Integrity Failures in Chi with Mutual Tls

Integrity failures in a Chi application when using Mutual TLS (mTLS) typically stem from configuration or routing issues that allow an endpoint to accept requests without requiring a valid client certificate, or to accept certificates that do not properly validate back to a trusted authority. When mTLS is intended to provide strong endpoint identity and message integrity, these failures undermine the trust chain and can permit unauthorized actors to interact with the service as if they were legitimate, authenticated clients.

Chi routes are often defined declaratively, and if the TLS configuration is not consistently applied to all routes, some paths may be served over TLS but without client certificate verification. This can expose admin or health-check paths to unauthenticated or spoofed requests, leading to data exposure or unauthorized actions. Even when mTLS is enforced at the listener level, integrity can be compromised if the certificate verification settings are incomplete, such as missing client CA configuration or improperly set verify_peer options.

Another common scenario involves upstream services or reverse proxies terminating TLS before forwarding to a Chi backend. In such setups, the backend may assume the connection is already authenticated, but if the proxy does not forward client certificate information or if the backend does not validate it, the integrity boundary is broken. Attackers can exploit this by sending requests that appear to come from authenticated clients, bypassing intended access controls and potentially modifying data or triggering unintended operations.

Real-world attack patterns include attempts to exploit weak or missing client certificate validation, similar to issues seen in services misconfigured around TLS termination and trust boundaries. While this is not a direct code vulnerability in Chi itself, the framework relies on the underlying HTTPS configuration to enforce mTLS. A misconfigured HTTPS server or load balancer can negate the protections that mTLS is meant to provide, allowing tampered requests to reach Chi handlers that assume a verified identity.

Using middleBrick’s OpenAPI/Swagger spec analysis, you can cross-reference your API definitions with runtime findings to detect mismatches between documented authentication requirements and actual enforcement. This is particularly useful when mTLS is defined in the spec but not correctly enforced in the Chi application or its deployment environment. The scanner’s checks for unauthenticated endpoints and improper validation help surface these integrity gaps before they can be exploited.

For teams using the Pro plan, continuous monitoring can alert you when a deployment introduces a configuration drift that weakens mTLS enforcement. The GitHub Action can fail a build if the scanned API’s security score drops due to missing authentication requirements on routes that should require client certificates. This helps maintain integrity across deployments and ensures that mTLS remains effective as configurations evolve.

To remediate integrity issues in Chi when using Mutual TLS, ensure that the HTTPS server is configured to request and validate client certificates for all relevant routes. This involves setting up the server with proper CA certificates, requiring peer verification, and rejecting connections that do not present a valid client certificate signed by an accepted authority.

Below are concrete code examples for configuring mTLS in a Chi application using the underlying HTTPS options. These examples assume you are using clack or a compatible HTTP server that supports TLS configuration via tls_server_options.

Chi HTTPS Server with Mutual TLS

(ql:quickload '(:chi :clack))

(defvar *trusted-ca-file* "/path/to/ca-bundle.pem")

(defapp my-secure-app ()
  (let ((router (make-instance 'chi:router)))
    (chi:GET "/admin" (req res) (setf (clack:response-body res) "Admin access"))
    (chi:GET "/health" (req res) (setf (clack:response-body res) "OK"))
    router))

(let ((tls-opts (clack:make-tls-server-options
                  :certificate "server.pem"
                  :private-key "server.key"
                  :ca-file *trusted-ca-file*
                  :verify-peer :require
                  :verify-depth 5)))
  (clack:listen "0.0.0.0" 8443 :app #'my-secure-app :tls-options tls-opts))

Client Certificate Request Example

Clients connecting to this Chi service must present a valid certificate signed by the CA specified in ca-bundle.pem. The following example shows how a client can be configured using clack:

(ql:quickload '(:clack))

(defvar *client-cert* "/path/to/client.pem")
(defvar *client-key* "/path/to/client.key")
(defvar *ca-cert* "/path/to/ca-bundle.pem")

(let ((client-opts (clack:make-tls-client-options
                    :certificate *client-cert*
                    :private-key *client-key*
                    :ca-file *ca-cert*
                    :verify-peer t)))
  (clack:call "https://localhost:8443/admin"
              :tls-options client-opts
              (lambda (res) (print (clack:response-body res)))))

Key Configuration Parameters

• :verify-peer :require ensures that the server demands a valid client certificate.
• :ca-file must point to a bundle of trusted CA certificates that can validate client certificates.
• :verify-depth limits the chain length to prevent overly complex or malicious chains.

When deploying behind a proxy or load balancer, ensure that the proxy is configured to request client certificates and forward them to the Chi backend, or terminate TLS at the proxy and enforce authentication before passing traffic to Chi. The middleBrick CLI can be used to scan your Chi endpoints from the terminal to verify that mTLS is correctly enforced:

middlebrick scan https://api.example.com

For ongoing compliance, the Pro and Enterprise plans support configurable scanning schedules and CI/CD integration. The GitHub Action can be added to fail builds if the scan detects missing authentication on routes that should require client certificates, helping to prevent regressions that could weaken integrity.