EXC07-J. Prevent exceptions while logging data

If an exception is thrown while logging is in progress, data may not be logged unless special care is taken. Failing to account for exceptions during the logging process can result in a multitude of security vulnerabilities, such as denial of service or vulnerabilities that allow the attacker to conceal critical security exceptions by preventing them from being logged.

Noncompliant Code Example

This noncompliant code example uses statements that can throw exceptions when logging is in progress. It attempts to log a SecurityException generated within the run() method, however, the original log message is not logged if an exception is thrown while logging is underway.

public class ExceptionLog implements Runnable {
  public void logMessage(String message) {
    FileOutputStream fo = null;
    FileLock lock = null;

    try {
      // This can throw an exception and prevent logging.
      fo = new FileOutputStream("log_file.txt", true); 

      // Lock the file so only one thread can write a log message at a time.
      lock = fo.getChannel().lock();

      // Output the log message.
      System.err.println(message);
      fo.write((message + "\n").getBytes());
    } 

    // If an exception is caught, the original message to log is lost
    catch (FileNotFoundException e){
      logMessage("File Not Found Exception."); 
    }
    catch(IOException e) {
      logMessage("IO Exception."); 
    }
    catch (OverlappingFileLockException e) {
      logMessage("Cannot access file.");
    }
    finally {
      // Clean up by releasing the file lock and closing the file.
      try {
        if (lock != null) {
          lock.release();
        }
        
        if (fo != null) {
          fo.close();
        }
      } catch (IOException e) {
        // This is unexpected.
        throw new RuntimeException(e);
      }
    }
  }

  public void run() {
    try {
      // Some security exception occurs here.
    } catch(SecurityException se) {
        logMessage("Security Exception has occurred!");
    }
  }

  public static void main(String[] args) {
    // Start multiple threads logging messages.
    for (int x = 1; x <= 20; x++) {
      (new Thread(new ExceptionLog())).start();
    }
  }    
}

This noncompliant code example throws an exception if there is a problem with the application's file system or if a thread attempts to write to the log file when the file is locked by another thread. An attacker can exploit these problems by:

Gaining access to the application's file system and deleting or changing the permissions of the log file so that a runtime exception results. While this appears to be inconceivable, it is possible that an attacker has access to the physical media from which the application reads data. If the application's code depends on such functionality, an attacker may induce runtime exceptions at the appropriate moment to circumvent logging. This vulnerability can particularly manifest in multi-threaded applications.
Exploiting the multi-threaded nature of the application by providing input to the application via normal input channels so that log messages are lost as a result of an OverlappingFileLockException. Because this attack vector makes use of standard input channels to perform the attack, it is much simpler to implement than the previous attack, which requires access to the application's file system.

Compliant Solution

This compliant solution executes several statements that can possibly throw exceptions prior to performing any security critical operations and uses the thread and exception safe java.util.logging.Logger class to implement logging. (See guideline EXC03-J. Use a logging API to log critical security exceptions for more information on the use of logging libraries.)

public class ExceptionLog implements Runnable {
  Logger logger;
  Integer id;

  public ExceptionLog(Integer i, Logger log) {
    logger = log;
    id = i;
  }

  public void logMessage(String message) {
    // Note that the Java Logger class does not throw exceptions
    // while logging a message.
    logger.log(Level.WARNING, "From " + id + ": " + message);
  }

  public void run() {
    try {
      // Some security exception occurs here.
    } catch(SecurityException se) {
        logMessage("Security Exception has occurred!");
    }
  }

  public static void main(String[] args) {
    try {
      // Set up the shared logger for use by the multiple threads
      Logger logger = Logger.getLogger("MyLog");
      FileHandler fh = new FileHandler("log_file.txt", true);
      logger.addHandler(fh);
      logger.setLevel(Level.ALL);
      SimpleFormatter formatter = new SimpleFormatter();
      fh.setFormatter(formatter);

      // Start multiple threads for logging messages
      for (int x = 1; x <= 20; x++) {
        (new Thread(new ExceptionLog(x, logger))).start();
      }
    } catch (SecurityException e) {
        // This is unexpected.
        throw new RuntimeException(e);
    } catch (IOException e) {
        // This is unexpected.
        throw new RuntimeException(e);
    }
  }    
}

As a result, exceptions do not result in failure to log a message or logging a different message than the intended one. While this is a stringent requirement, it is necessary in cases where an exception can be deliberately thrown to conceal an attacker's tracks. The logging mechanism must be robust and should be able to detect and handle all such cases. A slightly more expensive alternative is to support recursive logging.

While in this compliant solution an IOException is still possible, there is little that can be done when writing the data to the log file if the existence of the file itself is under question. Consequently, this recommendation does not prevent a program from reopening a closed log file. If an IOException results when doing so, it is likely that the log file was not properly protected using operating system level permissions.