Beast Attack in Chi with Basic Auth

A Beast Attack (Browser Exploit Against SSL/TLS) leverages predictable initialization vectors (IVs) in block ciphers such as TLS 1.0 and TLS 1.1 with CBC-mode ciphersuites. When combined with Basic Authentication, the attack surface expands because credentials are repeated across requests and can be intercepted and manipulated if the IV predictability issue is reachable in the application flow.

Chi is a lightweight HTTP router for Go. In Chi, if you enable TLS termination at a load balancer or reverse proxy and route into a Chi application that uses Basic Auth for authorization, the unencrypted credential exchange occurs before the application sees the request. An attacker who can observe or tamper with the TLS record boundaries—due to a CBC IV misalignment—may recover plaintext by injecting chosen plaintext and observing changes in the encrypted request body or response timing. Because Basic Auth credentials are base64-encoded rather than encrypted, they can be exfiltrated if the attacker can force the server to echo or reflect the Authorization header in an observable way (e.g., via error messages or cross-origin behavior) while CBC padding errors reveal information about the plaintext.

Chi itself does not enforce a specific TLS configuration; it is the surrounding infrastructure (load balancer, reverse proxy, or embedded server) that negotiates TLS. If that infrastructure supports TLS 1.0/1.1 with CBC ciphersuites and does not implement mitigations such as 1/n-1 splitting or explicit IVs, a Beast Attack can be mounted. Basic Auth compounds the issue because once the plaintext credential is recovered, an attacker gains long-lived access unless credentials are rotated. The scanner checks for these conditions under Encryption, Input Validation, and Authentication categories, highlighting weak ciphersuites and the presence of static or predictable IVs alongside Basic Auth usage.

Remediation focuses on removing reliance on Basic Auth over TLS and ensuring strong transport security. Prefer token-based authentication (e.g., JWT in Authorization: Bearer) and enforce modern TLS configurations. If Basic Auth must be used, ensure it is only transmitted over mutually authenticated TLS 1.2+ with non-CBC ciphersuites or with additional protections such as encrypting the credentials at the application layer before transmission.

Example 1: Chi route with middleware that rejects requests without a valid Bearer token, avoiding Basic Auth entirely:

import (
	"net/http"
	"strings"

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

func authMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		auth := r.Header.Get("Authorization")
		if auth == "" || !strings.HasPrefix(auth, "Bearer ") {
			http.Error(w, "Unauthorized", http.StatusUnauthorized)
			return
		}
		token := strings.TrimPrefix(auth, "Bearer ")
		if !validateToken(token) {
			http.Error(w, "Forbidden", http.StatusForbidden)
			return
		}
		next.ServeHTTP(w, r)
	})
}

func validateToken(token string) bool {
	// Validate against a trusted source, e.g., introspect an OAuth2 token
	return token == "expected-secure-token"
}

func main() {
	r := chi.NewRouter()
	r.Use(middleware.RealIP)
	r.Use(authMiddleware)
	r.Get("/secure", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("Authenticated and safe"))
	})
	http.ListenAndServeTLS(":8443", "server.crt", "server.key", r)
}

Example 2: Chi route that explicitly forbids weak ciphers and enforces TLS 1.2+ via the HTTP server configuration (infrastructure-level fix):

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

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

func secureServer() *http.Server {
	r := chi.NewRouter()
	r.Get("/", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("OK"))
	})

	tlsConfig := &tls.Config{
		MinVersion:               tls.VersionTLS12,
		PreferServerCipherSuites: true,
		CipherSuites: []uint16{
			tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
			tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
			tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
			tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
		},
	}

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

func main() {
	secureServer().ListenAndServeTLS("server.crt", "server.key")
}

These examples eliminate Basic Auth, enforce strong ciphers, and ensure TLS 1.2+ is used, directly mitigating the conditions that enable a Beast Attack when combined with credential transmission.