jwtmiddleware.go

package jwtmiddleware

import (
	"context"
	"errors"
	"fmt"
	"log"
	"net/http"
	"strings"

	"github.com/golang-jwt/jwt"
)

// A function called whenever an error is encountered
type errorHandler func(w http.ResponseWriter, r *http.Request, err string)

// TokenExtractor is a function that takes a request as input and returns
// either a token or an error.  An error should only be returned if an attempt
// to specify a token was found, but the information was somehow incorrectly
// formed.  In the case where a token is simply not present, this should not
// be treated as an error.  An empty string should be returned in that case.
type TokenExtractor func(r *http.Request) (string, error)

// Options is a struct for specifying configuration options for the middleware.
type Options struct {
	// The function that will return the Key to validate the JWT.
	// It can be either a shared secret or a public key.
	// Default value: nil
	ValidationKeyGetter jwt.Keyfunc
	// The name of the property in the request where the user information
	// from the JWT will be stored.
	// Default value: "user"
	UserProperty string
	// The function that will be called when there's an error validating the token
	// Default value:
	ErrorHandler errorHandler
	// A boolean indicating if the credentials are required or not
	// Default value: false
	CredentialsOptional bool
	// A function that extracts the token from the request
	// Default: FromAuthHeader (i.e., from Authorization header as bearer token)
	Extractor TokenExtractor
	// Debug flag turns on debugging output
	// Default: false
	Debug bool
	// When set, all requests with the OPTIONS method will use authentication
	// Default: false
	EnableAuthOnOptions bool
	// When set, the middelware verifies that tokens are signed with the specific signing algorithm
	// If the signing method is not constant the ValidationKeyGetter callback can be used to implement additional checks
	// Important to avoid security issues described here: https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/
	// Default: nil
	SigningMethod jwt.SigningMethod
}

type JWTMiddleware struct {
	Options Options
}

func OnError(w http.ResponseWriter, r *http.Request, err string) {
	http.Error(w, err, http.StatusUnauthorized)
}

// New constructs a new Secure instance with supplied options.
func New(options ...Options) *JWTMiddleware {

	var opts Options
	if len(options) == 0 {
		opts = Options{}
	} else {
		opts = options[0]
	}

	if opts.UserProperty == "" {
		opts.UserProperty = "user"
	}

	if opts.ErrorHandler == nil {
		opts.ErrorHandler = OnError
	}

	if opts.Extractor == nil {
		opts.Extractor = FromAuthHeader
	}

	return &JWTMiddleware{
		Options: opts,
	}
}

func (m *JWTMiddleware) logf(format string, args ...interface{}) {
	if m.Options.Debug {
		log.Printf(format, args...)
	}
}

// HandlerWithNext is a special implementation for Negroni, but could be used elsewhere.
func (m *JWTMiddleware) HandlerWithNext(w http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
	err := m.CheckJWT(w, r)

	// If there was an error, do not call next.
	if err == nil && next != nil {
		next(w, r)
	}
}

func (m *JWTMiddleware) Handler(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Let secure process the request. If it returns an error,
		// that indicates the request should not continue.
		err := m.CheckJWT(w, r)

		// If there was an error, do not continue.
		if err != nil {
			return
		}

		h.ServeHTTP(w, r)
	})
}

// FromAuthHeader is a "TokenExtractor" that takes a give request and extracts
// the JWT token from the Authorization header.
func FromAuthHeader(r *http.Request) (string, error) {
	authHeader := r.Header.Get("Authorization")
	if authHeader == "" {
		return "", nil // No error, just no token
	}

	// TODO: Make this a bit more robust, parsing-wise
	authHeaderParts := strings.Fields(authHeader)
	if len(authHeaderParts) != 2 || strings.ToLower(authHeaderParts[0]) != "bearer" {
		return "", errors.New("Authorization header format must be Bearer {token}")
	}

	return authHeaderParts[1], nil
}

// FromParameter returns a function that extracts the token from the specified
// query string parameter
func FromParameter(param string) TokenExtractor {
	return func(r *http.Request) (string, error) {
		return r.URL.Query().Get(param), nil
	}
}

// FromFirst returns a function that runs multiple token extractors and takes the
// first token it finds
func FromFirst(extractors ...TokenExtractor) TokenExtractor {
	return func(r *http.Request) (string, error) {
		for _, ex := range extractors {
			token, err := ex(r)
			if err != nil {
				return "", err
			}
			if token != "" {
				return token, nil
			}
		}
		return "", nil
	}
}

func (m *JWTMiddleware) CheckJWT(w http.ResponseWriter, r *http.Request) error {
	if !m.Options.EnableAuthOnOptions {
		if r.Method == "OPTIONS" {
			return nil
		}
	}

	// Use the specified token extractor to extract a token from the request
	token, err := m.Options.Extractor(r)

	// If debugging is turned on, log the outcome
	if err != nil {
		m.logf("Error extracting JWT: %v", err)
	} else {
		m.logf("Token extracted: %s", token)
	}

	// If an error occurs, call the error handler and return an error
	if err != nil {
		m.Options.ErrorHandler(w, r, err.Error())
		return fmt.Errorf("Error extracting token: %w", err)
	}

	// If the token is empty...
	if token == "" {
		// Check if it was required
		if m.Options.CredentialsOptional {
			m.logf("  No credentials found (CredentialsOptional=true)")
			// No error, just no token (and that is ok given that CredentialsOptional is true)
			return nil
		}

		// If we get here, the required token is missing
		errorMsg := "Required authorization token not found"
		m.Options.ErrorHandler(w, r, errorMsg)
		m.logf("  Error: No credentials found (CredentialsOptional=false)")
		return fmt.Errorf(errorMsg)
	}

	// Now parse the token
	parsedToken, err := jwt.Parse(token, m.Options.ValidationKeyGetter)

	// Check if there was an error in parsing...
	if err != nil {
		m.logf("Error parsing token: %v", err)
		m.Options.ErrorHandler(w, r, err.Error())
		return fmt.Errorf("Error parsing token: %w", err)
	}

	if m.Options.SigningMethod != nil && m.Options.SigningMethod.Alg() != parsedToken.Header["alg"] {
		message := fmt.Sprintf("Expected %s signing method but token specified %s",
			m.Options.SigningMethod.Alg(),
			parsedToken.Header["alg"])
		m.logf("Error validating token algorithm: %s", message)
		m.Options.ErrorHandler(w, r, errors.New(message).Error())
		return fmt.Errorf("Error validating token algorithm: %s", message)
	}

	// Check if the parsed token is valid...
	if !parsedToken.Valid {
		m.logf("Token is invalid")
		m.Options.ErrorHandler(w, r, "The token isn't valid")
		return errors.New("Token is invalid")
	}

	m.logf("JWT: %v", parsedToken)

	// If we get here, everything worked and we can set the
	// user property in context.
	newRequest := r.WithContext(context.WithValue(r.Context(), m.Options.UserProperty, parsedToken))
	// Update the current request with the new context information.
	*r = *newRequest
	return nil
}