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The default SecurityManager checks whether the caller of a particular method has sufficient permissions to proceed with an action. An action is a level of access; for instance, the actions for java.io.FilePermission are read, write, execute, and delete [API 2011]. The "Permission Descriptions and Risks" guide [Oracle 2008] enumerates the default permissions and the risks associated with granting these permissions to Java code.

Sometimes, stronger restrictions than those provided by the default security manager are necessary. Custom permissions prove to be more suitable for privilege separation in such cases. Failure to provide custom permissions when no corresponding default permissions exist can lead to privilege escalation vulnerabilities that enable untrusted callers to execute restricted operations or actions.

This guideline addresses the problem of excess privileges. See 16. Avoid granting excess privileges for another approach to solving this problem.

Noncompliant Code Example

This noncompliant code example contains a privileged block that is used to perform two sensitive operations: loading a library and setting the default exception handler. 

class LoadLibrary {
  private void loadLibrary() {
    AccessController.doPrivileged(new PrivilegedAction() {
      public Object run() {
        // Privileged code
        System.loadLibrary("myLib.so");
        // Perform some sensitive operation like setting the default exception handler
        MyExceptionReporter.setExceptionReporter(reporter); 
        return null; 
      }
    });		  
  }
} 

When used, the default security manager forbids the loading of the library unless the RuntimePermission loadLibrary.awt is granted in the policy file. However, the security manager does not automatically guard a caller from performing the second sensitive operation of setting the default exception handler because the permission for this operation is nondefault and consequently unavailable. This security weakness can be exploited, for example, by programming and installing an exception handler that reveals information that a legitimate handler would filter out.

Compliant Solution

This compliant solution defines a custom permission ExceptionReporterPermission with target exc.reporter to prohibit illegitimate callers from setting the default exception handler. This can be achieved by subclassing BasicPermission, which allows binary-style permissions (either allow or disallow). The compliant solution then uses a security manager to check whether the caller has the requisite permission to set the handler. The code throws a SecurityException if the check fails. The custom permission class ExceptionReporterPermission is also defined with the two required constructors.

class LoadLibrary {
  private void loadLibrary() {
    AccessController.doPrivileged(new PrivilegedAction() {
      public Object run() {
        // Privileged code
        System.loadLibrary("myLib.so");

        // Perform some sensitive operation like setting the default exception handler
        MyExceptionReporter.setExceptionReporter(reporter); 
        return null; 
      }
    });		  
  }
}

 
final class MyExceptionReporter extends ExceptionReporter {
  public void setExceptionReporter(ExceptionReporter reporter) {
    SecurityManager sm = System.getSecurityManager();
    if(sm != null) {
      sm.checkPermission(new ExceptionReporterPermission("exc.reporter"));
    }

    // Proceed to set the exception reporter
  }
 
  // ... other methods of MyExceptionReporter
}
 
final class ExceptionReporterPermission extends BasicPermission {
  public ExceptionReporterPermission(String permName) {
    super(permName);
  }

  // Even though the actions parameter is ignored, this constructor has to be defined
  public ExceptionReporterPermission(String permName, String actions) {
    super(permName, actions);
  }
}

Assuming that the preceding sources reside in the c:\package directory on a Windows-based system, for example, the policy file needs to grant two permissions: ExceptionReporterPermission exc.reporter and RuntimePermission loadlibrary.awt.

grant codeBase "file:/c:/package" {  // For *nix, file:${user.home}/package/ 
  permission ExceptionReporterPermission "exc.reporter";
  permission java.lang.RuntimePermission "loadLibrary.awt"; 
};

By default, permissions cannot be defined to support actions using BasicPermission, but the actions can be freely implemented in the subclass ExceptionReporterPermission if required. BasicPermission is abstract even though it contains no abstract methods; it defines all the methods that it extends from the Permission class. The custom-defined subclass of the BasicPermission class has to define two constructors to call the most appropriate (one- or two-argument) superclass constructor (the superclass lacks a default constructor). The two-argument constructor also accepts an action even though a basic permission does not use it. This behavior is required for constructing permission objects from the policy file. Note that the custom-defined subclass of the BasicPermission class is declared to be final in accordance with guideline VOID 15. OBJ56-JG. Either design classes for inheritance or declare them as final.

Applicability

Running Java code without defining custom permissions where default permissions are inapplicable can leave an application open to privilege escalation vulnerabilities.

Bibliography