, safe DynamoDB expression usage, and explicit route definitions—you reduce the risk of request smuggling and ensure that only well-formed requests affect your DynamoDB operations.