Fix JWT Vulnerabilities (Weak Signing, None Algo) in Gorilla
JWT implementation flaws in Go often stem from improper validation of the 'alg' header and hardcoded secrets. If you're using Gorilla/mux with golang-jwt, failing to enforce the signing method allows attackers to bypass auth via the 'none' algorithm or brute-force weak keys. This guide details how to harden your Keyfunc to prevent signature stripping and algorithm confusion.
The Vulnerable Pattern
func AuthMiddleware(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { tokenString := r.Header.Get("Authorization") // VULNERABLE: No validation of the 'alg' header. // An attacker can change 'alg' to 'none' or use a public key to spoof HMAC. token, _ := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) { return []byte("weak_secret_key"), nil })if token.Valid { next.ServeHTTP(w, r) } })
}
The Secure Implementation
The fix involves two critical steps. 1. Algorithm Enforcement: By performing a type assertion on `token.Method`, we ensure the library only accepts HMAC signatures. This effectively kills the 'alg: none' bypass and RSA/HMAC confusion attacks. 2. Secret Management: Hardcoded secrets are easily dumped via binaries or source leaks. Using `os.Getenv` with a high-entropy string prevents brute-forcing. Additionally, always check the `err` returned by `jwt.Parse`; failing to do so may result in nil-pointer dereferences or logic bypasses if `token` is nil.
func AuthMiddleware(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { tokenString := strings.TrimPrefix(r.Header.Get("Authorization"), "Bearer ")token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) { // SECURE: Explicitly validate the signing algorithm type if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok { return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"]) } // SECURE: Use environment variables for high-entropy secrets secret := os.Getenv("JWT_SIGNING_KEY") if secret == "" { return nil, errors.New("internal server error") } return []byte(secret), nil }) if err != nil || !token.Valid { http.Error(w, "Unauthorized", http.StatusUnauthorized) return } next.ServeHTTP(w, r) })
}
Your Gorilla API
might be exposed to JWT Vulnerabilities (Weak Signing, None Algo)
74% of Gorilla apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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