org.apache.logging.log4j.changelog.util.XmlUtils Maven / Gradle / Ivy

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.logging.log4j.changelog.util;

import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

final class XmlUtils {

    private XmlUtils() {}

    /**
     * @return a {@link DocumentBuilderFactory} instance configured with certain XXE protection measures
     * @see XML External Entity Prevention Cheat Sheet
     */
    static DocumentBuilderFactory createDocumentBuilderFactory() {
        final DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        String feature = null;
        try {

            // This is the PRIMARY defense.
            // If DTDs (doctypes) are disallowed, almost all XML entity attacks are prevented.
            // Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl
            feature = "http://apache.org/xml/features/disallow-doctype-decl";
            dbf.setFeature(feature, true);

            // If you can't completely disable DTDs, then at least do the following:
            // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-general-entities
            // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-general-entities
            // JDK7+ - http://xml.org/sax/features/external-general-entities
            // This feature has to be used together with the following one, otherwise it will not protect you from XXE for sure.
            feature = "http://xml.org/sax/features/external-general-entities";
            dbf.setFeature(feature, false);

            // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-parameter-entities
            // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-parameter-entities
            // JDK7+ - http://xml.org/sax/features/external-parameter-entities
            // This feature has to be used together with the previous one, otherwise it will not protect you from XXE for sure.
            feature = "http://xml.org/sax/features/external-parameter-entities";
            dbf.setFeature(feature, false);

            // Disable external DTDs as well
            feature = "http://apache.org/xml/features/nonvalidating/load-external-dtd";
            dbf.setFeature(feature, false);

            // and these as well, per Timothy Morgan's 2014 paper: "XML Schema, DTD, and Entity Attacks"
            dbf.setXIncludeAware(false);
            dbf.setExpandEntityReferences(false);

        } catch (final ParserConfigurationException error) {
            final String message = String.format("`%s` is probably not supported by your XML processor", feature);
            throw new RuntimeException(message, error);
        }
        return dbf;
    }

}