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This example demonstrates code that accepts a file name as an input argument on which operations are to be performed. An attacker can gain insights on the underlying file system structure by repeatedly passing different paths to fictitious files. When a file is not found, the FileInputStream constructor throws a FileNotFoundException. Other risks such as revelation of the user's home directory and as a result the user name also manifest themselves.

This example also violates the condition that user supplied input must never occur in file names or as constituting elements of file paths. This can lead to a brute force attack allowing the attacker to enumerate valid file names on a system by constantly monitoring the inputs that generate a system defined sanitized message.

import java.io.FileInputStream;
import java.io.FileNotFoundException;

class ExceptionExample {
  public static void main(String[] args) throws FileNotFoundException {
    FileInputStream fis = new FileInputStream("c:\\" + args[0]);
  }
}

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To overcome the problem, the exception must be caught while taking special care to sanitize the message before propagating it to the caller. In cases where the exception type itself can reveal too much, consider throwing a different exception altogether (with a different message, or possibly a higher level exception, referred to as exception translation). The MyExceptionReporter class described in EXC05-J. Use a class dedicated to reporting exceptions is a good choice, as exemplified in this compliant solution.

Notice how Throwable is caught instead of specific exceptions. This is a departure from commonly suggested best practices, but is critical in cases where runtime exceptions or errors can reveal sensitive information. Moreover, this solution overcomes the issue of the brute force attack described earlier by accepting a denumerable set of file name choices with the aid of a switch-case clause. The actual file names and paths are thus shielded from the user of the application.

class ExceptionExample {
  public static void main(String[] args) {
    try {
      FileInputStream fis=null;
      switch(Integer.valueOf(args[0])) {
        case 1: fis = new FileInputStream("c:\\somefolder\\file1"); break;
        case 2: fis = new FileInputStream("c:\\somefolder\\file2" + args[0]);
); break;
        //...
        default: System.out.println("Invalid option"); break;
      }      
    }
    catch(Throwable t) { 
      MyExceptionReporter.report(t); // Sanitizesanitize 
    } 
  }
}

While following this guideline, make sure that security exceptions such as java.security.AccessControlException and java.lang.SecurityException are not swallowed or masked in the process. This can lead to far more pernicious effects such as missed security event log entries. The MyExceptionReporter class prescribes a method to deal with this condition.

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