ERR09-J. Restrict calls to System.exit() to trusted code only

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.

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