Prevent XXE Attack with JAXB

java, jaxb, security, ws-security, xxe

Solution

JAXB

You can prevent the Xml eXternal Entity (XXE) attack by unmarshalling from an `XMLStreamReader` that has the `IS_SUPPORTING_EXTERNAL_ENTITIES` and/or `XMLInputFactory.SUPPORT_DTD` properties set to `false`.

JAX-WS

A JAX-WS implementation should take care of this for you. If it doesn't I would recommend opening a bug against the specific implmententation.

EXAMPLE

Demo

package xxe;

import javax.xml.bind.*;
import javax.xml.stream.*;
import javax.xml.transform.stream.StreamSource;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Customer.class);

        XMLInputFactory xif = XMLInputFactory.newFactory();
        xif.setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, false);
        xif.setProperty(XMLInputFactory.SUPPORT_DTD, false);
        XMLStreamReader xsr = xif.createXMLStreamReader(new StreamSource("src/xxe/input.xml"));

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Customer customer = (Customer) unmarshaller.unmarshal(xsr);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(customer, System.out);
    }

}

input.xml

This XML document contains an entity that has been setup to get the listing of files I used to create this example.

<?xml version="1.0"?>
<!DOCTYPE customer
[
<!ENTITY name SYSTEM "/Users/bdoughan/Examples/src/xxe/">
]
>
<customer>
  <name>&name;</name>
</customer>

Customer

package xxe;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Customer {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

Output - Default Configuration

By default the entity will be resolved.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <name>Customer.java
Demo.java
input.xml
</name>
</customer>

Output when `XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES` property is set to `false`

When this property is set the entity is not resolved.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <name></name>
</customer>

Output when `XMLInputFactory.SUPPORT_DTD` property is set to `false`

When this property is set an exception is thrown trying to resolve the entity.

Exception in thread "main" javax.xml.bind.UnmarshalException
 - with linked exception:
[javax.xml.stream.XMLStreamException: ParseError at [row,col]:[8,15]
Message: The entity "name" was referenced, but not declared.]
    at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.handleStreamException(UnmarshallerImpl.java:436)
    at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:372)
    at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:342)
    at xxe.Demo.main(Demo.java:18)
Caused by: javax.xml.stream.XMLStreamException: ParseError at [row,col]:[8,15]
Message: The entity "name" was referenced, but not declared.
    at com.sun.org.apache.xerces.internal.impl.XMLStreamReaderImpl.next(XMLStreamReaderImpl.java:598)
    at com.sun.xml.bind.v2.runtime.unmarshaller.StAXStreamConnector.bridge(StAXStreamConnector.java:196)
    at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:370)
    ... 2 more

Problem

Recently, we had a security audit on our code, and one of the problem is that our application is subject to the Xml eXternal Entity (XXE) attack. Basically, the application is a calculator that receives inputs as XML, through a Web-Service. Here is an example of such an XXE attack on our application: ``` <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"> <soapenv:Header/> <soapenv:Body> <foo:calculateStuff> <!--Optional:--> <xmlInput><![CDATA[<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <!DOCTYPE currency [ <!ENTITY include SYSTEM "file:///d:/" >]> <calcinput>...</calcinput> ]]></xmlInput> </foo:calculateStuff> </soapenv:Body> </soapenv:Envelope> ``` As you can see, we can refer to an entity that points to an external file (`"file:///d:/"`). Regarding the XML input itself (the `<calcinput>...</calcinput>` part) is unmarshalled with JAXB (v2.1). The web-service part is based on jaxws-rt (2.1). What do I need to do to secure my web-service?

Original source