Stack Overflow in Chi with Mutual Tls

In Chi, a typical HTTP server is created using a custom http.Server with TLS configuration. When mutual TLS (mTLS) is enabled, the server requests and validates client certificates. A Stack Overflow vulnerability can arise when a developer configures the server to accept any client certificate without proper verification, or when certificate validation is inadvertently skipped in favor of convenience during debugging. This misconfiguration can expose the application to authentication bypass, allowing unauthenticated or spoofed clients to interact with endpoints that should be restricted.

Chi routes are often defined with middleware that checks for client certificate presence or claims. If these checks are incomplete—for example, only verifying that a certificate exists but not validating the certificate chain or checking the subject’s distinguished name—an attacker can connect without a valid client certificate. In a Stack Overflow context, this could allow an unauthenticated user to access administrative or diagnostic routes that are mistakenly exposed in the same Chi router used for public-facing endpoints.

Additionally, when using mTLS with Chi, developers sometimes bind the same router to both authenticated and unauthenticated middleware stacks. If route definitions are not carefully segregated, an endpoint intended for authenticated mTLS clients may inadvertently inherit relaxed access rules. For instance, a route defined after a broad NoAuth() middleware can override stricter mTLS checks, leading to privilege escalation or information disclosure. This pattern is often discussed in Stack Overflow questions where developers observe that some routes accept client certificates while others do not, despite sharing the same server configuration.

The interaction with TLS settings in Chi further complicates this: if the server’s TLSConfig does not enforce ClientAuth as RequireAndVerifyClientCert, the server may accept connections without a client certificate at all. Even when mTLS is intended, incomplete certificate validation—such as not checking revocation via CRL or OCSP—can allow compromised certificates to be used. In practice, this means an attacker might present an expired or revoked client certificate that the server still accepts, leading to unauthorized access to sensitive API routes that should be protected by mTLS.

To properly enforce mutual TLS in Chi, you must ensure that the HTTP server’s TLS configuration requires and verifies client certificates, and that your route definitions do not bypass these checks. Below is a complete, working example that demonstrates correct mTLS setup in Chi.

// server.go
package main

import (
    "crypto/tls"
    "crypto/x509"
    "io/ioutil"
    "net/http"

    "github.com/go-chi/chi/v5"
    "github.com/go-chi/chi/v5/middleware"
)

func main() {
    // Load server certificate and key
    cert, err := tls.LoadX509KeyPair("server.crt", "server.key")
    if err != nil {
        panic(err)
    }

    // Load CA certificate to verify client certificates
    caCert, err := ioutil.ReadFile("ca.crt")
    if err != nil {
        panic(err)
    }
    caCertPool := x509.NewCertPool()
    caCertPool.AppendCertsFromPEM(caCert)

    // Configure TLS to require and verify client certificates
    tlsConfig := &tls.Config{
        Certificates: []tls.Certificate{cert},
        ClientCAs:    caCertPool,
        ClientAuth:   tls.RequireAndVerifyClientCert,
    }

    // Create Chi router
    r := chi.NewRouter()
    r.Use(middleware.RequestID)
    r.Use(middleware.RealIP)
    r.Use(middleware.Logger)
    r.Use(middleware.Recoverer)

    // Protected route requiring valid client certificate
    r.Get("/admin", func(w http.ResponseWriter, r *http.Request) {
        // At this point, the client certificate has been verified by the TLS layer
        w.Write([]byte("Admin area - access granted via mTLS"))
    })

    // Public route (if needed, on a separate server or port)
    // r.Get("/public", func(w http.ResponseWriter, r *http.Request) {
    //     w.Write([]byte("Public endpoint"))
    // })

    server := &http.Server{
        Addr:      ":8443",
        TLSConfig: tlsConfig,
        Handler:   r,
    }

    // Start HTTPS server with mTLS
    if err := server.ListenAndServeTLS("", ""); err != nil {
        panic(err)
    }
}

This example ensures that ClientAuth is set to tls.RequireAndVerifyClientCert, and that the server’s ClientCAs pool contains the trusted CA certificates. With this configuration, any request lacking a valid, verifiable client certificate will be rejected at the TLS layer before reaching Chi routes. It is also important to run this server on a dedicated port (e.g., 8443) to avoid conflating authenticated and unauthenticated traffic within the same handler stack.

For environments where you need both authenticated and public endpoints, consider running two separate Chi servers with distinct TLS configurations, or use port-based routing at the infrastructure level. Never define broad middleware that skips authentication before more specific mTLS checks, and always validate client certificates rather than relying on their mere presence.