Summary of PHP weak type safety issues

Some time ago, I did a question on the Network Attack and Defense Platform of Nanjing University of Posts and Telecommunications. After writing a writeup, I still need to summarize it. Since the questions are all web-type and all questions are written using PHP, many questions do not examine traditional vulnerabilities such as SQL injection and XSS. Many of them are problems with the syntax of PHP itself. Given that PHP is currently the best language in the world, problems with PHP itself can also be counted as an aspect of web security. Features in PHP training are weak typing and the loose handling of incoming parameters by built-in functions. This article is mainly to record the problems in PHP functions that I encountered when building an offensive and defensive platform, as well as the problems caused by PHP's weak types.
Introduction to PHP weak types
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 type at any time. Variables can also be converted into any other type of data.
Type conversion problem
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 operator
Type conversion
In the comparison of $a==$b
$a=null;$b=flase; //true
$a='';$b=null; //true
Like this There are many examples, and this kind of comparison is 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 different types of variables are compared, there will be a problem of variable conversion, and there may be problems after the conversion.
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 you encounter 0ed+this string, the 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 0ed+ is not satisfied, this pattern will not be equal. This question is tested in the md5 collision in the offensive and defensive platform.
Hex conversion
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 this string into decimal and then compare it. When 0×1240 is parsed into decimal, it is 123456, so the comparison with 123456 of int type and string type is 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;
Method 1: $item = (string)$var;
Method 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
Explain that when intval() is converted, it will Conversion is performed from the beginning of the string until a non-numeric character is encountered. Even if there is a string that cannot be converted, intval() will not report an error but return 0.
This feature of intval() 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...
The looseness of parameters of built-in functions
The looseness of built-in functions means that when calling a function, the function is passed parameter types that the function cannot accept. 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 ] ), and the required parameter in md5() is 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. This will cause the md5 values ​​of any two arrays to be equal. This feature of md5() is also considered in bypass again in the attack and defense platform.
strcmp()
strcmp() function is described in the PHP official manual as int strcmp ( string $str1 , string $str2 ), and two string type parameters need to be passed to strcmp(). If str1 is less than str2, -1 is returned, 0 is returned if equal, otherwise 1 is returned. The essence of the strcmp function comparing strings is to convert two variables into ascii, then perform a subtraction operation, and then determine the return value based on the operation result.
What if the parameter passed in to strcmp() is a number?
$array=[1,2,3];
var_dump(strcmp($array,'123')); //null, in a sense, null is equivalent to false.
This feature of strcmp has been tested in the pass check in the attack and defense platform.
switch()
If switch is a case judgment of numeric type, switch will convert the parameters into int type. As follows:
$i ="2abc";
switch ($i) {
case 0:
case 1:
case 2:
echo "i is less than 3 but not negative";
break;
case 3:
echo "i is 3";
}
At this time, the program output is i is less than 3 but not negative. This is because the switch() function performs type conversion on $i, and the conversion result is 2.
in_array()
In the PHP manual, the explanation of the in_array() function is bool in_array ( mixed $needle , array $haystack [, bool $strict = FALSE ] ). If the strict parameter is not provided, then in_array will use loose comparison To determine whether $needle is in $haystack. When the value of stringe is true, in_array() will compare the type of needsls and the type in haystack to see whether they are the same.
$array=[0,1,2,'3'];
var_dump(in_array('abc', $array)); //true
var_dump(in_array('1bc', $array)); //true
You can see that the above situation returns true, because 'abc' will be converted to 0 and '1bc' will be converted to 1.
array_search() and in_array() have the same problem.