EXC32-J. Catch specific exceptions as opposed to the more general RuntimeException

It is highly unlikely that a method is built to deal with all possible runtime exceptions. Consequently, no method should ever catch RuntimeException. If a method catches RuntimeException, it may receive exceptions it was not designed to handle, such as NullPointerException. Many catch clauses simply log or ignore their error, and execution resumes. Runtime exceptions indicate bugs in the program that should be fixed by the developer. They almost always lead to control flow vulnerabilities.

Likewise, a method should never catch Exception or Throwable, because this implies catching RuntimeException.

Noncompliant Code Example

The following function takes a String and returns true if it consists of a capital letter succeeded by lowercase letters. To handle corner cases, it merely wraps the code in a try-catch block and reports any exceptions that arise.

boolean isCapitalized(String s) {
  try {
    if (s.equals("")) {
      return true;
    }
    String first = s.substring( 0, 1);
    String rest = s.substring( 1);
    return (first.equals (first.toUpperCase()) &&
	    rest.equals (rest.toLowerCase()));
  } catch (RuntimeException exception) {
    ExceptionReporter.report (exception);
  }
  return false;
}

This code reports errors if s is a null pointer. However, it also unintentionally catches other errors that are unlikely to be handled properly, such as if the string belongs to a different thread.

Compliant Solution

Instead of catching RuntimeException, a program should be as specific in catching exceptions as possible.

boolean isCapitalized(String s) {
  try {
    if (s.equals("")) {
      return true;
    }
    String first = s.substring( 0, 1);
    String rest = s.substring( 1);
    return (first.equals (first.toUpperCase()) &&
	    rest.equals (rest.toLowerCase()));
  } catch (NullPointerException exception) {
    ExceptionReporter.report (exception);
  }
  return false;
}

This code only catches those exceptions that are intended to be caught. For example, a concurrency based exception is not caught.

Noncompliant Code Example

In this noncompliant code example, a divide by zero exception is handled initially. Instead of the specific exception type ArithmeticException, a more generic type Exception is caught. This is dangerous as any future exception declaration updates to the method signature (such as, addition of IOException) may no longer require the developer to provide a handler. Consequently, the recovery process may not be tailored to the specific exception type that is thrown. Additionally, unchecked exceptions under RuntimeException are also unintentionally caught whenever the top level Exception class is caught.

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 example improves by catching a specific divide-by-zero exception but fails on the premise that it unscrupulously accepts other undesirable runtime exceptions, by catching Exception.

try {
  division(200,5);
  division(200,0); //divide by zero        
} catch (ArithmeticException ae) { throw new DivideByZeroException(); }
  // DivideByZeroException extends Exception so is checked
  catch (Exception e) { System.out.println("Exception occurred :" + e.getMessage()); }	

Compliant Solution

To be compliant, catching specific exception types is advisable especially when the types differ significantly. Here, Arithmetic Exception and IOException have been unbundled as they belong to very diverse categories.

import java.io.IOException;

public class DivideException {
  public static void main(String[] args) {
    try {
      division(200,5);
      division(200,0); //divide by zero        
    } catch (ArithmeticException ae) { throw new DivideByZeroException(); }
      // DivideByZeroException extends Exception so is checked
      catch (IOException ie) { System.out.println("I/O Exception occurred :" + ie.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);   	
  }
}

Exceptions

EXC32-J-EX1: A secure application must also abide by [EXC01-J. Do not allow exceptions to transmit sensitive information]. To follow this rule, an application might find it necessary to catch all exceptions at some top-level to sanitize (or suppress) them. This is also summarized in the CWE entries, CWE 7 and CWE 388. If exceptions need to be caught, it is better to catch Throwable instead of Exception [[Roubtsov 03]].

Risk Assessment

Catching RuntimeException traps several types of exceptions not intended to be caught. This prevents them from being handled properly.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[MITRE 09]] CWE ID 396 "Declaration of Catch for Generic Exception", CWE ID 7 "J2EE Misconfiguration: Missing Error Handling", CWE ID 537 "Information Leak Through Java Runtime Error Message", CWE ID 536 "Information Leak Through Servlet Runtime Error Message"
[[Schweisguth 03]]
[[JLS 05]] Chapter 11, Exceptions
[[Tutorials 08]] Exceptions
[[Doshi 03]]
[[Muller 02]]

EXC31-J. Handle checked exceptions that can be thrown within a finally block      12. Exceptional Behavior (EXC)      EXC33-J. Throw specific exceptions as opposed to the more general RuntimeException or Exception