Missing Default Case in Multiple Condition Expression
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.
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.
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:
Instead a default label should be used for unaccounted conditions:
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.
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.
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.
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.
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.
The recommended approach is to add a default case that captures any unexpected result conditions and properly handles them.
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").
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.
Weaknesses in this category are related to comparison.
Weaknesses in this category are related to the CISQ Quality Measures for Maintainability. Presence of these weaknesses could reduce the maintainability of the software.
Weaknesses in this category are related to coding practices that are deemed unsafe and increase the chances that an exploitable vulnerability will be present in the ap...
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