The code does not have a default case in an expression with multiple conditions, such as a switch statement.
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.
Threat Mapped score: 0.0
Industry: Finiancial
Threat priority: Unclassified
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| Phase | Note |
|---|---|
| Implementation | N/A |
Intro: 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:
Body: 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; } // program execution continues... ...Intro: 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.
Body: 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; } return result; }Intro: 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.
Body: 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.") # program execution continues...Intro: 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.
Body: 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; } // program execution continues...Intro: 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").
Body: The case statement does not include a default to handle the scenario when the user provides inputs of 3'h6 and 3'h7. Those inputs push the system to an undefined state and might cause a crash (denial of service) or any other unanticipated outcome. Adding a default statement to handle undefined inputs mitigates this issue. This is shown in the "Good" code snippet below. The default statement is in bold.
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