Base weakness

Draft

CWE-478: 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.

Recorded CVEs

With this primary weakness

KEVs

In the CISA KEV catalog

Not rated

Likelihood of exploit

Per MITRE

Base

Abstraction

MITRE classification

Overview

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.

Code examples

Illustrative examples from MITRE showing how the weakness appears in code.

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:

Vulnerable example

// program execution continues...
#define FAILED 0

Safe example

#define FAILED 0

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.

Vulnerable example

java

public static final String INTEREST_RATE_AT_ZERO_POINTS = "5.00";

Safe example

java

public static final String INTEREST_RATE_AT_ZERO_POINTS = "5.00";

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.

Vulnerable example

python

result = process_data(data)

Safe example

python

result = process_data(data)

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.

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.

Vulnerable example

javascript

let step = input1 + input2;

Safe example

javascript

let step = input1 + input2;

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").

Vulnerable example

verilog

module fsm_1(out, user_input, clk, rst_n);

Safe example

verilog

case (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.

CWE-478: Missing Default Case in Multiple Condition Expression

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Terminology & mappings

Mapped taxonomies

Frequently asked questions

What is CWE-478?

What CVEs are caused by CWE-478?

How do you prevent CWE-478?

How is CWE-478 detected?

What are the consequences of CWE-478?

Is CWE-478 actively exploited?

References

Stay ahead of CWE-478

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Related weaknesses

Parent

Terminology & mappings

Mapped taxonomies

Frequently asked questions

What is CWE-478?

What CVEs are caused by CWE-478?

How do you prevent CWE-478?

How is CWE-478 detected?

What are the consequences of CWE-478?

Is CWE-478 actively exploited?

References

Stay ahead of CWE-478