Expression is Always False

Draft Base

Structure: Simple

Description

This vulnerability occurs when code contains a conditional expression that can never evaluate to true, causing a section of the program to be permanently unreachable.

Extended Description

An 'always false' expression is a type of dead code that often results from logical errors, incorrect assumptions about data, or leftover debugging checks. For example, comparing a variable against a fixed value it can never hold, or checking a condition after it has already been definitively ruled out earlier in the logic. This creates a maintenance hazard, as developers might waste time debugging non-functional code, and it can mask deeper logical flaws that affect the program's real behavior. While the unreachable code itself might not directly cause a crash, it indicates flawed reasoning that could lead to security issues elsewhere. It can prevent security-critical validation routines from executing, cause resource leaks if cleanup code is skipped, or create unexpected program flow that attackers could analyze to understand the codebase. Regular code reviews and static analysis tools are effective at catching these expressions before they reach production.

Common Consequences 1

Scope: Other

Impact: Quality DegradationVaries by Context

Detection Methods 1

Automated Static AnalysisHigh

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Potential Mitigations 1

Phase: Testing

Use Static Analysis tools to spot such conditions.

Demonstrative Examples 3

In the following Java example the updateUserAccountOrder() method used within an e-business product ordering/inventory application will validate the product number that was ordered and the user account number. If they are valid, the method will update the product inventory, the user account, and the user order appropriately.

Code Example:Bad
Java
java

However, the method never sets the isValidAccount variable after initializing it to false so the isValidProduct is mistakenly used twice. The result is that the expression "isValidProduct && isValidAccount" will always evaluate to false, so the updateAccountOrder() method will never be invoked. This will create serious problems with the product ordering application since the user account and inventory databases will be updated but the order will not be updated.

This can be easily corrected by updating the appropriate variable.

Code Example:Good
Java
java

In the following example, the hasReadWriteAccess method uses bit masks and bit operators to determine if a user has read and write privileges for a particular process. The variable mask is defined as a bit mask from the BIT_READ and BIT_WRITE constants that have been defined. The variable mask is used within the predicate of the hasReadWriteAccess method to determine if the userMask input parameter has the read and write bits set.

Code Example:Bad
C
c

However the bit operator used to initialize the mask variable is the AND operator rather than the intended OR operator (Use of Incorrect Operator), this resulted in the variable mask being set to 0. As a result, the if statement will always evaluate to false and never get executed.

The use of bit masks, bit operators and bitwise operations on variables can be difficult. If possible, try to use frameworks or libraries that provide appropriate functionality and abstract the implementation.

In the following example, the updateInventory method used within an e-business inventory application will update the inventory for a particular product. This method includes an if statement with an expression that will always evaluate to false. This is a common practice in C/C++ to introduce debugging statements quickly by simply changing the expression to evaluate to true and then removing those debugging statements by changing expression to evaluate to false. This is also a common practice for disabling features no longer needed.

Code Example:Bad
C
c

Using this practice for introducing debugging statements or disabling features creates dead code that can cause problems during code maintenance and potentially introduce vulnerabilities. To avoid using expressions that evaluate to false for debugging purposes a logging API or debugging API should be used for the output of debugging messages.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Improper Adherence to Coding Standards (CWE-710)

CanPrecede:

Dead Code (CWE-561)

Taxonomy Mapping

CERT C Secure Coding
Software Fault Patterns