Poodle Attack in Chi with Jwt Tokens

The Poodle attack (Padding Oracle On Downgraded Legacy Encryption) targets block ciphers in CBC mode and exploits how systems handle padding errors during decryption. In the context of Chi and JWT tokens, this typically arises when JWT decryption or validation is performed using a block cipher in CBC mode without adequate protections against padding oracle behavior.

Chi is a common server-side web framework for the Go language. When JWT tokens are parsed and decrypted inside Chi middleware, developers may inadvertently use cipher configurations that are susceptible to Poodle-style attacks. For example, if a JWT library or custom implementation decrypts tokens with AES-CBC and returns distinguishable padding errors (e.g., invalid padding versus invalid signature), an attacker can iteratively submit modified ciphertexts and observe differences in error responses or timing. This leakage allows recovery of plaintext, including sensitive claims or keys, even when the token is transmitted over HTTPS.

In practice, the risk surface appears when:

• JWT decryption in Chi routes or middleware uses AES-CBC without integrity protection prior to decryption.
• Error handling exposes padding validation failures as distinct HTTP responses or logs.
• The application does not enforce authenticated encryption (e.g., AES-GCM) and relies on ad hoc CBC-based schemes.

middleBrick can detect scenarios where unauthenticated endpoints expose JWT-related behavior that could be leveraged in a Poodle context by analyzing runtime responses and OpenAPI specifications. Findings include severity ratings and remediation guidance mapped to frameworks like OWASP API Top 10 and PCI-DSS.

To mitigate Poodle-related risks with JWT tokens in Chi, use authenticated encryption and avoid manual padding handling. Prefer libraries that implement AES-GCM or use high-level JWT parsing functions that validate integrity before decryption. Below are concrete, idiomatic examples for Chi using Go.

Example 1: Secure JWT parsing with github.com/golang-jwt/jwt

package main

import (
	"github.com/golang-jwt/jwt/v5"
	"github.com/go-chi/chi/v5"
	"net/http"
)

func main() {
	r := chi.NewRouter()
	r.Use(func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			auth := r.Header.Get("Authorization")
			if auth == "" {
				http.Error(w, `{"error": "missing authorization header"}`, http.StatusUnauthorized)
				return
			}
			// Expecting Bearer <token>
			const prefix = "Bearer "
			if len(auth) < len(prefix) || auth[:len(prefix)] != prefix {
				http.Error(w, `{"error": "invalid authorization format"}`, http.StatusUnauthorized)
				return
			}
			tokenString := auth[len(prefix):]
			// Use Parse with a key function; the library handles verification and parsing safely
			_, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
				// TODO: select the appropriate key based on token header kid
				return []byte("your-256-bit-secret"), nil
			})
			if err != nil {
				http.Error(w, `{"error": "invalid token"}`, http.StatusUnauthorized)
				return
			}
			next.ServeHTTP(w, r)
		})
	})

	r.Get("/protected", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte(`{"ok": true}`))
	})

	http.ListenAndServe(":8080", r)
}

Example 2: Enforcing JWT validation in Chi middleware with claims checks

package main

import (
	"time"
	"github.com/golang-jwt/jwt/v5"
	"github.com/go-chi/chi/v5"
	"net/http"
)

type CustomClaims struct {
	Scope string `json:"scope"`
	jwt.RegisteredClaims
}

func main() {
	r := chi.NewRouter()
	r.Use(JwtMiddleware)
	r.Get("/data", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte(`{"data": "secure"}`))
	})

	http.ListenAndServe(":8080", r)
}

func JwtMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		tokenString := r.Header.Get("Authorization")
		if tokenString == "" {
			http.Error(w, `{"error": "authorization required"}`, http.StatusUnauthorized)
			return
		}
		claims := &CustomClaims{}
		token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
			return []byte("your-256-bit-secret"), nil
		})
		if err != nil || !token.Valid {
			http.Error(w, `{"error": "invalid token"}`, http.StatusUnauthorized)
			return
		}
		// Example: enforce expiration and audience
		if claims.ExpiresAt != nil && claims.ExpiresAt.Before(time.Now()) {
			http.Error(w, `{"error": "token expired"}`, http.StatusUnauthorized)
			return
		}
		next.ServeHTTP(w, r)
	})
}

Key remediation points:

• Use authenticated encryption (e.g., AES-GCM) instead of raw AES-CBC for JWT encryption.
• Always validate the token signature and claims before using any payload data.
• Standardize error responses to avoid leaking padding or validation details; return a generic 401 for invalid tokens.
• Leverage established libraries (like github.com/golang-jwt/jwt) rather than implementing cryptographic primitives yourself.

These practices reduce the attack surface for padding oracle and related cryptographic attacks while keeping JWT handling robust within Chi applications.