Auth Bypass in Chi with Mutual Tls

Mutual Transport Layer Security (mTLS) requires both the client and the server to present valid certificates during the TLS handshake. In Chi, a common Go HTTP handler framework, developers often configure mTLS by setting up a custom tls.Config and attaching it to the server. If the server verifies the client certificate but does not properly enforce authorization checks based on the certificate’s subject, common name (CN), or organizational unit (OU), an authentication bypass can occur.

For example, suppose the server only checks that a client certificate is valid and present, but does not validate that the certificate belongs to an allowed identity. An attacker who possesses any valid client certificate issued by the same trusted Certificate Authority (CA) could authenticate successfully without being the intended principal. This is an Auth Bypass via insufficient identity validation: the system authenticates the client but does not properly authorize it.

Another scenario specific to Chi arises when route handlers rely on request context values that are set earlier in middleware. If the mTLS verification middleware sets user identity based on the certificate but subsequent handlers do not re-verify permissions or assume the identity is trustworthy, an attacker may exploit insecure handler logic to access or modify resources they should not. This can map to common implementation mistakes such as missing role checks or incorrectly scoped claims, similar to patterns seen in OWASP API Top 10:2023 — Broken Function Level Authorization (BOLA).

Operational tooling like middleBrick can detect these issues by scanning the unauthenticated attack surface and, when provided with a site map or API inventory, flagging missing authorization checks on authenticated routes. It runs checks in parallel for Authentication and Authorization, highlighting cases where authentication succeeds but authorization is missing or inconsistent. Note that middleBrick detects and reports these findings and provides remediation guidance; it does not fix or block requests.

Real-world attack patterns include leveraging stolen or misissued client certificates, abusing weak certificate issuance policies, or chaining missing authorization checks with other endpoints to achieve privilege escalation. These illustrate why mTLS deployments in Chi must couple cryptographic authentication with explicit, per-request authorization decisions rather than assuming trust after the handshake.

To remediate Auth Bypass risks in Chi when using mTLS, ensure that certificate validation is followed by explicit identity and authorization checks in each handler or middleware. Below are concrete, working code examples that demonstrate a secure approach.

First, configure the server with a tls.Config that requests and verifies client certificates:

// server.go
package main

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

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

func main() {
	// Load server certificate and key
	serverCert := []byte(`-----BEGIN CERTIFICATE-----
MIIC...server cert...
-----END CERTIFICATE-----`)
	serverKey := []byte(`-----BEGIN ENCRYPTED PRIVATE KEY-----
MIIEvQIBADAN...server key...
-----END ENCRYPTED PRIVATE KEY-----`)

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

	tlsConfig := &tls.Config{
		Certificates: []tls.Certificate{loadTLSCert(serverCert, serverKey)},
		ClientCAs:    caCertPool,
		ClientAuth:   tls.RequireAndVerifyClientCert,
		MinVersion:   tls.VersionTLS12,
	}

	r := chi.NewRouter()
	r.Use(mTlsVerifyMiddleware(caCertPool))
	// secure routes here
	http.ListenAndServeTLS("":8443", "server.crt", "server.key", r.WithTLSConfig(tlsConfig))
}

func loadTLSCert(certPEM, keyPEM []byte) tls.Certificate {
	cert, err := tls.X509KeyPair(certPEM, keyPEM)
	if err != nil {
		panic(err)
	}
	return cert
}

Second, implement a middleware in Chi that extracts and validates client identity from the verified certificate. Do not rely solely on the TLS state; explicitly map certificate fields to an internal principal:

// middleware.go
package main

import (
	"crypto/x509"
	"net/http"

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

func mTlsVerifyMiddleware(caPool *x509.CertPool) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			clientCert := r.TLS.VerifiedChains[0][0]
			// Validate against CA is already done by VerifyChains,
			// now perform authorization-sensitive checks:
			allowed := isAllowedClient(clientCert, caPool)
			if !allowed {
				http.Error(w, "forbidden", http.StatusForbidden)
				return
			}
			// Set identity in request context for downstream handlers
			ctx := context.WithValue(r.Context(), "principal", clientCert.Subject.CommonName)
			next.ServeHTTP(w, r.WithContext(ctx))
		})
	}
}

func isAllowedClient(cert *x509.Certificate, caPool *x509.CertPool) bool {
	// Example: enforce CN prefix or OU membership
	if len(cert.Subject.OrganizationalUnit) == 0 {
		return false
	}
	for _, ou := range cert.Subject.OrganizationalUnit {
		if ou == "api-clients" {
			return true
		}
	}
	return false
}

Third, ensure handlers validate permissions on each request rather than trusting middleware context alone:

// handlers.go
package main

import (
	"net/http"

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

func secureHandler(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		principal, ok := r.Context().Value("principal").(string)
		if !ok || !hasPermission(principal, chi.URLParam(r, "resourceID")) {
			http.Error(w, "forbidden", http.StatusForbidden)
			return
		}
		next.ServeHTTP(w, r)
	})
}

func hasPermission(user, resource string) bool {
	// Implement your authorization logic here, e.g., check a mapping or policy
	return user == "alice" && resource == "123"
}

These examples show how to couple mTLS authentication with explicit authorization in Chi. Additional recommendations include rotating certificates via a secure process, monitoring for unexpected client certificates, and integrating findings from scans by middleBrick to iteratively tighten authorization logic.