Missing Default Case in Multiple Condition Expression

Description

If a multiple-condition expression (such as a switch in C) omits the default case but does not consider or handle all possible values that could occur, then this might lead to complex logical errors and resultant weaknesses. Because of this, further decisions are made based on poor information, and cascading failure results. This cascading failure may result in any number of security issues, and constitutes a significant failure in the system.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following does not properly check the return code in the case where the security_check function returns a -1 value when an error occurs. If an attacker can supply data that will invoke an error, the attacker can bypass the security check:

#define FAILED 0
#define PASSED 1
int result;
...
result = security_check(data);
switch (result) {

  case FAILED:
    printf("Security check failed!\n");
    exit(-1);
    //Break never reached because of exit()
    break;

  case PASSED:
    printf("Security check passed.\n");
    break;


}
// program execution continues...
...

Instead a default label should be used for unaccounted conditions:

#define FAILED 0
#define PASSED 1
int result;
...
result = security_check(data);
switch (result) {

  case FAILED:
    printf("Security check failed!\n");
    exit(-1);
    //Break never reached because of exit()
    break;

  case PASSED:
    printf("Security check passed.\n");
    break;

  default:
    printf("Unknown error (%d), exiting...\n",result);
    exit(-1);


}

This label is used because the assumption cannot be made that all possible cases are accounted for. A good practice is to reserve the default case for error handling.

Example Two

In the following Java example the method getInterestRate retrieves the interest rate for the number of points for a mortgage. The number of points is provided within the input parameter and a switch statement will set the interest rate value to be returned based on the number of points.

public static final String INTEREST_RATE_AT_ZERO_POINTS = "5.00";
public static final String INTEREST_RATE_AT_ONE_POINTS = "4.75";
public static final String INTEREST_RATE_AT_TWO_POINTS = "4.50";
...
public BigDecimal getInterestRate(int points) {

  BigDecimal result = new BigDecimal(INTEREST_RATE_AT_ZERO_POINTS);

  switch (points) {

    case 0:
      result = new BigDecimal(INTEREST_RATE_AT_ZERO_POINTS);
      break;

    case 1:
      result = new BigDecimal(INTEREST_RATE_AT_ONE_POINTS);
      break;

    case 2:
      result = new BigDecimal(INTEREST_RATE_AT_TWO_POINTS);
      break;


  }
  return result;

}

However, this code assumes that the value of the points input parameter will always be 0, 1 or 2 and does not check for other incorrect values passed to the method. This can be easily accomplished by providing a default label in the switch statement that outputs an error message indicating an invalid value for the points input parameter and returning a null value.

public static final String INTEREST_RATE_AT_ZERO_POINTS = "5.00";
public static final String INTEREST_RATE_AT_ONE_POINTS = "4.75";
public static final String INTEREST_RATE_AT_TWO_POINTS = "4.50";
...
public BigDecimal getInterestRate(int points) {

  BigDecimal result = new BigDecimal(INTEREST_RATE_AT_ZERO_POINTS);

  switch (points) {

    case 0:
      result = new BigDecimal(INTEREST_RATE_AT_ZERO_POINTS);
      break;

    case 1:
      result = new BigDecimal(INTEREST_RATE_AT_ONE_POINTS);
      break;

    case 2:
      result = new BigDecimal(INTEREST_RATE_AT_TWO_POINTS);
      break;

    default:
      System.err.println("Invalid value for points, must be 0, 1 or 2");
      System.err.println("Returning null value for interest rate");
      result = null;


  }

  return result;

}

Example Three

In the following Python example the match-case statements (available in Python version 3.10 and later) perform actions based on the result of the process_data() function. The expected return is either 0 or 1. However, if an unexpected result (e.g., -1 or 2) is obtained then no actions will be taken potentially leading to an unexpected program state.

result = process_data(data)
match result:
  case 0:
    print("Properly handle zero case.")

  case 1:
    print("Properly handle one case.")


# program execution continues...

The recommended approach is to add a default case that captures any unexpected result conditions, regardless of how improbable these unexpected conditions might be, and properly handles them.

result = process_data(data)
match result:
  case 0:
    print("Properly handle zero case.")

  case 1:
    print("Properly handle one case.")

  case _:
    print("Properly handle unexpected condition.")


# program execution continues...

Example Four

In the following JavaScript example the switch-case statements (available in JavaScript version 1.2 and later) are used to process a given step based on the result of a calcuation involving two inputs. The expected return is either 1, 2, or 3. However, if an unexpected result (e.g., 4) is obtained then no action will be taken potentially leading to an unexpected program state.

let step = input1 + input2;
switch(step) {
  case 1:
    alert("Process step 1.");
    break;

  case 2:
    alert("Process step 2.");
    break;

  case 3:
    alert("Process step 3.");
    break;


}
// program execution continues...

The recommended approach is to add a default case that captures any unexpected result conditions and properly handles them.

let step = input1 + input2;
switch(step) {
  case 1:
    alert("Process step 1.");
    break;

  case 2:
    alert("Process step 2.");
    break;

  case 3:
    alert("Process step 3.");
    break;

  default:
    alert("Unexpected step encountered.");


}
// program execution continues...

Example Five

The Finite State Machine (FSM) shown in the "bad" code snippet below assigns the output ("out") based on the value of state, which is determined based on the user provided input ("user_input").

module fsm_1(out, user_input, clk, rst_n);
input [2:0] user_input;
input clk, rst_n;
output reg [2:0] out;
reg [1:0] state;
always @ (posedge clk or negedge rst_n )

  begin

    if (!rst_n)

      state = 3'h0;
    else
    case (user_input)
      3'h0:
      3'h1:
      3'h2:
      3'h3: state = 2'h3;
      3'h4: state = 2'h2;
      3'h5: state = 2'h1;

    endcase
  end
  out <= {1'h1, state};

endmodule

case (user_input)

  3'h0:
  3'h1:
  3'h2:
  3'h3: state = 2'h3;
  3'h4: state = 2'h2;
  3'h5: state = 2'h1;
  default: state = 2'h0;
endcase

See Also