SEC53-JG. Do not expose methods that use the immediate caller's class loader instance

Class loaders allow a Java application to be dynamically extended at runtime by loading 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 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. A class loader that loads untrusted code must be prevented from interacting with trusted code that invokes any of the methods from the following table:

In practice, the trusted code's class loader frequently allows these methods to be invoked whereas 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.

A problem arises because the class loader delegation model is fundamental to many Java implementations and frameworks. So the best advice is to avoid exposing the methods listed above to untrusted code.

Consider, for example, an attack scenario where untrusted code is attempting to load a privileged class. If its class loader is permitted to delegate the class loading to a trusted class's class loader, privilege escalation can occur, because the untrusted code's class loader may lack permission to load the requested privileged class on its own. Furthermore, if the trusted code accepts tainted inputs, the trusted code's class loader could load additional privileged — or even malicious — classes on behalf of the untrusted code.

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

A security-sensitive class loader typically employs the security manager to enforce a security policy before loading new classes. For example, the applet class loader ensures that an applet cannot directly invoke methods of classes present in the com.sun.* package. A 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). A security manager complements, rather than superseding, the security offered by the class loader architecture. Consequently, APIs that perform security manager checks may still violate this guideline at the class loader level when exposed to untrusted callers.

With the exception of the loadLibrary() and load() methods, the tabulated methods fail to perform any security manager checks. The loadLibrary() and load() APIs are typically used from within a doPrivileged block; in that case, unprivileged callers can directly invoke them without requiring any special permissions. Consequently, the security manager checks are curtailed at the immediate caller. The security manager fails to examine the entire call stack and so fails to enhance security. Accepting tainted inputs from untrusted code and allowing them to be used by these APIs may expose vulnerabilities.

This guideline is an instance of 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 (CVE-2013-0422)

Java 1.7.0u10 was 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 a Class object. This method delegated its work to the loadClass() method, whose source code is shown:

/**
 * 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. The forName() method only checks the immediate caller and uses its trusted state to return a Class object of a potentially sensitive class.

Compliant Solution (CVE-2013-0422)

Oracle mitigated this vulnerability in Java 1.7.0p11 by adding an access check to the 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)
// ...

Noncompliant Code Example

In this noncompliant code example a call to System.loadLibrary() is embedded in a doPrivileged block. This is insecure because a library can be loaded on behalf of untrusted code. In essence, the untrusted code's class loader may be able to indirectly load a library even though it lacks sufficient permissions to do so directly. After loading the library, untrusted code can call native methods from the library if those methods are accessible. This is possible because the doPrivileged block stops security manager checks being applied to callers further up the execution chain.

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

Nonnative library code can also be susceptible to related security flaws. Loading a nonnative safe library may not directly expose a vulnerability. However, after loading an additional unsafe library, an attacker can easily exploit the safe library if it contains other vulnerabilities. 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 method load() 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

A method that passes untrusted inputs to the Class.forName() method might permit an attacker to access classes with escalated privileges. The single argument Class.forName() method is another API method that uses its immediate caller's class loader to load a requested class. Untrusted code can misuse this API to indirectly manufacture classes that have the same privileges as those of the attacker's immediate caller.

public Class loadClass(String className) {
  // className may be the name of a privileged or even a malicious class
  return Class.forName(className);
}

Compliant Solution

This compliant solution hard-codes the class's name.

public Class loadClass() {
  return Class.forName("Foo");
}

Noncompliant Code Example

This noncompliant code example returns an instance of java.sql.Connection from trusted to untrusted code. Untrusted code that lacks the permissions required to create a SQL connection can bypass these restrictions by using the acquired instance directly.

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

Compliant Solution

The getConnection() method is unsafe because it uses the url to indicate a class to be loaded; this class serves as the database driver. This compliant solution prevents a malicious user 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);
}

Applicability

Allowing untrusted code to carry out actions using the immediate caller's class loader may allow the untrusted code to execute with the same privileges as the immediate caller.

It is permissible to use APIs that do not use the immediate caller's class loader instance. For example, the three-argument java.lang.Class.forName() method requires an explicit argument that specifies the class loader instance to use. Do not use the immediate caller's class loader as the third argument when instances must be returned to untrusted code.

public static Class forName(String name, boolean initialize,  ClassLoader loader) /* explicitly specify the class loader to use */ throws ClassNotFoundException

Risk Assessment

Related Guidelines

Bibliography

[API 2011] Class ClassLoader

Oracle Security Alert for CVE-2013-0422
Manion, Art, Anatomy of Java Exploits, CERT/CC Blog, January 15, 2013 2:00 PM