Use of Wrong Operator in String Comparison

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.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

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.

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:

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

Example Two

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.

<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";
}

<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 Three

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.

var $i = 65;
var $s1 = "65";

if ($i === $s1)
{

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

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

var $i = 65;
var $s1 = "65";

if ($i == $s1)
{

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

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

See Also