CVE-2020-26217

Depending on the version of XStream used there are various usage patterns that mitigate this flaw, though we would strongly recommend using the allow list approach if at all possible as there are likely more class combinations the deny list approach may not address.

Allow list approach

java
XStream xstream = new XStream();
XStream.setupDefaultSecurity(xstream);
xstream.allowTypesByWildcard(new String[] {"com.misc.classname"})

Deny list for XStream 1.4.13

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

Deny list for XStream 1.4.7 -> 1.4.12

java
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 });

Deny list for versions prior to XStream 1.4.7

java
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_LOW);