Information Disclosure in Chi with Mutual Tls

Information Disclosure in the context of an API security scan in Chi with Mutual TLS (mTLS) occurs when an API response or infrastructure metadata inadvertently reveals sensitive data that should remain protected. Even when mTLS is enforced to authenticate and encrypt traffic, misconfigurations or implementation gaps can lead to sensitive information appearing in responses, logs, or error messages. This can expose data such as personally identifiable information (PII), internal hostnames, version strings, stack traces, or business logic details.

Mutual TLS ensures both the client and server present valid certificates, which strongly authenticates endpoints and protects data in transit. However, mTLS does not inherently prevent an API from returning more data than necessary or from exposing information through verbose error messages, debug headers, or misconfigured serialization formats. During a scan, middleBrick tests the unauthenticated attack surface and, where mTLS is required, simulates presenting valid client certificates to assess endpoints. The checks for Data Exposure and Encryption specifically look for sensitive content in responses, regardless of the transport security, and flag cases where information is returned without adequate filtering or minimization.

In Chi, where services may rely on mTLS for inter-service communication, Information Disclosure can arise from several common patterns. For example, an endpoint might return full database entity representations that include internal IDs, timestamps, or foreign keys that should be opaque to the caller. Another scenario involves APIs that include server or framework version headers (e.g., X-Powered-By) or detailed error payloads during validation failures. These details, while helpful for debugging, become sensitive when exposed to less-trusted clients. The presence of mTLS may give a false sense of security, leading developers to overlook output-level data exposure because they assume encrypted channels alone are sufficient.

Consider an endpoint that returns user profile details. A vulnerable implementation might include the internal user UUID, creation date, and role flags in the JSON response. Even with mTLS ensuring that only authenticated services can call the endpoint, this response can leak information if the client does not need all fields. middleBrick’s Data Exposure checks examine the structure and content of responses to detect unnecessary data fields, potential PII, and indicators of sensitive business logic. When paired with the Encryption check, which verifies that responses are served over TLS and that certificates are valid, the scanner highlights gaps where encryption is present but data exposure remains.

Additionally, mTLS configurations can inadvertently contribute to information leakage if client certificate validation is inconsistently applied across endpoints. An API might enforce mTLS on some routes while leaving others unauthenticated, creating an asymmetry that attackers can probe. During the scan, middleBrick evaluates whether endpoints correctly require client certificates and whether the server responds with informative errors when certificates are missing or invalid. These errors, if verbose, can disclose whether mTLS is enforced and which certificate details the server expects, aiding an attacker in crafting requests.

To mitigate Information Disclosure in Chi with mTLS, focus on minimizing the data returned by each endpoint and ensuring error messages are generic and non-specific. Validate that mTLS is consistently enforced and that certificate validation does not leak diagnostic details. Review response schemas to remove fields that are not required by clients, and apply the principle of least privilege to both data access and certificate permissions. middleBrick’s findings include remediation guidance tailored to detected exposures, such as redacting sensitive fields and tightening error handling, helping you reduce the risk of information leakage even when strong transport security is in place.

Remediation for Information Disclosure in Chi with Mutual TLS centers on tightening both the mTLS configuration and the API response content. The goal is to ensure that authenticated communication does not become a vector for exposing unnecessary data. Begin by reviewing and standardizing certificate validation across all endpoints, ensuring that missing or invalid client certificates result in generic, non-informative errors that do not reveal server or configuration details.

On the server side, configure your application to strip or omit sensitive fields from JSON responses. For example, in a Chi application using Ring and Compojure, you can define a response middleware that removes fields like internal IDs or version headers before sending data to the client. Below is a concrete code example that demonstrates how to filter a response map in a Chi handler, ensuring only the required fields are returned while mTLS is enforced at the connection level.

(ns myapp.handler
  (:require [compojure.core :refer [GET defroutes]]))

(defn filter-user-response [user-map]
  (select-keys user-map [:public-id :username :email]))

(defn user-handler [request]
  (let [full-user-data {:public-id "usr-123"
                        :internal-id 456
                        :username "jdoe"
                        :email "[email protected]"
                        :created-at "2023-01-01T00:00:00Z"
                        :roles #{:admin :user}}
        safe-response (filter-user-response full-user-data)]
    {:status 200
     :headers {"Strict-Transport-Security" "max-age=31536000; includeSubDomains"}
     :body safe-response}))

(defroutes app-routes
  (GET "/user" [] user-handler))

This handler returns only the fields that are safe and necessary for the client. The mTLS setup is handled at the server and connection level, ensuring that each request presents a valid client certificate. By combining this output filtering with proper TLS configuration, you reduce the risk of information disclosure through over-inclusive responses.

Additionally, ensure that error messages do not include stack traces or internal paths when mTLS authentication fails. In Chi, you can customize exception handling to return a generic 400 or 401 response without revealing whether the failure was due to certificate issues or malformed requests. Below is an example of a centralized error handler that avoids leaking details while still supporting mTLS-based authentication.

(ns myapp.middleware
  (:require [cheshire.core :as json]))

(defn wrap-safe-errors [handler]
  (fn [request]
    (try
      (handler request)
      (catch Exception _
        {:status 400
         :headers {"Content-Type" "application/json"}
         :body (json/generate-string {:error "Bad request"})}))))

;; Apply this middleware alongside your TLS/mTLS setup
(def app
  (-> app-routes
      wrap-safe-errors))

These examples illustrate concrete steps you can take in Chi to address Information Disclosure when using Mutual TLS. The key is to align transport security with disciplined output handling, ensuring that authenticated communication does not expose more data than necessary. middleBrick’s scans can help identify remaining exposure points by checking response contents and error behavior, and the remediation guidance provided with each finding supports targeted fixes.