Header Injection in Chi with Dynamodb

Header injection in the context of an API built with the Chi routing library and backed by Amazon DynamoDB arises when user-controlled input is reflected into HTTP response headers or is used to construct DynamoDB query conditions without proper validation or encoding. Chi is a lightweight Clojure routing library that encourages explicit parameter handling; however, if developer code passes unchecked parameters from HTTP requests into response headers or into DynamoDB expression attribute values, the API can leak internal behavior or become susceptible to injection-style manipulation at the protocol or data layer.

One common pattern is reading a header such as X-User-Id from the request and using it directly as a partition key value in a DynamoDB query. If the header value is not validated, an attacker can supply crafted input that modifies the intended query structure, for example by injecting conditional expressions or special characters that affect how the query is interpreted. Another scenario involves reflection of header values in custom response headers (e.g., X-Client-Role) without sanitization, which can enable header manipulation or metadata leakage. Because DynamoDB operations often rely on exact key matches, malformed input can lead to unexpected query results or errors that inadvertently reveal whether certain items exist, aiding enumeration attacks.

The risk is compounded when API responses include verbose error messages that surface raw query expressions or validation logic. An attacker can send headers containing malformed or oversized values to probe how the service parses and stores data in DynamoDB. Although Chi does not inherently introduce injection, the combination of dynamic header usage and direct DynamoDB expression construction can expose injection-like outcomes, such as altered query paths or inconsistent authorization checks. These issues map closely to the OWASP API Top 10 categories: Broken Object Level Authorization (BOLA) and Injection.

Real-world examples include an API endpoint that builds a DynamoDB key condition expression using string concatenation from header values without placeholders, or an endpoint that sets response headers based on user input without normalization. In such cases, an attacker might send a header containing characters that affect header parsing or inject conditional logic that changes the query semantics. Because DynamoDB relies on strict attribute type expectations, malformed input can also trigger unexpected type coercion or validation errors, which may be surfaced to the client and used to infer backend behavior.

To assess this specific combination, middleBrick performs checks across multiple security domains in parallel, including Input Validation, Authentication, and Property Authorization. The scanner looks for places where headers influence DynamoDB queries or response metadata without rigorous validation, and it reports findings with severity levels and remediation guidance. Notably, middleBrick does not fix these issues but provides detailed guidance so teams can harden their Chi routes and DynamoDB interactions.

Remediation focuses on strict input validation, canonical encoding, and avoiding direct concatenation of user-controlled data into DynamoDB expressions or headers. In Chi, prefer using explicit route parameters and validated request maps instead of raw headers for sensitive values. When headers must be used, normalize and sanitize them before any further processing.

For DynamoDB interactions, use the AWS SDK for JavaScript (v3) with strongly typed commands and expression builders that rely on placeholder attributes rather than string concatenation. Define a small validation layer that ensures header values conform to expected patterns before they are used in queries or stored.

Example: safe header reading and DynamoDB query construction in a Chi handler.

(ns myapp.handler
  (:require [cheshire.core :as json]
            [compojure.core :refer [GET]]
            [ring.util.response :as resp]
            [aws.sdk.dynamodb :as ddb]))

(defn sanitize-header-value [value]
  (when (and (string? value)
             (re-matches #"^[a-zA-Z0-9-_]{1,64}$" value))
    value))

(defn query-user-by-header [req]
  (let [user-id (sanitize-header-value (get-in req [:headers "x-user-id"]))]
    (if (nil? user-id)
      (resp/bad-request (json/generate-string {:error "Invalid or missing user-id"}))
      (let [params {:table-name "Users"
                    :key-condition-expression "pk = :uid"
                    :expression-attribute-values {":uid" {:s user-id}}}]
        (try
          (let [response (ddb/query params)]
            (resp/json-response (:items response)))
          (catch Exception e
            (resp/internal-server-response (json/generate-string {:error "Query failed"}))))))))

In this example, the header value is validated against a strict alphanumeric pattern with a bounded length before it is used as a DynamoDB expression attribute value. The query uses a placeholder :uid rather than concatenating the header value into the expression string, which prevents injection-style manipulations of the query structure.

Example: setting a response header safely after validation.

(defn safe-response-handler [req]
  (let [role (get-in req [:headers "x-client-role"])
        safe-role (when (and (string? role)
                            (#{"admin" "user" "guest"} role))
                    role)]
    (if (nil? safe-role)
      (resp/bad-request (resp/response "Invalid role"))
      (resp/response "ok"))))

Here, the header value is checked against an allowlist before being used, ensuring that only expected values reach any downstream logic or metadata. This approach reduces the likelihood of header injection or logic bypass via manipulated headers.

For continuous protection, integrate middleBrick into your workflows using the CLI to scan from terminal with middlebrick scan <url>, add API security checks to your CI/CD pipeline with the GitHub Action to fail builds if risk scores drop below your chosen threshold, or scan APIs directly from your AI coding assistant using the MCP Server. These integrations help catch header misuse and DynamoDB handling issues early, without altering the runtime behavior of your services.