Example for jwt.Parse uses incorrect method for validating algorithm methods

[mattt]

The ExampleParse_hmac function in hmac_example_test.go provides the following example code:

jwt/hmac_example_test.go

Lines 51 to 59 in bc8bdca

	token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
	// Don't forget to validate the alg is what you expect:
	if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
	return nil, fmt.Errorf("Unexpected signing method: %v", token.Header["alg"])
	}
	// hmacSampleSecret is a []byte containing your secret, e.g. []byte("my_secret_key")
	return hmacSampleSecret, nil
	})

The validation performed on line 53 seems at odds with the recommendations in the Parse function docs:

jwt/parser.go

Lines 218 to 225 in bc8bdca

	// Parse parses, validates, verifies the signature and returns the parsed token.
	// keyFunc will receive the parsed token and should return the cryptographic key
	// for verifying the signature. The caller is strongly encouraged to set the
	// WithValidMethods option to validate the 'alg' claim in the token matches the
	// expected algorithm. For more details about the importance of validating the
	// 'alg' claim, see
	// https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/
	func Parse(tokenString string, keyFunc Keyfunc, options ...ParserOption) (*Token, error) {

In this case, the type assertion tests whether token.Method can be safely converted to the *jwt.SigningMethodHMAC type. But a more specific check for HS256 seems more appropriate.

token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
	// hmacSampleSecret is a []byte containing your secret, e.g. []byte("my_secret_key")
	return hmacSampleSecret, nil
}, jwt.WithValidMethods([]string{jwt.SigningMethodHS256.Alg()}))

Or for all HMAC signing methods:

validMethods := []string{
	jwt.SigningMethodHS256.Alg(),
	jwt.SigningMethodHS384.Alg(),
	jwt.SigningMethodHS512.Alg(),
}
token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
	// hmacSampleSecret is a []byte containing your secret, e.g. []byte("my_secret_key")
	return hmacSampleSecret, nil
}, jwt.WithValidMethods(validMethods))