XiaoHui
PHP + MYSQL programming. I have learned some knowledge about php SQL injection attacks. So I wrote this article to sum up my experience. In my opinion, the main cause of SQL injection attacks is the following two reasons:
1. The magic_quotes_gpc option in the php configuration file php. ini is disabled.
2. The developer does not check and escape the data type.
But in fact, the second point is the most important. In my opinion, it should be the most basic quality for web programmers to check the data types entered by users and submit the correct data types to MYSQL. But in reality, many small Web developers often forget this, leading to a wide open backdoor.
Why is the second most important? Without the second guarantee, the magic_quotes_gpc option may cause SQL injection attacks, whether on or off. The following describes the technical implementation:
1. injection attack when magic_quotes_gpc = Off
Magic_quotes_gpc = Off is a very insecure option in php. The new php version has changed the default value to On. However, there are still a considerable number of server options that are off. After all, some old servers are also used.
When magic_quotes_gpc = On, it automatically adds all (single quotes), "(double sign), (backslash), and white space characters in the submitted variables to the front. The official instructions for php are as follows:
Magic_quotes_gpc boolean
Sets the magic_quotes state for GPC (Get/Post/Cookie) operations. when magic_quotes are on, all (single-quote), "(double quote), (backslash) and NULs are escaped with a backslash automatically
If there is no escape, that is, off, attackers can take advantage of it. The following test script is used as an example:
Http://www.xiaohui.com/weekly/20070314.htm
<?
If (isset ($ _ POST ["f_login"])
{
// Connect to the database...
//... Code omitted...
// Check whether the user exists
$ T_strUname = $ _ POST ["f_uname"];
$ T_strPwd = $ _ POST ["f_pwd"];
$ T_strSQL = "SELECT * FROM tbl_users WHERE username = $ t_strUname AND password = $ t_strPwd LIMIT 0, 1 ";
If ($ t_hRes = mysql_query ($ t_strSQL ))
{
// Processing after successful query...
}
}
?>
<Html> <Body>
<Form method = post action = "">
Username: <input type = "text" name = "f_uname" size = 30> <br>
Password: <input type = text name = "f_pwd" size = 30> <br>
<Input type = "submit" name = "f_login" value = "Logon">
</Form>
</Body>
In this script, when the user enters the normal user name and password, assuming the values are zhang3 and abc123, the SQL statement submitted is as follows:
SELECT * FROM tbl_users
WHERE username = zhang3 AND password = abc123 LIMIT 0, 1
If the attacker enters zhang3 OR 1 = 1 # In the username field and abc123 in the password field, the submitted SQL statement is changed to the following:
SELECT * FROM tbl_users
WHERE username = zhang3 OR 1 = 1 # AND password = abc123 LIMIT 0, 1
Because # Is a comment character in mysql, # the subsequent statement is not executed, and the above statement is implemented:
SELECT * FROM tbl_users
WHERE username = zhang3 OR 1 = 1
In this way, attackers can bypass authentication. If attackers know the database structure, it is more dangerous to construct a union select statement:
Assume that in username, enter: zhang3 OR 1 = 1 UNION select cola, colb, cold FROM tbl_ B #
Enter abc123 in password,
The submitted SQL statement is changed:
SELECT * FROM tbl_users
WHERE username = zhang3
OR 1 = 1 UNION select cola, colb, cold FROM tbl_ B # AND password = abc123 LIMIT 0, 1
This is quite dangerous. If the agic_quotes_gpc option is on and the quotation marks are escaped, the attack statement constructed by the above attacker will become like this and thus cannot achieve its purpose:
SELECT * FROM tbl_users
WHERE username = zhang3 OR 1 = 1 #
AND password = abc123
LIMIT 0, 1
SELECT * FROM tbl_users
WHERE username = zhang3 OR 1 = 1 UNION select cola, colb, cold FROM tbl_ B #
AND password = abc123 LIMIT 0, 1
2. injection attack when magic_quotes_gpc = On
When magic_quotes_gpc = On, attackers cannot perform SQL Injection On fields of the stable type. This does not mean this is safe. In this case, you can use numeric fields for SQL injection.
In MYSQL 5.x of the latest version, data type input is strictly specified, and automatic type conversion is disabled by default. A numeric field cannot be a quote type marked with quotation marks. That is to say, if the uid is numeric, in the previous mysql version, such a statement is legal:
Insert into tbl_user SET uid = "1 ";
SELECT * FROM tbl_user WHERE uid = "1 ";
In the latest MYSQL 5.x, the preceding statement is not legal and must be written as follows:
Insert into tbl_user SET uid = 1;
SELECT * FROM tbl_user WHERE uid = 1;
In this case, I think it is correct. As a developer, it is the most basic requirement to submit correct data types that comply with the Rules to the database.
How can attackers attack magic_quotes_gpc = On? It is very simple, that is, SQL injection to numeric fields. The following php script is used as an example:
<?
If (isset ($ _ POST ["f_login"])
{
// Connect to the database...
//... Code omitted...
// Check whether the user exists
$ T_strUid = $ _ POST ["f_uid"];
$ T_strPwd = $ _ POST ["f_pwd"];
$ T_strSQL = "SELECT * FROM tbl_users WHERE uid = $ t_strUid AND password = $ t_strPwd LIMIT 0, 1 ";
If ($ t_hRes = mysql_query ($ t_strSQL ))
{
// Processing after successful query...
}
}
?>
<Html> <Body>
<Form method = post action = "">
User ID: <input type = "text" name = "f_uid" size = 30> <br>
Password: <input type = text name = "f_pwd" size = 30> <br>
<Input type = "submit" name = "f_login" value = "Logon">
</Form>
</Body>
The above script requires the user to enter userid and password to log on. A normal statement. The user inputs 1001 and abc123. The SQL statement submitted is as follows:
SELECT * FROM tbl_users WHERE userid = 1001 AND password = abc123 LIMIT
If the attacker inputs 1001 OR 1 = 1 # At userid, the SQL statement injected is as follows:
SELECT * FROM tbl_users WHERE userid = 1001 OR 1 = 1 # AND password = abc123 LIMIT
The attacker achieved the goal.
Iii. How to Prevent php SQL injection attacks
How to Prevent php SQL injection attacks? In my opinion, the most important thing is to check and escape data types. The following rules are summarized:
The display_errors option in php. ini should be set to display_errors = off. In this way, php scripts do not output errors on web pages, so that attackers can analyze the information.
When calling mysql functions such as mysql_query, add @, I .e. @ mysql_query (...), so that mysql errors will not be output. Similarly, attackers may not analyze useful information. In addition, some programmers are used to output errors and SQL statements when mysql_query errors during development, such:
$ T_strSQL = "SELECT a from B ....";
If (mysql_query ($ t_strSQL ))
{
// Correct Handling
}
Else
{
Echo "error! SQL statement: $ t_strSQL error message ". mysql_query ();
Exit;
}
This approach is quite dangerous and stupid. To do this, you 'd better set a global variable or define a macro in the website configuration file and set the debug flag:
In the global configuration file:
Define ("DEBUG_MODE", 0); // 1: debug mode; 0: RELEASE MODE
// In the call script:
$ T_strSQL = "SELECT a from B ....";
If (mysql_query ($ t_strSQL ))
{
// Correct Handling
}
Else
{
If (DEBUG_MODE)
Echo "error! SQL statement: $ t_strSQL error message ". mysql_query ();
Exit;
}
Run the convert and type checks on the submitted SQL statements.
4. I wrote a security parameter to obtain the function.
To prevent incorrect data and php + mysql injection, I wrote a function PAPI_GetSafeParam () to obtain the safe parameter values:
Define ("XH_PARAM_INT", 0 );
Define ("XH_PARAM_TXT", 1 );
Function PAPI_GetSafeParam ($ pi_strName, $ pi_Def = "", $ pi_iType = XH_PARAM_TXT)
{
If (isset ($ _ GET [$ pi_strName])
$ T_Val = trim ($ _ GET [$ pi_strName]);
Else if (isset ($ _ POST [$ pi_strName])
$ T_Val = trim ($ _ POST [$ pi_strName]);
Else
Return $ pi_Def;
// INT
If (XH_PARAM_INT = $ pi_iType)
{
If (is_numeric ($ t_Val ))
Return $ t_Val;
Else
Return $ pi_Def;
}
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