Page History

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

Non-Compliant Code Example

(DoS) attacks. For example, a call to System.exit() that is embedded in Java Server Pages (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 cleanup actions when forcibly terminated (for example, by using the Windows Task Manager, POSIX kill command, or other mechanisms).

Noncompliant Code Example

This noncompliant code example uses This non-compliant example calls System.exit() aiming to forcefully shutdown shut down the JVM and terminate the running process. There is no The program lacks a security manager check which is highly inadvisable; 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..out.println("Regular code block");
    System.exit(1);  //abrupt Abrupt exit call
    System.out.println("This is never executedexecutes");
  }
}	

Compliant Solution

The This compliant solution installs a custom security manager PasswordSecurityManager that overrides the checkExist checkExit() method defined in the 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 thus exits gracefully.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.

public class PasswordSecurityManager extends SecurityManager {
  private boolean flagisExitAllowedFlag; 
  
  public PasswordSecurityManager(){
    super();
    flagisExitAllowedFlag = false;  
  }
 
  public boolean isExitAllowed(){
   if(flag == true)
     return trueisExitAllowedFlag;	 
  }
 else
     return false;	 
 }@Override 
 
 public void checkExit(int status) {
    if (!isExitAllowed()) {
      throw new SecurityException();
    }
 
 public void setExitAllowed(boolean f) {super.checkExit(status);
   if(f == true)}
 
  public void setExitAllowed(boolean flag = true;
   elsef) {
     flagisExitAllowedFlag = falsef; 	 
  }
}

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 Abrupt exit call
    }
    catch (Throwable x) {
      if (x instanceof SecurityException) {
        System.out.println("Intercepted System.exit()");
        // Log exception
      } else {
        // Forward to exception handler
  x.printStackTrace();    }
    }

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

Non-Compliant 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. Since the JVM is in a sensitive state, some precautions must be taken:

• Hook threads should be light-weight and simple
• They should be thread safe
• They should not rely on system services as they themselves may be shutting down

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
  }
}

References

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 (that is, false). Because this flag is not initially set, normal exception processing bypasses the initial call to System.exit(). The program catches the SecurityException and performs mandatory cleanup operations, including logging the exception. The System.exit() method is enabled only after cleanup is complete.

Exceptions

ERR09-J-EX0: It is permissible for a command-line utility to call System.exit(), for example, when the required number of arguments are not input [Bloch 2008], [ESA 2005].

Risk Assessment

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

Automated Detection

Related Guidelines

Android Implementation Details

On Android, System.exit() should not be used because it will terminate the virtual machine abruptly, ignoring the activity life cycle, which may prevent proper garbage collection.

Bibliography

...

Image Added Image Added Image AddedCovert Java, Chapter 16 Intercepting a Call to System.exit
Java Documentation http://java.sun.com/j2se/1.4.2/docs/api/java/lang/SecurityManager.html#checkExit(int^{Image Removed})
Custom security managers, http://java.sun.com/developer/onlineTraining/Programming/JDCBook/signed2.html^{Image Removed}