private String validateSection(String section, String sectionName) throws ValidateException {
        section = section.trim();

        if (section.isEmpty())
            throw new ValidateException(String.format(
                "Malformed JWT provided; expected at least three sections (header, payload, and signature), but %s section missing",
                sectionName));

        return section;
    }
Returns the third section--the JWT signature--in its base 64-encoded form.

@return Signature section of the JWT
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/secured/SerializedJwt.java