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The Java classes used by a program are not necessarily loaded upon program startup. Many Java Virtual Machines (JVMs) load classes only when they need them.

If untrusted code is permitted to load classes, it may possess the ability to load a malicious class. This is a class that shares a fully-qualified name with a benign class that is required by trusted code. When the trusted code tries to load its benign class, the JVM provides it with the malicious class instead. As a result, if a program permits untrusted code to load classes, it must first preload any benign classes it needs. Once loaded, these benign classes cannot be replaced by untrusted code.

Noncompliant Code Example (Tomcat)

This noncompliant code example shows a vulnerability present in several versions of the Tomcat HTTP web server (fixed in version 6.0.20) that allows untrusted web applications to override the default XML parser used by the system to process web.xml, context.xml and tag library descriptor (TLD) files of other web applications deployed on the Tomcat instance. Consequently, untrusted web applications that install a parser could view and/or alter these files under certain circumstances.

The noncompliant code example shows the code associated with initialization of a new Digester instance in the org.apache.catalina.startup.ContextConfig class. "A Digester processes an XML input stream by matching a series of element nesting patterns to execute Rules that have been added prior to the start of parsing" [Tomcat 2009]. The code to initialize the Digester follows:

protected static Digester webDigester = null;

if (webDigester == null) {
  webDigester = createWebDigester();
}

The createWebDigester() method is responsible for creating the Digester. This method calls createWebXMLDigester(), which invokes the method DigesterFactory.newDigester(). This method creates the new digester instance and sets a boolean flag useContextClassLoader to true.

// This method exists in the class DigesterFactory and is called by 
// ContextConfig.createWebXmlDigester().
// which is in turn called by ContextConfig.createWebDigester()
// webDigester finally contains the value of digester defined
// in this method.
public static Digester newDigester(boolean xmlValidation,
 

Security manager checks may be allowed to get bypassed depending on the immediate caller's class loader. When an API (See table below) is invoked on a class object, a comparison is run between the immediate caller's class loader and that of the class object. The class object is the object on which an API is invoked. For instance, in the presence of a security manager, the getSystemClassLoader and getParent methods succeed only if the caller's class loader is the delegation ancestor of the current class loader or if the caller's class loader is the same as the current one or if the code in the current execution context has the RunTimePermission, namely "getClassLoader".

Non-Compliant Code Example

The createInstance method is the immediate caller of java.lang.Class.newInstance <TO BE CONFIRMED> in this non-compliant example. The newInstance method is being invoked on the dateClass class object. The issue is that the untrustedCode method can trigger the instantiation of a new class even though it should not have the permission to do so. This behavior is not caught by the security manager.

import java.util.Date;

public class exception
{
    public static void untrustedCode()
    {
    	 Date now = new Date();
         Class<?>   dateClass = now.getClass();
         createInstance(dateClass);
    }
    public static void createInstance(Class<?> dateClass)boolean xmlNamespaceAware,
    {
        try
        {   // Create another Date object using the Date Class
    RuleSet rule) {
  Digester digester =  Object o = dateClass.newInstance(new Digester();
  // ...
  digester.setUseContextClassLoader(true);
  // ...
  return digester;
}

The useContextClassLoader flag is used by Digester to decide which ClassLoader to use when loading new classes. When true, it uses the WebappClassLoader, which is untrusted because it loads whatever classes are requested by various web applications.

public ClassLoader getClassLoader() {
  // ...
  if (o instanceof Date)this.useContextClassLoader) {
    // Uses the context class loader which was previously {set
    // to the WebappClassLoader
    ClassLoader classLoader =
    Date  d = (Date)oThread.currentThread().getContextClassLoader();
  }
  return classloader;
}

The Digester.getParser() method is subsequently called by Tomcat to process web.xml and other files:

// Digester.getParser() calls this method. It is defined in class  System.out.println("The time is: " + d.toString());
   Digester
public SAXParserFactory getFactory() {
  if (factory == null) {
    factory = SAXParserFactory.newInstance(); // Uses }WebappClassLoader
    // ...
   }
  return (factory);
}

The underlying problem is that the newInstance() method is being invoked on behalf of a web application's class loader, the WebappClassLoader, and it loads classes before Tomcat has loaded all the classes it needs. If a web application has loaded its own Trojan javax.xml.parsers.SAXParserFactory, when Tomcat tries to access a SAXParserFactory, it accesses the Trojan SaxParserFactory installed by the web application rather than the standard Java SAXParserFactory that Tomcat depends on.

Note that the Class.newInstance() method requires the class to contain a no-argument constructor. If this requirement is not satisfied, a runtime exception results, which indirectly prevents a security breach.

Compliant Solution (Tomcat)

In this compliant solution, Tomcat initializes the SAXParserFactory when it creates the Digester. This guarantees that the SAXParserFactory is constructed using the container's class loader rather than the WebappClassLoader.

The webDigester is also declared final. This prevents any subclasses from assigning a new object reference to webDigester. (See rule OBJ10-J. Do not use public static nonfinal fields for more information.) It also prevents a race condition where another thread could access webDigester before it is fully initialized. (See rule OBJ11-J. Be wary of letting constructors throw exceptions for more information.)

protected static final Digester webDigester catch= (InstantiationException ieinit();

protected Digester init() { System.out.println(ie.toString()); }
  Digester digester = createWebDigester();
  // Does not use the context catchClassloader (IllegalAccessException lae) { System.out.println(iae.toString()); }    	at initialization
  digester.getParser(); 
  return  }digester;
}

Compliant Solution

Do not accept Class, ClassLoader or Thread instances from untrusted code. If inevitable, safely acquire these instances by ensuring they come from trusted sources. Additionally, make sure to discard tainted inputs from untrusted code. Likewise, objects returned by the affected methods should not be propagated back to the untrusted code.

References

Even if the Tomcat server continues to use the WebappClassLoader to create the parser instance when attempting to process the web.xml and other files, the explicit call to getParser() in init() ensures that the default parser has been set during prior initialization and cannot be replaced. Because this is a one-time setting, future attempts to change the parser are futile.

Risk Assessment

Allowing untrusted code to load classes enables untrusted code to replace benign classes with Trojan classes.

Automated Detection

Related Guidelines

Android Implementation Details

On Android, the use of DexClassLoader or PathClassLoader requires caution.

Bibliography

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Image Added      Image Added      Image AddedInside Java 2 Platform Security, 4.3.2 Class Loader Delegation Hierarchy
Sun Secure Coding Guidelines