ERR09-J. Prevent inadvertent calls to System.exit() or forced shutdown

Invocation of System.exit() terminates the JVM, consequently terminating all programs and threads running thereon. This can result in denial-of-service attacks. For example, a call to System.exit() that is embedded in JSP code can cause a web server to terminate, preventing further service for users. Programs must prevent both inadvertent and malicious calls to System.exit(). Additionally, programs should perform necessary clean-up actions when forcibly terminated (via ctrl + c or the kill command, for example).

Noncompliant Code Example

This noncompliant code example uses System.exit() to forcefully shutdown the JVM and terminate the running process. The program lacks a security manager; consequently, it lacks the capability to check whether the caller is permitted to invoke System.exit().

public class InterceptExit {
  public static void main(String[] args) {
    // ...
    System.exit(1);  // Abrupt exit 
    System.out.println("This never executes");
  }
}	

Compliant Solution

This compliant solution installs a custom security manager PasswordSecurityManager that overrides the checkExit() method defined in the SecurityManager class. This override is required to enable invocation of cleanup code before allowing the exit. The default checkExit() method in the SecurityManager class lacks this facility.

class PasswordSecurityManager extends SecurityManager {
  private boolean isExitAllowedFlag; 
  
  public PasswordSecurityManager(){
    super();
    isExitAllowedFlag = false;  
  }
 
  public boolean isExitAllowed(){
    return isExitAllowedFlag;	 
  }
 
  @Override public void checkExit(int status) {
    if(!isExitAllowed()) {
      throw new SecurityException();
    }
    super.checkExit(status);
  }
 
  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.exit(1);  // Abrupt exit call
    } catch (Throwable x) {
      if (x instanceof SecurityException) {
        System.out.println("Intercepted System.exit()");
        // Log exception
      } else {
        // Forward to exception handler
      }
    }

    // ...
    secManager.setExitAllowed(true);  // Permit exit
    // System.exit() will work subsequently
    // ...
  }
}

This implementation uses an internal flag to track whether the exit is permitted. The method setExitAllowed() sets this flag. The checkExit method throws a SecurityException when the flag is unset (e.g., false). Consequently, normal exception processing bypasses the initial call to System.exit(). The program catches the SecurityException and performs mandatory clean-up operations, including logging the exception. The setExitAllowed() method is invoked only after clean-up is complete. Consequently, the program exits gracefully.

Noncompliant Code Example

When a user forcefully exits a program by pressing the ctrl + c key or by using the kill command, the JVM terminates abruptly. Although this event cannot be captured, the program should nevertheless perform any mandatory clean-up operations before exiting. This noncompliant code example fails to do so.

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 never executes");
  }
}

Compliant Solution

Use the addShutdownHook() method of java.lang.Runtime to assist with performing clean-up operations in the event of abrupt termination. The JVM starts the shutdown hook thread when abrupt termination is initiated; the shutdown hook runs concurrently with other JVM threads.

According to the Java API [[API 2006]] 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.

Some precautions must be taken because the JVM is in a sensitive state during shutdown. Shutdown hook threads should:

• be light-weight and simple
• be thread safe
• hold locks when accessing data and release those locks when done

• lack reliance on system services, as the services 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 2006]].

This compliant solution shows the standard method to install a hook.

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

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

The JVM can abort for external reasons, such as an external SIGKILL signal (UNIX) or the TerminateProcess call (Microsoft Windows), or memory corruption caused by native methods. Shutdown hooks may fail to execute as expected in such cases, because the JVM cannot guarantee that they will be executed as intended.

Exceptions

EXC09-EX1: It is permissible for a command line utility to call System.exit() or terminate prematurely; for example, when the required number of arguments are not input [[Bloch 2008]] and [[ESA 2005]].

Risk Assessment

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

Related Vulnerabilities

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

Bibliography

[[API 2006]] method checkExit(), Class Runtime, method addShutdownHook
[[Austin 2000]] Writing a Security Manager
[[Darwin 2004]] 9.5 The Finalize Method
[[ESA 2005]] Rule 78: Restrict the use of the System.exit method
[[Goetz 2006]] 7.4. JVM Shutdown
[[Kalinovsky 2004]] Chapter 16 Intercepting a Call to System.exit
[[MITRE 2009]] CWE ID 382 "J2EE Bad Practices: Use of System.exit()"

ERR07-J. Prevent exceptions while logging data      06. Exceptional Behavior (ERR)      ERR10-J. Do not let code throw undeclared checked exceptions