JWT Method Check

Not verifying the JWT algorithim present in a JWT token when verifying its signature may result in algorithim confusion.

Recommendation

Before presenting a key to verify a signature, ensure that the key corresponds to the algorithim present in the JWT token.

Example

The following code uses the asymmetric public key to verify the JWT token and assumes that the algorithim is RSA. By not checking the signature, an attacker can specify the HMAC option and use the same public key, which is assumed to be public and often at endpoint /jwt/jwks.jso, to sign the JWT token and bypass authentication.

package main

import (
	"fmt"

	"github.com/golang-jwt/jwt/v5"
)

type JWT struct {
	privateKey []byte
	publicKey  []byte
}

func (j JWT) Validate(token string) (interface{}, error) {
	key, err := jwt.ParseRSAPublicKeyFromPEM(j.publicKey)
	if err != nil {
		return "", fmt.Errorf("validate: parse key: %w", err)
	}

	tok, err := jwt.Parse(token, func(jwtToken *jwt.Token) (interface{}, error) {

		return key, nil
	})
	if err != nil {
		return nil, fmt.Errorf("validate: %w", err)
	}

	claims, ok := tok.Claims.(jwt.MapClaims)
	if !ok || !tok.Valid {
		return nil, fmt.Errorf("validate: invalid")
	}

	return claims["dat"], nil
}

References

• Pentesterlab: Exploring Algorithm Confusion Attacks on JWT.
• Common Weakness Enumeration: CWE-347.