Summary of security issues that need to be paid attention to when using PHP weak types

Weakly typed language is also called weakly typed language. The opposite of strongly typed definition. Languages ​​such as VB and PHP are weakly typed. This article will give you a detailed introduction to the security issues of PHP's weak types. You can refer to them if necessary. Let's take a look together.

Preface

I believe everyone knows that PHP is the best language in the world, and the problems of PHP itself can also be regarded as web security. One aspect. The characteristics in PHP are weak types and built-in functions loose handling of incoming parameters.

This article is mainly to record the problems in PHP functions that I encountered on the offensive and defensive platform, as well as the problems caused by PHP's weak types. It has certain reference value for everyone when learning or using PHP. Let’s take a look together.

Introduction to PHP weak types

The reason why the editor thinks that PHP is very powerful is because PHP provides many unique features for developers to use , one of which is the PHP weak type mechanism.

In PHP, you can perform the following operations.

$param = 1;
$param = array();
$param = "stringg";

Weakly typed languages ​​have no restrictions on the data type of variables. You can assign variables to any other types of variables at any time, and the variables can also be converted to any Other types of data.

Type conversionProblem

Type conversion is an unavoidable problem. For example, when you need to convert GET or POST parameters into int type, or when the two variables do not match, PHP will automatically convert the variables. However, PHP is a weakly typed language, which leads to many unexpected problems when performing type conversion.

Comparison operators

Type conversion

at$a==$ There are many examples of

$a=null;$b=flase ; //true
$a='';$b=null; //true

in the comparison of b

, and these comparisons are all equal.

There are also type conversion problems when using comparison operators, as follows:

0=='0' //true
0 == 'abcdefg' //true
0 === 'abcdefg' //false
1 == '1abcdef' //true

When variables of different types are compared, there will be variable conversion problems. During conversion There may be problems later.

Hash comparison

In addition to the above method, there are also problems when performing hash comparison. As follows:

"0e132456789"=="0e7124511451155" //true
"0e123456abc"=="0e1dddada" //false
"0e1abc"=="0"  //true

When performing comparison operations, if a string like 0e\d+ is encountered, this string will be parsed into scientific notation. Therefore, the values ​​of the two numbers in the above example are both 0 and they are equal. If 0e\d+ is not satisfied, this pattern will not be equal. This question is tested in the md5 collision in the offensive and defensive platform.

HexadecimalConversion

There is also a problem when comparing hexadecimal remainder strings.

Examples are as follows:

"0x1e240"=="123456" //true
"0x1e240"==123456 //true
"0x1e240"=="1e240" //false

When one of the strings starts with 0x, PHP will parse the string into decimal and then compare it. When 0x1240 is parsed into decimal, it is 123456, so Comparisons with 123456 of int type and string type are all equal. The difficulty in naming in the offensive and defensive platform is due to this characteristic of inspection.

Type conversion

Common conversions are mainly converting int to string and string to int.

int to string:

$var = 5;
方式1：$item = (string)$var;
方式2：$item = strval($var);

string to int: intval() function.

For this function, you can look at 2 examples first.

var_dump(intval('2')) //2
var_dump(intval('3abcd')) //3
var_dump(intval('abcd')) //0

Explanationintval()When converting, the conversion will be performed from the beginning of the string until a non-numeric character is encountered. Even if a string that cannot be converted appears, intval() will not report an error but return 0.

intval() This feature is tested in the question of MYSQL in the offensive and defensive platform.

At the same time, programmers should not use the following code when programming:

if(intval($a)>1000) {
 mysql_query("select * from news where id=".$a)
}

At this time, the value of $a may be 1002 union…..

Built-inParameters of functionsThe looseness of

The looseness of built-in functions means that when calling a function, it is passed to the function Function cannot accept parameter type. The explanation is a bit confusing, so let’s illustrate the problem directly through practical examples. Below we will focus on a few such functions.

md5()

$array1[] = array(
 "foo" => "bar",
 "bar" => "foo",
);
$array2 = array("foo", "bar", "hello", "world");
var_dump(md5($array1)==var_dump($array2)); //true

The description of the md5() function in the PHP manual is string md5 ( string $str [, bool $raw_output = false ] ) , md5() needs to be a string type parameter. But when you pass an array, md5() will not report an error, and knowledge will not be able to correctly calculate the md5 value of the array, which will cause the md5 values ​​of any two arrays to be equal. This md5() feature is also considered in bypass again in the attack and defense platform.

strcmp()

strcmp()函数在PHP官方手册中的描述是int strcmp ( string $str1 , string $str2 ) ,需要给strcmp()传递2个string类型的参数。如果str1小于str2,返回-1，相等返回0，否则返回1。strcmp函数比较字符串的本质是将两个变量转换为ascii，然后进行减法运算，然后根据运算结果来决定返回值。

如果传入给出strcmp()的参数是数字呢？

$array=[1,2,3];
var_dump(strcmp($array,'123')); //null,在某种意义上null也就是相当于false。

strcmp这种特性在攻防平台中的pass check有考到。

switch()

如果switch是数字类型的case的判断时，switch会将其中的参数转换为int类型。如下：

$i ="2abc";
switch ($i) {
case 0:
case 1:
case 2:
 echo "i is less than 3 but not negative";
 break;
case 3:
 echo "i is 3";
}

这个时候程序输出的是i is less than 3 but not negative，是由于switch()函数将$i进行了类型转换，转换结果为2。

in_array()

在PHP手册中，in_array()函数的解释是bool in_array ( mixed $needle , array $haystack [, bool $strict = FALSE ] ) ,如果strict参数没有提供，那么in_array就会使用松散比较来判断$needle是否在$haystack中。当strince的值为true时，in_array()会比较needls的类型和haystack中的类型是否相同。

$array=[0,1,2,'3'];
var_dump(in_array('abc', $array)); //true
var_dump(in_array('1bc', $array)); //true

可以看到上面的情况返回的都是true,因为'abc'会转换为0，'1bc'转换为1。

<a href="http://www.php.cn/wiki/1007.html" target="_blank">array_search</a>()与in_array()也是一样的问题。

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