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Likewise, programs must not catch RuntimeException or its ancestors Exception or Throwable. Few, if any, methods are capable of handling all possible runtime exceptions. When a method catches RuntimeException, it may receive exceptions unanticipated by the designer, including NullPointerException and ArrayIndexOutOfBoundsException. Many catch clauses simply log or ignore the enclosed exceptional condition and attempt to resume normal execution; this practice often violates rule ERR00-J. Do not suppress or ignore checked exceptions. Runtime exceptions often indicate bugs in the program that should be fixed by the developer, and often cause control flow vulnerabilities.

Noncompliant Code Example (NullPointerException)

This noncompliant code example defines an isName() method that takes a String argument and returns true if the given string is a valid name. A valid name is defined as two capitalized words separated by one or more spaces. Rather than checking to see whether the given string is null, the method catches NullPointerException and returns false.

boolean isName(String s) {
  try {
    String names[] = s.split(" ");

    if (names.length != 2) {
      return false;
    }
    return (isCapitalized(names[0]) && isCapitalized(names[1]));
  } catch (NullPointerException e) {
    return false;
  }
}

Compliant Solution

This compliant solution explicitly checks the String argument for null rather than catching NullPointerException.

boolean isName(String s) {
  if (s == null) {
    return false;
  }
  String names[] = s.split(" ");
  if (names.length != 2) {
    return false;
  }
  return (isCapitalized(names[0]) && isCapitalized(names[1]));
}

Compliant Solution

This compliant solution performs no null check and permits a NullPointerException to be thrown.

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Omitting the null check means that the program fails more quickly than if the program had returned false and lets an invoking method discover the null value. A function that throws a NullPointerException without a null check must provide a precondition that the argument being passed to it is not null.

Noncompliant Code Example (Explicit Null Checks)

This noncompliant code example is derived from the logging service null object design pattern described by Henney \[[Henney 2003|AA. Bibliography#Henney 03]\]. The logging service is composed of two classes: one that prints the triggering activity's details to a disk file using the {{FileLog}} class, and another that prints to the console using the {{ConsoleLog}} class. An interface, {{Log}}, defines a {{write()}} method that is implemented by the respective log classes. Method selection occurs polymorphically at runtime.  The logging infrastructure is subsequently used by a {{Service}} class.

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This design choice suppresses genuine occurrences of NullPointerException. It also violates the design principle that exceptions should be used only for exceptional conditions; ignoring a null Log object is part of ordinary operation for a server.

Compliant Solution (Null Object Pattern)

The null object design pattern provides an alternative to the use of explicit null checks in code. It reduces the need for explicit null checks through the use of an explicit, safe null object rather than a null reference.

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Code that uses this pattern must be clearly documented to ensure that security-critical messages are never discarded because the pattern has been misapplied.

Noncompliant Code Example (Division)

In this noncompliant code example, the original version of the division() method was declared to throw only ArithmeticException. However, the caller catches a more general type (Exception) to report arithmetic problems rather than catching the specific exception type (ArithmeticException). This practice is insecure because future changes to the method signature could add to the list of potential exceptions the caller must handle. In this example, a newer version of the division method can potentially throw IOException in addition to ArithmeticException. However, the compiler cannot tell the caller's developer that he must provide a corresponding handler because his existing code already catches IOException as a result of catching Exception. Consequently, the recovery process may be inappropriate for the specific exception type that is thrown. Furthermore, the developer has failed to anticipate that catching Exception also catches unchecked exceptions.

public class DivideException {
  public static void main(String[] args) {
    try {
      division(200, 5);
      division(200, 0); // Divide by zero
    } catch (Exception e) {
      System.out.println("Divide by zero exception : " + e.getMessage());
    }
  }

  public static void division(int totalSum, int totalNumber) throws ArithmeticException, IOException  {
    int average  = totalSum / totalNumber;
    // Additional operations that may throw IOException...
    System.out.println("Average: " + average);
  }
}

Noncompliant Code Example

This noncompliant code example attempts improvement by specifically catching ArithmeticException. However, it continues to catch Exception and consequently catches both unanticipated checked exceptions and also unanticipated runtime exceptions.

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Note that DivideByZeroException is a custom exception type that extends Exception.

Compliant Solution

This compliant solution catches only the specific anticipated exceptions (ArithmeticException and IOException). All other exceptions are permitted to propagate up the call stack.

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The ExceptionReporter class is documented in ERR00-J. Do not suppress or ignore checked exceptions.

Compliant Solution (Java 1.7)

Java 1.7 allows a single catch block to catch multiple exceptions. This allows one catch block to handle exceptions of different types, which prevents redundant code. This compliant solution catches the specific anticipated exceptions (ArithmeticException and IOException) and handles them with one catch clause. All other exceptions are permitted to propagate to the next nearest dynamically enclosing catch clause of a try statement.

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Note that DivideByZeroException is a custom exception type that extends Exception.

Exceptions

EXC14-EX0: A catch block may catch all exceptions to process them before rethrowing them (filtering sensitive information from exceptions before the call stack leaves a trust boundary, for example). Refer to ERR06-J. Do not allow exceptions to expose sensitive information, as well as CWE 7 and CWE 388. In such cases, a catch block should catch Throwable rather than Exception or RuntimeException.

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• A real time control system that catches and logs all exceptions at the outermost layer, followed by warm-starting the system so that real time control can continue. Such approaches are clearly justified when program termination would have safety-critical or mission-critical consequences.
• A system that catches all exceptions that propagate out of each major subsystem, logs the exceptions for later debugging, and subsequently shuts down the failing subsystem (perhaps replacing it with a much simpler, limited-functionality version) while continuing other services.

Risk Assessment

Catching NullPointerException may mask an underlying null dereference, degrade application performance, and result in code that is hard to understand and maintain. Likewise, catching RuntimeException may unintentionally trap other exception types and prevent them from being handled properly.

Automated Detection

Automated detection of code that catches RuntimeException, Exception, or Throwable is trivial. Sound automated determination of whether such code complies with the exceptions to this rule is infeasible. Heuristic techniques may be helpful.

Related Vulnerabilities

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

Related Guidelines

Bibliography

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