EXC04-J. Prevent against inadvertent calls to System.exit() or forced shutdown

When System.exit() is invoked, all programs and threads running on the JVM terminate. This can lead to denial-of-service attacks, for example, a web server can stop servicing users on encountering an untimely exit.

Noncompliant Code Example

This noncompliant example calls System.exit() aiming to forcefully shutdown the JVM and terminate the running process. No security manager checks have been installed to check whether the program has sufficient permissions to exit.

public class InterceptExit {
  public static void main(String[] args) {
    System.out.println("Regular code block");
    System.exit(1);  //abrupt exit call
    System.out.println("This is never executed");
  }
}	

Compliant Solution

The compliant solution installs a custom security manager PasswordSecurityManager that overrides the checkExist method defined in SecurityManager class. An internal flag is used to keep track of whether the exit is permitted or not. The method setExitAllowed is used to set this flag to true. If the flag is false, a SecurityException is thrown. The System.exit call is not permitted to execute by catching the SecurityException in a try-catch block. After intercepting and performing mandatory clean-up operations, the setExitAllowed method is invoked. The program as a result exits gracefully.

class PasswordSecurityManager extends SecurityManager{
  private boolean isExitAllowedFlag; 
  
  public PasswordSecurityManager(){
    super();
    isExitAllowedFlag = false;  
  }
 
 public boolean isExitAllowed(){
   return isExitAllowedFlag;	 
 }
 
 public void checkExit() {
   if(!isExitAllowed())
     throw new SecurityException();
   }
 
 public void setExitAllowed(boolean f) {
   isExitAllowedFlag = f; 	 
 }
}

public class InterceptExit {
  public static void main(String[] args) {
    PasswordSecurityManager secManager = new PasswordSecurityManager();
    System.setSecurityManager(secManager);
    try {
      System.out.println("Regular code block");
      System.exit(1);  //abrupt exit call
    }
    catch (Throwable x) {
      if (x instanceof SecurityException)
        System.out.println("Intercepted System.exit()");
      else
        x.printStackTrace();
    }

    System.out.println("Executing code block...");
    secManager.setExitAllowed(true);  //permit exit
    System.out.println("Finished block, exiting...");  //exit finally 
  }
}

Noncompliant Code Example

If a user forcefully exits a program by pressing the ctrl + c key or uses the kill command, the JVM terminates abruptly. Although this event cannot be captured, the code should be able to react to it. This is missing in this non compliant code example.

public class InterceptExit {
  public static void main(String[] args) {
    System.out.println("Regular code block");
    //abrupt exit such as ctrl + c key pressed
    System.out.println("This is never executed");
  }
}

Compliant Solution

The addShutdownHook method of java.lang.Runtime helps to perform clean-up operations in the unusual termination scenario. When the shutdown is initiated, the hook thread starts to run concurrently with other JVM threads.

According to [[API 06]] Class Runtime, method addShutdownHook:

A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine. Once the shutdown sequence has begun it is impossible to register a new shutdown hook or de-register a previously-registered hook.

Since the JVM is in a sensitive state during this stage, some precautions must be taken:

• Hook threads should be light-weight and simple
• They should be thread safe
• They should hold locks while accessing data

• They should not rely on system services as they themselves may be shutting down (for example, the logger may shutdown from another hook). Instead of one service it may be better to run a series of shutdown tasks from one thread by using a single shutdown hook. [[Goetz 06]]

This compliant solution shows the method to install a hook.

public class Hook {
  public static void main(String[] args) {
  Runtime.getRuntime().addShutdownHook(new Thread() {
  public void run() {
    hookShutdown();
  }
  });
		
  //other code
  }

  public static void hookShutdown() {
    // Log shutdown and close all resources
  }
}

It is still possibly for the JVM to abort due to external issues, such as an external SIGKILL signal (UNIX) or the TerminateProcess call (Microsoft Windows), or memory corruption caused by native methods. In such cases, it is not guaranteed that the hooks will execute as expected.

Exceptions

EX1: It is permissible for a command line utility to call System.exit() or terminate prematurely. [[Bloch 08]] and [[ESA 05]]

Risk Assessment

Allowing inadvertent calls to System.exit() may lead to denial-of-service attacks.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] method checkExit(), Class Runtime, method addShutdownHook
[[Kalinovsky 04]] Chapter 16 Intercepting a Call to System.exit
[[Austin 00]] Writing a Security Manager
[[Darwin 04]] 9.5 The Finalize Method
[[Goetz 06]] 7.4. JVM Shutdown
[[ESA 05]] Rule 78: Restrict the use of the System.exit method
[[MITRE 09]] CWE ID 382 "J2EE Bad Practices: Use of System.exit()"

EXC03-J. Try to recover gracefully from system errors      11. Exceptional Behavior (EXC)      EXC05-J. Use a class dedicated to reporting exceptions