Command Injection in Chi with Mutual Tls

Command injection occurs when an attacker can control part of a system command executed by the application. In Chi (a minimalistic Go HTTP router), this typically arises when user input is passed to OS commands via functions like exec.Command without proper validation or escaping. Enabling mutual TLS (mTLS) secures transport-layer communication by requiring client certificates, but it does not affect how the server handles untrusted input. Therefore, mTLS can create a false sense of security: operators assume encrypted and authenticated channels protect against all classes of attacks, while command injection remains possible at the application logic layer.

With mutual TLS, the server verifies client certificates before processing requests. This means an attacker must present a valid client cert to reach the vulnerable endpoint if strict mTLS enforcement is applied. However, if the server allows certificate-based authorization to bypass input checks (e.g., mapping a CN or OU to an admin role), the presence of mTLS may actually widen the attack surface by encouraging developers to relax input validation. Once an authenticated session passes user-controlled data into a shell or external command, the command injection vulnerability is fully exploitable over the mTLS channel.

Real-world patterns include using environment variables or certificate fields (such as CommonName) in command construction, for example building a filename or tool argument from certificate metadata without sanitization. Because mTLS ensures identity, developers might skip normalization or allowlist checks, inadvertently enabling injection via seemingly trusted sources. The combination therefore does not introduce command injection by itself, but mTLS can inadvertently encourage insecure coding practices that lead to exploitable command injection paths.

Remediation focuses on two goals: enforcing mutual TLS correctly in Chi and ensuring user input is never used to construct system commands. Below are concrete, idiomatic Go examples that demonstrate secure practices.

1) Configure mTLS in Chi without relaxing input validation. Use http.StripPrefix and proper middleware to require client certs, and always treat request parameters, headers, and certificate fields as untrusted.

import (
	"crypto/tls"
	"net/http"

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

func main() {
	r := chi.NewRouter()

	// Enforce client certificate verification
	r.Use(middleware.RequestID)
	r.Use(middleware.RealIP)
	r.Use(func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			// Ensure client cert is present and valid
			if len(r.TLS.PeerCertificates) == 0 {
				http.Error(w, "client certificate required", http.StatusUnauthorized)
				return
			}
			// Example: inspect certificate fields but do not use them to build commands
			cert := r.TLS.PeerCertificates[0]
			_ = cert.Subject.CommonName // read-only, not used in command construction
			next.ServeHTTP(w, r)
		})
	})

	tlsConfig := &tls.Config{
		ClientAuth: tls.RequireAndVerifyClientCert,
		ClientCAs:  loadCertPool(), // load your CA pool
	}

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

	server.ListenAndTLS()
}

func loadCertPool() *tls.CertPool {
	pool := tls.NewCertPool()
	// load PEM CA certs, e.g., from file or embedded
	return pool
}

2) Avoid command injection by never passing user or certificate-derived data to shell commands. Use statically defined paths and arguments, and prefer Go-native operations over invoking external processes. If external commands are unavoidable, use exec.Command with explicit arguments and no shell involvement.

import (
	"os/exec"
)

// Unsafe pattern to avoid:
// cmd := exec.Command("sh", "-c", "echo "+userInput)

// Secure alternative:
cmd := exec.Command("/usr/bin/some-tool", "--option", "fixed-value")
// If you must incorporate bounded input, validate with a strict allowlist:
allowedNames := map[string]bool{"fileA": true, "fileB": true}
if !allowedNames[userInput] {
	http.Error(w, "invalid input", http.StatusBadRequest)
	return
}
cmd := exec.Command("/usr/bin/process", userInput)

3) Apply defense in depth: combine mTLS with strict input validation, logging, and runtime restrictions. Do not rely on mTLS to sanitize data. Regularly audit dependencies and commands for indirect injections (e.g., via wrappers or environment variables).