Format String in Chi with Dynamodb

A format string vulnerability occurs when user-controlled input is passed directly into a formatted output function without proper sanitization or type conversion. In a Chi-based Clojure application that interacts with DynamoDB, this typically arises when constructing request parameters or log messages using unchecked string interpolation. For example, if a route extracts an identifier from request parameters and uses it to build a DynamoDB query key via str or format, a malicious payload like %s or %x can cause unintended memory reads or writes in the underlying Java runtime, since Clojure ultimately delegates to Java libraries for HTTP and data formatting.

When the application builds a DynamoDB GetItem or Query request using unchecked string formatting, such as embedding an ID directly into a condition expression or key schema string, the format string flaw can distort the intended query structure. Consider a route that uses Integer/parseInt on user input to construct a partition key; if the input contains format specifiers, the parsing or subsequent string building may consume extra arguments, leading to exceptions or information disclosure. In the context of DynamoDB, this can result in malformed requests that expose stack traces or internal state through error messages, aiding an attacker in mapping the service behavior.

Chi routes often handle parameters via :params in the request map. If these values are forwarded to a logging or monitoring function that uses format without explicit type-safe conversion, the format string vulnerability becomes actionable. An attacker can supply inputs such as \%d \%d to manipulate how many arguments are consumed, potentially causing the application to read adjacent memory or skip expected values. When combined with DynamoDB client calls that rely on precisely structured keys, this can lead to unexpected query behavior or exposure of unauthenticated endpoints if error handling inadvertently reveals sensitive data paths.

The LLM/AI security checks included in middleBrick specifically test for system prompt leakage and output scanning for PII or code, which can help detect whether format string induced errors expose sensitive information in logs or responses. Since DynamoDB operations often return detailed error messages for malformed requests, an application lacking input validation may leak internal schema or endpoint details. By correlating format string probes with DynamoDB request patterns, middleBrick can identify whether user input directly influences query construction or logging in a way that violates secure coding practices.

To mitigate, always validate and convert user input before it reaches formatting or DynamoDB client construction. Use Clojure’s Integer/parseInt within a controlled scope and avoid format for building query strings or keys. Prefer parameterized data structures that do not rely on string interpolation for DynamoDB attribute values. middleBrick’s dashboard and CLI can scan unauthenticated endpoints to surface these risks, while the Pro plan supports continuous monitoring to detect regressions when routes are updated.

Remediation focuses on strict input validation, avoiding string interpolation for DynamoDB keys and condition expressions, and using the AWS SDK’s built-in parameterization. In Chi, handle parameters early, convert them to the expected types, and construct DynamoDB requests using maps rather than concatenated strings.

Instead of building a key via (str "/table/" id), use a map structure expected by the DynamoDB client and validate id as a non-negative integer. For example:

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

(defn parse-positive-int [s]
  (try
    (let [v (Integer/parseInt s)]
      (when (pos? v) v))
    (catch NumberFormatException _ nil)))

(defn get-item-handler [request]
  (let [id-param (get-in request [:params :id])
        id (parse-positive-int id-param)]
    (if (nil? id)
      {:status 400 :body (json/generate-string {:error "invalid id"})}
      (let [response (ddb/get-item {:db "my-table"
                                    :key {"id" {:n (str id)}}})
            item (:body response)]
        {:status 200 :body (json/generate-string item)}))))

This approach ensures that id is a valid integer before it is converted to a string for the DynamoDB attribute value, eliminating format specifiers from the input. The key map uses the AWS SDK’s expected shape, avoiding any need for manual string formatting that could be vulnerable to injection or malformed requests.

For logging, avoid format with user input. Use structured logging with explicit fields:

(ns app.logging
  (:require [taoensso.timbre :as log]))

(defn safe-log [id status]
  (log/info :event "get-item"
            :user-id id
            :status status))

In the route definition, call safe-log with already-validated values, ensuring no format strings are constructed from request parameters. middleBrick’s output scanning can detect whether log messages contain PII or code, and the dashboard tracks findings across scans to verify that remediation reduces the attack surface.

When using the CLI (middlebrick scan <url>) or GitHub Action to integrate checks into CI/CD, configure thresholds to fail builds if high-severity format string patterns are detected in API routes. The Pro plan enables continuous monitoring so that any future changes to DynamoDB interaction code are automatically reassessed. Remember that middleBrick detects and reports issues but does not modify code; developers must apply the described fixes to achieve a secure implementation.