Most methods lack security manager checks because they do not provide access to sensitive parts of the system, such as the file system. Most methods that do provide security manager checks verify that every class and method in the call stack is authorized before they proceed. This security model allows restricted programs, such as Java applets, to have full access to the core Java library. It also prevents a sensitive method from acting on behalf of a malicious method that hides behind trusted methods in the call stack.

However, certain methods use a reduced-security check that checks only that the calling method is authorized rather than checking every method in the call stack. Any code that invokes these methods must guarantee that they cannot be invoked on behalf of untrusted code. These methods are listed in the following table.

Because the java.lang.reflect.Field.setAccessible() and getAccessible() methods are used to instruct the Java Virtual Machine (JVM) to override the language access checks, they perform standard (and more restrictive) security manager checks and consequently lack the vulnerability described by this guideline. Nevertheless, these methods should also be used with extreme caution. The remaining set* and get* field reflection methods perform only the language access checks and are consequently vulnerable.

Class Loaders

Class loaders allow a Java application to be dynamically extended at runtime by loading additional classes. For each class that is loaded, the JVM tracks the class loader that was used to load the class. When a loaded class first refers to another class, the virtual machine requests that the referenced class be loaded by the same class loader that was used to load the referencing class. Java's class loader architecture controls interaction between code loaded from different sources by allowing the use of different class loaders. This separation of class loaders is fundamental to the separation of code: it prevents malicious code from gaining access to and subverting trusted code.

Several methods that are charged with loading classes delegate their work to the class loader of the class of the method that called them. The security checks associated with loading classes are performed by class loaders. Consequently, any method that invokes one of these class loading methods must guarantee that these methods cannot act on behalf of untrusted code. These methods are listed in the following table.

With the exception of the loadLibrary() and load() methods, the tabulated methods do not perform any security manager checks; they delegate security checks to the appropriate class loader. 

In practice, the trusted code's class loader frequently allows these methods to be invoked, whereas the untrusted code's class loader may lack these privileges. However, when the untrusted code's class loader delegates to the trusted code's class loader, the untrusted code gains visibility to the trusted code. In the absence of such a delegation relationship, the class loaders would ensure namespace separation; consequently, the untrusted code would be unable to observe members or to invoke methods belonging to the trusted code.

The class loader delegation model is fundamental to many Java implementations and frameworks. Avoid exposing the methods listed in the preceding tables to untrusted code. Consider, for example, an attack scenario where untrusted code is attempting to load a privileged class. If its class loader lacks permission to load the requested privileged class on its own, but the class loader is permitted to delegate the class loading to a trusted class's class loader, privilege escalation can occur. Furthermore, if the trusted code accepts tainted inputs, the trusted code's class loader could be persuaded to load privileged, malicious classes on behalf of the untrusted code.

Classes that have the same defining class loader will exist in the same namespace, but they can have different privileges depending on the security policy. Security vulnerabilities can arise when privileged code coexists with unprivileged code (or less privileged code) that was loaded by the same class loader. In this case, the less privileged code can freely access members of the privileged code according to the privileged code's declared accessibility. When the privileged code uses any of the tabulated APIs, it bypasses security manager checks (with the exception of loadLibrary() and load()).

This guideline is similar to SEC03-J. Do not load trusted classes after allowing untrusted code to load arbitrary classes. Many examples also violate SEC00-J. Do not allow privileged blocks to leak sensitive information across a trust boundary.

Noncompliant Code Example

In this noncompliant code example, a call to System.loadLibrary() is embedded in a doPrivileged block.

public void load(String libName) {
  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() { 
      System.loadLibrary(libName);
      return null; 
    }
  });
}

This code is insecure because it could load a library on behalf of untrusted code. In essence, the untrusted code's class loader may be able to use this code to load a library even though it lacks sufficient permissions to do so directly. After loading the library, the untrusted code can call native methods from the library, if those methods are accessible, because the doPrivileged block stops any security manager checks from being applied to callers further up the execution stack.

Nonnative library code can also be susceptible to related security flaws. Suppose there exists a library that contains a vulnerability that is not directly exposed, perhaps because it lies in an unused method. Loading this library may not directly expose a vulnerability. However, an attacker could then load an additional library that exploits the first library's vulnerability. Moreover, nonnative libraries often use doPrivileged blocks, making them attractive targets.

Compliant Solution

This compliant solution hard codes the name of the library to prevent the possibility of tainted values. It also reduces the accessibility of the load() method from public to private. Consequently, untrusted callers are prohibited from loading the awt library. 

private void load() {
  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() { 
      System.loadLibrary("awt");
      return null; 
    }
  });
}

Noncompliant Code Example

This noncompliant code example returns an instance of java.sql.Connection from trusted to untrusted code.

public Connection getConnection(String url, String username, String password) {
  // ...
  return DriverManager.getConnection(url, username, password);
}

Untrusted code that lacks the permissions required to create a SQL connection can bypass these restrictions by using the acquired instance directly. The getConnection() method is unsafe because it uses the url argument to indicate a class to be loaded; this class serves as the database driver.

Compliant Solution

This compliant solution prevents malicious users from supplying their own URL to the database connection, thereby limiting their ability to load untrusted drivers.

private String url = // Hardwired value

public Connection getConnection(String username, String password) {
  // ...
  return DriverManager.getConnection(this.url, username, password);
}

Noncompliant Code Example (CERT Vulnerability 636312)

CERT Vulnerability Note VU#636312 describes a vulnerability in Java 1.7.0 update 6 that was widely exploited in August 2012. The exploit actually used two vulnerabilities; the other one is described in SEC05-J. Do not use reflection to increase accessibility of classes, methods, or fields.)

The exploit runs as a Java applet. The applet class loader ensures that an applet cannot directly invoke methods of classes present in the com.sun.* package. A normal security manager check ensures that specific actions are allowed or denied depending on the privileges of all of the caller methods on the call stack (the privileges are associated with the code source that encompasses the class).

The first goal of the exploit code was to access the private sun.awt.SunToolkit class. However, invoking class.forName() directly on the name of this class would cause a SecurityException to be thrown. Consequently, the exploit code used the following method to access any class, bypassing the security manager:

private Class GetClass(String paramString)
    throws Throwable
{
    Object arrayOfObject[] = new Object[1];
    arrayOfObject[0] = paramString;
    Expression localExpression = new Expression(Class.class, "forName", arrayOfObject);
    localExpression.execute();
    return (Class)localExpression.getValue();
}

The java.beans.Expression.execute() method delegates its work to the following method:

private Object invokeInternal() throws Exception {
  Object target = getTarget();
  String methodName = getMethodName();

  if (target == null || methodName == null) {
    throw new NullPointerException(
      (target == null ? "target" : "methodName") + 
       " should not be null");
  }

  Object[] arguments = getArguments();
  if (arguments == null) {
    arguments = emptyArray;
  }
  // Class.forName() won't load classes outside
  // of core from a class inside core, so it
  // is handled as a special case.
  if (target == Class.class && methodName.equals("forName")) {
    return ClassFinder.resolveClass((String)arguments[0], this.loader);
  }

// ...

The com.sun.beans.finder.ClassFinder.resolveClass() method delegates its work to the findClass() method:

public static Class<?> findClass(String name) 
   throws ClassNotFoundException {
  try {
    ClassLoader loader = Thread.currentThread().getContextClassLoader();
    if (loader == null) {
      loader = ClassLoader.getSystemClassLoader();
    }
    if (loader != null) {
      return Class.forName(name, false, loader);
    }
  } catch (ClassNotFoundException exception) {
    // Use current class loader instead
  } catch (SecurityException exception) {
    // Use current class loader instead
  }
  return Class.forName(name);
}

Although this method is called in the context of an applet, it uses Class.forName() to obtain the requested class. Class.forName() delegates the search to the calling method's class loader. In this case, the calling class (com.sun.beans.finder.ClassFinder) is part of core Java, so the trusted class loader is used in place of the more restrictive applet class loader, and the trusted class loader loads the class, unaware that it is acting on behalf of malicious code.

Compliant Solution (CVE-2012-4681)

Oracle mitigated this vulnerability in Java 1.7.0 update 7 by patching the com.sun.beans.finder.ClassFinder.findClass() method. The checkPackageAccess() method checks the entire call stack to ensure that Class.forName(), in this instance only, fetches classes only on behalf of trusted methods.

public static Class<?> findClass(String name) 
   throws ClassNotFoundException {
  checkPackageAccess(name);
  try {
    ClassLoader loader = Thread.currentThread().getContextClassLoader();
    if (loader == null) {
      // Can be null in IE (see 6204697)
      loader = ClassLoader.getSystemClassLoader();
    }
    if (loader != null) {
      return Class.forName(name, false, loader);
    }

  } catch (ClassNotFoundException exception) {
    // Use current class loader instead
  } catch (SecurityException exception) {
    // Use current class loader instead
  }
  return Class.forName(name);
}

Noncompliant Code Example (CVE-2013-0422)

Java 1.7.0 update 10 was widely exploited in January 2013 because of several vulnerabilities. One vulnerability in the MBeanInstantiator class granted unprivileged code the ability to access any class regardless of the current security policy or accessibility rules. The MBeanInstantiator.findClass() method could be invoked with any string and would attempt to return the Class object named after the string. This method delegated its work to the loadClass() method, whose source code is shown here:

/**
 * Load a class with the specified loader, or with this object
 * class loader if the specified loader is null.
 **/
static Class<?> loadClass(String className, ClassLoader loader)
    throws ReflectionException {

    Class<?> theClass;
    if (className == null) {
        throw new RuntimeOperationsException(new
            IllegalArgumentException("The class name cannot be null"),
                          "Exception occurred during object instantiation");
    }
    try {
        if (loader == null)
            loader = MBeanInstantiator.class.getClassLoader();
        if (loader != null) {
            theClass = Class.forName(className, false, loader);
        } else {
            theClass = Class.forName(className);
        }
    } catch (ClassNotFoundException e) {
        throw new ReflectionException(e,
        "The MBean class could not be loaded");
    }
    return theClass;
}

This method delegates the task of dynamically loading the specified class to the Class.forName() method, which delegates the work to its calling method's class loader. Because the calling method is MBeanInstantiator.loadClass(), the core class loader is used, which provides no security checks.

Compliant Solution (CVE-2013-0422)

Oracle mitigated this vulnerability in Java 1.7.0 update 11 by adding an access check to the MBeanInstantiator.loadClass() method. This access check ensures that the caller is permitted to access the class being sought:

// ...
    if (className == null) {
        throw new RuntimeOperationsException(new
            IllegalArgumentException("The class name cannot be null"),
                          "Exception occurred during object instantiation");
    }
    ReflectUtil.checkPackageAccess(className);
    try {
        if (loader == null)
// ...

Applicability

Allowing untrusted code to invoke methods with reduced-security checks can result in privilege escalation. Likewise, allowing untrusted code to perform actions using the immediate caller's class loader may allow the untrusted code to execute with the same privileges as the immediate caller.

Methods that avoid using the immediate caller's class loader instance fall outside the scope of this guideline. For example, the three-argument java.lang.Class.forName() method requires an explicit argument that specifies the class loader instance to use.

public static Class forName(String name, boolean initialize,
       ClassLoader loader) throws ClassNotFoundException

Do not use the immediate caller's class loader as the third argument when instances must be returned to untrusted code.

Bibliography