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Common Weakness Enumeration

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ID

CWE-597: Use of Wrong Operator in String Comparison

Weakness ID: 597
Abstraction: Variant
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The product uses the wrong operator when comparing a string, such as using "==" when the .equals() method should be used instead.
+ Extended Description
In Java, using == or != to compare two strings for equality actually compares two objects for equality rather than their string values for equality. Chances are good that the two references will never be equal. While this weakness often only affects program correctness, if the equality is used for a security decision, the unintended comparison result could be leveraged to affect program security.
+ Relationships
Section HelpThis table 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
ChildOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.480Use of Incorrect Operator
ChildOfVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.595Comparison of Object References Instead of Object Contents
Section HelpThis table 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 "Software Development" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.133String Errors
Section HelpThis table 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 "CISQ Quality Measures (2020)" (CWE-1305)
NatureTypeIDName
ChildOfVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.595Comparison of Object References Instead of Object Contents
+ Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
PhaseNote
Implementation
+ Common Consequences
Section HelpThis table 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
Other

Technical Impact: Other

+ Demonstrative Examples

Example 1

In the example below, two Java String objects are declared and initialized with the same string values. An if statement is used to determine if the strings are equivalent.

(bad code)
Example Language: Java 
String str1 = new String("Hello");
String str2 = new String("Hello");
if (str1 == str2) {
System.out.println("str1 == str2");
}

However, the if statement will not be executed as the strings are compared using the "==" operator. For Java objects, such as String objects, the "==" operator compares object references, not object values. While the two String objects above contain the same string values, they refer to different object references, so the System.out.println statement will not be executed. To compare object values, the previous code could be modified to use the equals method:

(good code)
 
if (str1.equals(str2)) {
System.out.println("str1 equals str2");
}

Example 2

In the example below, three JavaScript variables are declared and initialized with the same values. Note that JavaScript will change a value between numeric and string as needed, which is the reason an integer is included with the strings. An if statement is used to determine whether the values are the same.

(bad code)
Example Language: JavaScript 
<p id="ieq3s1" type="text">(i === s1) is FALSE</p>
<p id="s4eq3i" type="text">(s4 === i) is FALSE</p>
<p id="s4eq3s1" type="text">(s4 === s1) is FALSE</p>

var i = 65;
var s1 = '65';
var s4 = new String('65');

if (i === s1)
{
document.getElementById("ieq3s1").innerHTML = "(i === s1) is TRUE";
}

if (s4 === i)
{
document.getElementById("s4eq3i").innerHTML = "(s4 === i) is TRUE";
}

if (s4 === s1)
{
document.getElementById("s4eq3s1").innerHTML = "(s4 === s1) is TRUE";
}

However, the body of the if statement will not be executed, as the "===" compares both the type of the variable AND the value. As the types of the first comparison are number and string, it fails. The types in the second are int and reference, so this one fails as well. The types in the third are reference and string, so it also fails.

While the variables above contain the same values, they are contained in different types, so the document.getElementById... statement will not be executed in any of the cases.

To compare object values, the previous code is modified and shown below to use the "==" for value comparison so the comparison in this example executes the HTML statement:

(good code)
Example Language: JavaScript 
<p id="ieq2s1" type="text">(i == s1) is FALSE</p>
<p id="s4eq2i" type="text">(s4 == i) is FALSE</p>
<p id="s4eq2s1" type="text">(s4 == s1) is FALSE</p>

var i = 65;
var s1 = '65';
var s4 = new String('65');

if (i == s1)
{
document.getElementById("ieq2s1").innerHTML = "(i == s1) is TRUE";
}

if (s4 == i)
{
document.getElementById("s4eq2i").innerHTML = "(s4 == i) is TRUE";
}

if (s4 == s1)
{
document.getElementById("s4eq2s1").innerHTML = "(s4 == s1) is TRUE";
}

Example 3

In the example below, two PHP variables are declared and initialized with the same numbers - one as a string, the other as an integer. Note that PHP will change the string value to a number for a comparison. An if statement is used to determine whether the values are the same.

(bad code)
Example Language: PHP 
var $i = 65;
var $s1 = "65";

if ($i === $s1)
{
echo '($i === $s1) is TRUE'. "\n";
}
else
{
echo '($i === $s1) is FALSE'. "\n";
}

However, the body of the if statement will not be executed, as the "===" compares both the type of the variable AND the value. As the types of the first comparison are number and string, it fails.

While the variables above contain the same values, they are contained in different types, so the TRUE portion of the if statement will not be executed.

To compare object values, the previous code is modified and shown below to use the "==" for value comparison (string converted to number) so the comparison in this example executes the TRUE statement:

(good code)
Example Language: PHP 
var $i = 65;
var $s1 = "65";

if ($i == $s1)
{
echo '($i == $s1) is TRUE'. "\n";
}
else
{
echo '($i == $s1) is FALSE'. "\n";
}
+ Potential Mitigations

Phase: Implementation

Within Java, use .equals() to compare string values.

Within JavaScript, use == to compare string values.

Within PHP, use == to compare a numeric value to a string value. (PHP converts the string to a number.)

Effectiveness: High

+ Memberships
Section HelpThis 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
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.847The CERT Oracle Secure Coding Standard for Java (2011) Chapter 4 - Expressions (EXP)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.998SFP Secondary Cluster: Glitch in Computation
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1136SEI CERT Oracle Secure Coding Standard for Java - Guidelines 02. Expressions (EXP)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1181SEI CERT Perl Coding Standard - Guidelines 03. Expressions (EXP)
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
The CERT Oracle Secure Coding Standard for Java (2011)EXP03-JDo not use the equality operators when comparing values of boxed primitives
The CERT Oracle Secure Coding Standard for Java (2011)EXP03-JDo not use the equality operators when comparing values of boxed primitives
SEI CERT Perl Coding StandardEXP35-PLCWE More SpecificUse the correct operator type for comparing values
Software Fault PatternsSFP1Glitch in computation
+ References
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Typos", Page 289. 1st Edition. Addison Wesley. 2006.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2006-12-15CWE Content TeamMITRE
+ Modifications
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Description, Relationships
2008-10-14CWE Content TeamMITRE
updated Relationships
2009-05-27CWE Content TeamMITRE
updated Demonstrative_Examples
2011-03-29CWE Content TeamMITRE
updated Demonstrative_Examples, Description, Potential_Mitigations
2011-06-01CWE Content TeamMITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Taxonomy_Mappings
2019-01-03CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2020-02-24CWE Content TeamMITRE
updated Relationships
2020-08-20CWE Content TeamMITRE
updated Relationships
2021-03-15CWE Content TeamMITRE
updated Demonstrative_Examples, Description, Potential_Mitigations, Relationships
+ Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Erroneous String Compare
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Page Last Updated: July 20, 2021