CWE-597: Use of Wrong Operator in String Comparison
Weakness ID: 597
Vulnerability Mapping:ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction:
VariantVariant - 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.
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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.
Common Consequences
This 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.
Scope
Impact
Likelihood
Other
Technical Impact: Other
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
Relationships
This 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)
Nature
Type
ID
Name
ChildOf
Base - 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.
Variant - 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.
This 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)
Nature
Type
ID
Name
ChildOf
Variant - 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.
The 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.
Phase
Note
Implementation
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";
}
Detection
Methods
Automated Static Analysis
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.)
Effectiveness: High
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.
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
(this CWE ID may be used to map to real-world vulnerabilities)
Reason:
Acceptable-Use
Rationale:
This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Comments:
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
Taxonomy
Mappings
Mapped Taxonomy Name
Node ID
Fit
Mapped Node Name
The CERT Oracle Secure Coding Standard for Java (2011)
EXP03-J
Do not use the equality operators when comparing values of boxed primitives
The CERT Oracle Secure Coding Standard for Java (2011)
EXP03-J
Do not use the equality operators when comparing values of boxed primitives
SEI CERT Perl Coding Standard
EXP35-PL
CWE More Specific
Use the correct operator type for comparing values
Software Fault Patterns
SFP1
Glitch 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.
Description
This 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.
