Common Weakness Enumeration

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CWE-457: Use of Uninitialized Variable

Use of Uninitialized Variable
Weakness ID: 457 (Weakness Variant)Status: Draft
+ Description

Description Summary

The code uses a variable that has not been initialized, leading to unpredictable or unintended results.

Extended Description

In some languages such as C and C++, stack variables are not initialized by default. They generally contain junk data with the contents of stack memory before the function was invoked. An attacker can sometimes control or read these contents. In other languages or conditions, a variable that is not explicitly initialized can be given a default value that has security implications, depending on the logic of the program. The presence of an uninitialized variable can sometimes indicate a typographic error in the code.

+ Time of Introduction
  • Implementation
+ Applicable Platforms


C: (Sometimes)

C++: (Sometimes)

Perl: (Often)

PHP: (Often)


+ Modes of Introduction

In C, using an uninitialized char * in some string libraries will return incorrect results, as the libraries expect the null terminator to always be at the end of a string, even if the string is empty.

+ Common Consequences

Technical Impact: Other

Initial variables usually contain junk, which can not be trusted for consistency. This can lead to denial of service conditions, or modify control flow in unexpected ways. In some cases, an attacker can "pre-initialize" the variable using previous actions, which might enable code execution. This can cause a race condition if a lock variable check passes when it should not.


Technical Impact: Other

Strings that are not initialized are especially dangerous, since many functions expect a null at the end -- and only at the end -- of a string.

+ Likelihood of Exploit


+ Demonstrative Examples

Example 1

This code prints a greeting using information stored in a POST request:

(Bad Code)
Example Language: PHP 
if (isset($_POST['names'])) {
$nameArray = $_POST['names'];
echo "Hello " . $nameArray['first'];

This code checks if the POST array 'names' is set before assigning it to the $nameArray variable. However, if the array is not in the POST request, $nameArray will remain uninitialized. This will cause an error when the array is accessed to print the greeting message, which could lead to further exploit.

Example 2

The following switch statement is intended to set the values of the variables aN and bN before they are used:

(Bad Code)
Example Language:
int aN, Bn;
switch (ctl) {
case -1:
aN = 0;
bN = 0;
case 0:
aN = i;
bN = -i;
case 1:
aN = i + NEXT_SZ;
bN = i - NEXT_SZ;
aN = -1;
aN = -1;
repaint(aN, bN);

In the default case of the switch statement, the programmer has accidentally set the value of aN twice. As a result, bN will have an undefined value. Most uninitialized variable issues result in general software reliability problems, but if attackers can intentionally trigger the use of an uninitialized variable, they might be able to launch a denial of service attack by crashing the program. Under the right circumstances, an attacker may be able to control the value of an uninitialized variable by affecting the values on the stack prior to the invocation of the function.

+ Observed Examples
Uninitialized variable leads to code execution in popular desktop application.
Crafted input triggers dereference of an uninitialized object pointer.
Crafted audio file triggers crash when an uninitialized variable is used.
Uninitialized random seed variable used.
+ Potential Mitigations

Phase: Implementation

Strategy: Identify and Reduce Attack Surface

Assign all variables to an initial value.

Phase: Build and Compilation

Strategy: Compilation or Build Hardening

Most compilers will complain about the use of uninitialized variables if warnings are turned on.

Phases: Implementation; Operation

When using a language that does not require explicit declaration of variables, run or compile the software in a mode that reports undeclared or unknown variables. This may indicate the presence of a typographic error in the variable's name.

Phase: Requirements

The choice could be made to use a language that is not susceptible to these issues.

Phase: Architecture and Design

Mitigating technologies such as safe string libraries and container abstractions could be introduced.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class398Indicator of Poor Code Quality
Seven Pernicious Kingdoms (primary)700
ChildOfWeakness BaseWeakness Base665Improper Initialization
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory998SFP Secondary Cluster: Glitch in Computation
Software Fault Pattern (SFP) Clusters (primary)888
MemberOfViewView630Weaknesses Examined by SAMATE
Weaknesses Examined by SAMATE (primary)630
CanFollowWeakness BaseWeakness Base456Missing Initialization of a Variable
Development Concepts699
Research Concepts1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPUninitialized variable
7 Pernicious KingdomsUninitialized Variable
Software Fault PatternsSFP1Glitch in computation
+ White Box Definitions

A weakness where the code path has:

1. start statement that defines variable

2. end statement that accesses the variable

3. the code path does not contain a statement that assigns value to the variable

+ References
mercy. "Exploiting Uninitialized Data". Jan 2006. <>.
Microsoft Security Vulnerability Research & Defense. "MS08-014 : The Case of the Uninitialized Stack Variable Vulnerability". 2008-03-11. <>.
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 8: C++ Catastrophes." Page 143. McGraw-Hill. 2010.
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 7, "Variable Initialization", Page 312.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submission DateSubmitterOrganizationSource
CLASPExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-08-01KDM AnalyticsExternal
added/updated white box definitions
2008-09-08CWE Content TeamMITREInternal
updated Applicable_Platforms, Common_Consequences, Description, Relationships, Observed_Example, Other_Notes, References, Taxonomy_Mappings
2009-01-12CWE Content TeamMITREInternal
updated Common_Consequences, Demonstrative_Examples, Potential_Mitigations
2009-03-10CWE Content TeamMITREInternal
updated Demonstrative_Examples
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated References, Relationships
2012-10-30CWE Content TeamMITREInternal
updated Demonstrative_Examples
2013-02-21CWE Content TeamMITREInternal
updated Applicable_Platforms, Description, Other_Notes, Potential_Mitigations, Relationships
2014-06-23CWE Content TeamMITREInternal
updated Modes_of_Introduction, Other_Notes
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Uninitialized Variable
Page Last Updated: July 30, 2014