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Common Weakness Enumeration

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ID

CWE-457: Use of Uninitialized Variable

Weakness ID: 457
Abstraction: Variant
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
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.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfClassClass665Improper Initialization
CanFollowBaseBase456Missing Initialization of a Variable
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
ChildOfClassClass665Improper Initialization
CanFollowBaseBase456Missing Initialization of a Variable
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
ImplementationIn 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.
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

C: (Sometimes Prevalent)

C++: (Sometimes Prevalent)

Perl: (Often Prevalent)

PHP: (Often Prevalent)

(Language-Independent classes): (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Availability
Integrity
Other

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.
Authorization
Other

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
High
+ Demonstrative Examples

Example 1

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

(bad)
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)
Example Language:
int aN, Bn;
switch (ctl) {
case -1:
aN = 0;
bN = 0;
break;

case 0:
aN = i;
bN = -i;
break;

case 1:
aN = i + NEXT_SZ;
bN = i - NEXT_SZ;
break;

default:
aN = -1;
aN = -1;
break;

}
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
ReferenceDescription
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: Attack Surface Reduction

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.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory3987PK - Code Quality
MemberOfCategoryCategory998SFP Secondary Cluster: Glitch in Computation
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPUninitialized variable
7 Pernicious KingdomsUninitialized Variable
Software Fault PatternsSFP1Glitch in computation
CERT Perl Secure CodingDCL33-PLImpreciseDeclare identifiers before using them
+ References
[REF-436] mercy. "Exploiting Uninitialized Data". 2006-01. <http://www.felinemenace.org/~mercy/papers/UBehavior/UBehavior.zip>.
[REF-437] Microsoft Security Vulnerability Research & Defense. "MS08-014 : The Case of the Uninitialized Stack Variable Vulnerability". 2008-03-11. <http://blogs.technet.com/swi/archive/2008/03/11/the-case-of-the-uninitialized-stack-variable-vulnerability.aspx>.
[REF-44] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 8: C++ Catastrophes." Page 143. McGraw-Hill. 2010.
[REF-62] 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
Submissions
Submission DateSubmitterOrganizationSource
CLASP
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-08-01KDM Analytics
added/updated white box definitions
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Common_Consequences, Description, Relationships, Observed_Example, Other_Notes, References, Taxonomy_Mappings
2009-01-12CWE Content TeamMITRE
updated Common_Consequences, Demonstrative_Examples, Potential_Mitigations
2009-03-10CWE Content TeamMITRE
updated Demonstrative_Examples
2009-05-27CWE Content TeamMITRE
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated References, Relationships
2012-10-30CWE Content TeamMITRE
updated Demonstrative_Examples
2013-02-21CWE Content TeamMITRE
updated Applicable_Platforms, Description, Other_Notes, Potential_Mitigations, Relationships
2014-06-23CWE Content TeamMITRE
updated Modes_of_Introduction, Other_Notes
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated References, Relationships, Taxonomy_Mappings, White_Box_Definitions
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Uninitialized Variable

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Page Last Updated: November 14, 2017