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OWASP Backend Security Project PHP Security Programming

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Revision as of 21:13, 24 July 2008 by Dbellucci (talk | contribs)

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Overview

PHP developer should be aware about threats that can be exposed on vulnerable PHP code. To this aims we're going to address two of them in the following sections:

  • SQL Injection
  • LDAP Injection

Since both of them are well known attacks vectors we're going to address some valid techniques to defend against:

  • Escaping Quotes on both input parameters and HTTP Request Header
  • Usage of Prepared Statements to query backend DBMS
  • Data Validation
  • Safe error handling

But that's not all. Since a backend datastore should be accessed though a authentication developer should be aware on where to store such a access credentials as well.

Recent research activities has shown that is possible to detect intrusion attempts from a WEB Application by embedding an Application Layer IDS inside your application.

SQL Injection

Here follows a tipical Login Form where users credentials are stored on a Backend DBMS. To successfull validate user credentials an authenticated connection to Backend DBMS is needed. Application developer decided to store the Database Connection String parameters in a .inc file as shown in the example:


auth.php

<?php
include('./db.inc');

function sAuthenticateUser($username, $password){
  $authenticatedUserName="";
  if ($link = iMysqlConnect()) {

    $query  = "SELECT username FROM users";
    $query .=                " WHERE username = '".$username."'";
    $query .=                " AND   password = md5('".$password."')";
    $result = mysql_query($query);

    if ($result) {
      if ($row = mysql_fetch_row($result)) {
         $authenticatedUserName =  $row[0];
      }
    }
  }

  return $authenticatedUserName;

}

if ($sUserName = sAuthenticateUser($_POST["username"], 
				   $_POST["password"])) {

  /* successfull authentication code goes here */
   ...
   ...
 } else {

  /* unsuccessfull authentication code goes here */
   ...
   ...
 }

?>


db.inc

<?php

define('DB_HOST',     "localhost");
define('DB_USERNAME', "user");
define('DB_PASSWORD', "password");
define('DB_DATABASE', "owasp");


function iMysqlConnect(){
  $link = mysql_connect(DB_HOST,
  	  DB_USERNAME,
			DB_PASSWORD);

  if ($link && mysql_select_db(DB_DATABASE))
    return $link;
  return FALSE;
}

?>

Vulnerability

  • SQL Injection
    by using some inference techniques it's possible to enumerate a backend Database and even worst it could be possible to interact with underlying operating system
  • Authentication Bypass
    by exploiting a SQL Injection vulnerability Authentication an evil user can bypass authentication by suppliying :
    username ' OR 1=1 #
    password anything
  • Information Disclosure
    db.inc contents could be retrieved by an evil user


Remediation

  • Make .inc unavailable to remote user
    As we'll see it's possible to avoid .inc file retrieval from a remote user
  • Escaping Quotes
    when evil users supplies username ' OR 1=1 # quotes should be escaped in such in username \' OR 1=1 #
  • Prepared Statements
    Prepared Statements prevents SQL Injection attacks by giving to the backend DBMS an execution plan of a query where parameters are replaced by variables. Variables will be instantiated with values and query will be executed. Since running an execution plan with parameters containing SQL Expressions, Operators or statements fails, we can get ride of Injection.
  • Data Validation
    Input parameters should be validated in both data type and value. Developers should be aware that PHP is not a strongly type language. Everything it's a string when talking about Input Parameters and Headers.
  • Embedd an Application Layer IDS
    PHPIDS it's a promising Open Source Project well mantained and with interesting features. An optimazed RegExp engine can be embedded in your PHP code. Benchmark performed by project developers shows that there is no performance penality at all.


As you can see there are many way to protect your PHP Code against SQL Injection Attack vectors. Wich ones to use is up to you by the way as stated by the Security in Depth it would be great to use all of them.

LDAP Injection

Here follows a tipical Login Form where users credentials are stored on a Backend LDAP Directory. To successfull validate user credentials an authenticated connection to Backend DBMS is needed. Application developer decided to store the LDAP Connection String parameters in a .inc file as shown in the example:

ldap.inc:

<?php
  define(LDAP_DIRECTORY_ADMIN , "cn=admin,dc=domain,dc=com");
  define(LDAP_DIRECTORY_PASS  , "pw$d0");
  define(LDAP_USER_BASEDN     , "ou=Users,dc=domain, dc=com");
?>


auth.php:

include('ldap.inc');

function authLdap($sUsername, $sPassword) {
  $ldap_ch=ldap_connect("localhost"); 

  if (!$ldap_ch) {
    return FALSE;
  }

  $bind = ldap_bind($ldap_ch, LDAP_DIRECTORY_ADMIN, LDAP_DIRECTORY_PASS);

  if (!$bind) {
    return FALSE;
  }

  $sSearchFilter = "(&(uid=$sUsername)(userPassword=$sPassword))";
  $result = ldap_search($ldap_ch, " dc=domain,dc=com", $sSearchFilter);

  if (!$result) {
    return FALSE;
  }
  
  $info = ldap_get_entries($ldap_ch, $result);

  if (!($info) || ($info["count"] == 0)) {
    return FALSE;
  }

  return TRUE;

}

$sUsername = $_GET['username'];
$sPassword = $_GET['password'];

$bIsAuth=authLdap($sUsername, $sPassword);

if (! $bIsAuth ) {
   /* Unauthorized access, handle exception */
   ...
   ...
} 

/* User has been succesfull authenticated */



Vulnerability

  • Authentication Bypass
    by exploiting a LDAP Injection vulnerability evil user can bypass authentication by suppliying username * and password *
  • Information Disclosure
    ldap.inc contents could be retrieved by an evil user


Remediation

  • Make .inc unavailable to remote user
    As we'll see it's possible to avoid .inc file retrieval from a remote user
  • Authenticate Users through LDAP Bind
    Since LDAP define a BIND method which requires a valid user credential we can use ldap_bind() rather than setting up a complex machinery with ldap_search()
  • Data Validation
    Input parameters should be validated in both data type and value. Developers should be aware that PHP is not a strongly type language. Everything it's a string when talking about Input Parameters and Headers.
  • Embedd an Application Layer IDS
    PHPIDS it's a promising Open Source Project well mantained and with interesting features. An optimazed RegExp engine can be embedded in your PHP code. Benchmark performed by project developers shows that there is no performance penality at all.


As you can see there are many way to protect your PHP Code against LDAP Injection Attack vectors. Wich ones to use is up to you by the way as stated by the Security in Depth it would be great to use all of them.

Description

PHP preventing SQL Injection

When talking about preventing SQL Injection in PHP we shall take in consideration whatever to use old Database connector of newset PHP Portable Data Objects interface. To this aim will take in consideration both of them since:

  • PHP PDO has been introduced in PHP 5.1
  • PHP PDO represents a Database abstract layer
  • Experienced developers may want to develop a custom Abstract Layer for theire demand

Reader should be aware that at the moment PDO OCI used as a Oracle Driver is experimental.


DBMS authentication credentials

When working with a DBMS through an authenticated connection application developers should be very carefull on how, and subsequently where, store authentication credentials to query such a backend engine. A configuration file with .inc extension should be avoided if left world wide readable by a web server since it's content can be easy retrieved. Such issue can be avoided by :

  • Denying remote access to .inc files
     <Files ~ “\.inc$”>
        Order allow,deny
        Deny from all
     </Files>

requires user intervention on Apache configuration!


  • Adding a security token
<?php

if (defined('SECURITY_INCLUDE_TOKEN') && SECURITY_INCLUDE_TOKEN != 'WfY56#!5150'){
     define ('DBMS_CONNECTION_STRING','mysql://owaspuser:[email protected]:3306');
     ....
}

<?php
define('SECURITY_INCLUDE_TOKEN', 'WfY56#!5150');
include 'dbms_handler.php';
..
?>


  • configuring php_ini settings in apache
/etc/apache2/sites-enabled/000-owasp 

<VirtualHost *>
     DocumentRoot /var/www/apache2/
     php_value mysql.default_host 127.0.0.1
     php_value mysql.default_user owaspuser
     php_value mysql.default_password owasppassword
     ....
     ....
     ....
</VirtualHost>


dbmshandler.php 

<?php
    function iMySQLConnect() {
        return mysql_connect();

    }
?> 
at the moment of write it only works when backend Database Engine is MySQL


  • using Apache SetEnv directive
/etc/apache2/sites-enabled/000-owasp 

<VirtualHost *>
     DocumentRoot /var/www/apache2/
     SetEnv DBHOST "127.0.0.1"
     SetEnv DBUSER "owaspuser"
     SetEnv DBPASS "owasppassword"
   
     ....
     ....
     ....
</VirtualHost>


dbmshandler.php 

<?php
    function iMySQLConnect() {
        return mysql_connect(getenv("DBHOST"),
                             getenv("DBUSER"),
                             getenv("DBPASS"));
    }
?>



requires user intervention on Apache configuration


Wich one to use is up to you. You should take into account whetever a user interventation is needed on WEB Server or not and if you want to guarantee application portability as well between different DB Backend Engine.

Example using PDO MySQL driver:

/etc/apache2/sites-enabled/000-owasp 

<VirtualHost *>
     DocumentRoot /var/www/apache2/
     SetEnv PDO_DSN "mysql:host=localhost;dbname=owasp"
     SetEnv PDO_USER "owaspuser"
     SetEnv PDO_PASS "owasppassword"
   
     ....
     ....
     ....
</VirtualHost>


dbmshandler.php 

<?php
function SQLConnect() {
    $oPdo = NULL;
    try {
            $oPdo = new PDO(getenv("PDO_DSN"),
                            getenv("PDO_USER"),
                            getenv("PDO_PASS"));

            /* Throws an exception when subsequent errors occour */
            $oPdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            /* handle PDO connection success */
               ...
               ...
            return $oPdo;
   } catch (PDOException $e) {
            /* handle PDO connection error */
               ...
               ...
           return NULL;
   }
} 
?>



Escaping Quotes

magic_quotes_gpc escapes quotes from HTTP Request by examing both GET/POST data and Cookie value. The truth is that any other data in HTTP Request isn't escaped and an evil user may attempt to exploit a SQL Injection vulnerability on other HTTP Request data such as User-Agent value. Another drawback is that while it performs well on MySQL (as example) it doesn't works with Microsoft SQL Server where single quote should be escaped with ''' rather than \'

Since every application should be portable across WEB Servers we want to rollaback from magic_quotes_gpc each time a php script is running on WEB Server:

function magic_strip_slashes() {
    if (get_magic_quotes()) {

       // GET
       if (is_array($_GET)) {
           foreach ($_GET as $key => $value) {
               $_GET[$key] = stripslashes($value);
           }
       }

       // POST
       if (is_array($_POST)) {
           foreach ($_GET as $key => $value) {
               $_POST[$key] = stripslashes($value);
           }
       }

       // COOKIE
       if (is_array($_COOKIE)) {
           foreach ($_GET as $key => $value) {
               $_COOKIE[$key] = stripslashes($value);
           }
       }
    }
}


and use a DBMS related function to escape quotes such as:

  • MySQL: mysql_real_escape_string
  • PostgreSQL: pg_escape_string
function sEscapeString($sDatabase, $sQuery) {
    $sResult=NULL;
 
    switch ($sDatabase) {
    case "mysql":
        $sResult = mysql_real_escape_string($sQuery);
        break;
  
    case "postgresql":
        $sResult = pg_escape_string($sQuery);
        break;

    case "mssql":
        $sResult = str_replace("'", "''",$sQuery);
        break;

    case "oracle":
        $sResult = str_replace("'", "''",$sQuery);
        break;
    }

    return $sResult;
}
}

Since both Oracle and Microsoft SQL Server connectors doesn't have a real escape_string function software developer can create his own escapeing functions or use addslasshes().

With properly quotes escaping we can prevent Authentication Bypass vulnerability in Example 1:


auth.php

<?php
include('./dbmshandler.php);

function sAuthenticateUser($username, $password){
  $authenticatedUserName="";
  if ($link = iMysqlConnect()) {

    $query  = "SELECT username FROM users";
    $query .=                " WHERE username = '".$username."'";
    $query .=                " AND   password = md5('".$password."')";

    /* escape quotes */
    $result = sEscapeString("mysql", $query);

    if ($result) {
      if ($row = mysql_fetch_row($result)) {
         $authenticatedUserName =  $row[0];
      }
    }
  }

  return $authenticatedUserName;
}

/* start by rollback magic_quotes_gpc action (if any) */

magic_strip_slashes();


if ($sUserName = sAuthenticateUser($_POST["username"], 
                                   $_POST["password"])) {

  /* successfull authentication code goes here */
   ...
   ...
 } else {

  /* unsuccessfull authentication code goes here */
   ...
   ...
 }


PHP Portable Data Objects implements a quote() method on PDO class but it's worst noticing that not all underlying PDO Drivers implements this method. On the other side PDO::query() method by defualt escape quotes on SQL query string as shown in following example.


Example using PDO MySQL driver:

auth.php

<?php
include('./dbmshandler.php');

function sAuthenticateUser($username, $password){
  $authenticatedUserName=NULL;
  if ($oPdo = SQLConnect()) {

    $query  = "SELECT username FROM users";
    $query .=                " WHERE username = '".$username."'";
    $query .=                " AND   password = md5('".$password."')";

    try {
         $row = $oPdo->query($query)->fetch();
         if ($row) {
              return $row['username'];
         } 
    } catch (PDOException e) {
         /* handle execption and SQL Injection Attempt */
         ....
         ....
         return NULL;
    }    
}


/* start by rollback magic_quotes_gpc action (if any) */

magic_strip_slashes();


if ($sUserName = sAuthenticateUser($_POST["username"], 
                                   $_POST["password"])) {

  /* successfull authentication code goes here */
   ...
   ...
} else {

  /* unsuccessfull authentication code goes here */
   ...
   ...
}


Prepared Statements

Prepared Statements is the ability to preparse and generate an execution plan for SQL Queries. Such an execution plan will be instantiated with typed parameters. If params are of incorrect type or contains a nested query the execution of plan will fails.

<?php
function getBookByID($id) {
    $aBook = NULL;
    $link = mysqli_connect();

    $stmt = $link->stmt_init();
    if ($stmt->prepare("SELECT * FROM books WHERE ID =?")) {
        $stmt->bind_param("i",$id);
        $stmt->execute();
        
        /* Retrieves book entry and fill $aBook array */
        ...
        ...

        /* Free prepared statement allocated resources */
        $stmt->close();
    }

    return $aBook;

} 

/* MAIN */

/* Cast GET 'id' variable to integer */
$iID = (int)$_GET['id'];

$aBookEntry = getBookByID($iID);

if ($aBookEntry) {
    /* Display retrieved book entry */
    ...
    ...
}

?>

PHP Portable Data Objects emulate prepared statements for drivers with no natively supports. Here follows an example of prepared statements usage with PHP PDO


Example using PDO:

<?php
include('./dbmshandler.php');

function getBookByID($id) {
    $aBook = NULL;
    $oPdo = SQLConnect();

    if ($oPdo) {
        $stmt = $oPdo->prepare("SELECT * FROM books WHERE ID =?");
        $stmt->bindParam(1, $id, PDO::PARAM_INT);
        if ($smmt->execute()) {
            $aBook = $stmt->fetch(PDO::FETCH_ASSOC);
        }
    }

    return $aBook;

} 

Data Validation

Modern WEB Applications are supposed to interacts with users throught input data. Input data can be supplied through a HTML Form and WEB Application retrieves such a data through a GET/POST variable. Input data can contains malicious values to exploit some security flaws in WEB Applications. As a general rule data validation should be performed on both input and output values, since they both depends on each other. data should be rejected unless it matches a criteria. Developers should define a restriced range for valid data and reject everything else. Such a criteria will include:

  • Data Type
  • Data Length;
  • Data Value

A typical Data Validation workflow will be:

  • Get the data to be validated
  • Check if it should be a numerical or string
  • Look at it's size in byte to avoid errors when database table columns has some constraint in value size
  • Check if data contains a valid value (EMail, phone number, date, and so on).

To this aims PHP can help developers with :

  • casting operators
  • regexp functions


Numeric Data

PHP is not a strongly typed languages it means that every input data is a string by default. If you want to validate a numeric value you should casting operator. Casting an input data to int ensure that:

  • if data is numeric you get its value
  • if data doesn't contains a number casting will returns 0
  • if data includes a number casting will returns its numeric portion

Example

 
 ...
    $iId = (int)$_GET['id'];
    if ( $iId != $_GET['id']) {
      /* User supplied data is not numeric, handle exception */
      ...
      return;
    } 

    if ($iId > MAX_ID_VALUE || $iId < MIN_ID_VALUE) {
      /* User supplied data is numerica but it doesn't contains an allowed value, handle exception */

    }

    /* $iId is safe */

    


String Data

Strings data validation is a bit tricker since it can contains malicious values. It means that it should be validated on what data is supposed to include. Data can contains:

  • EMail Address
  • Phone Number
  • URL
  • Name
  • Date

and so on.

To this aim WEB Developers should match Input Data against a Regular Expression to match what Data is supposed to inclues. Here follows some example.

Example: Validating an Email Address

 
    ...
    $sEmail = $_POST['email'];
    if (! preg_match("/^[\w-]+(?:\.[\w-]+)*@(?:[\w-]+\.)+[a-zA-Z]{2,7}$/", $sEmail)) {
      /* User supplied data is not a valid email address,  handle exception */
      ...
      return;
    }

    /* $sEmail is safe, check len */

    if (strlen($sEmail) > MAX_EMAIL_LEN) {
       /* User supplied data is to big for backend database, handle exception */
      ...
      return;
    }





Example: Validating an Italian Phone Number

 
    ...
    $sPhoneNumber = $_POST['phonenumber'];

    if (! preg_match(, "/[0-9]+[-\/ ]?[0-9]+$/", $sPhoneNumber)) {
      /* User supplied data is not a phone number,  handle exception */
      ...
      return;
    }

    /* $sPhoneNumber is safe, check len */

    if (strlen($sPhoneNumber) > MAX_PHONENUMBER_LEN) {
       /* User supplied data is to big for backend database, handle exception */
      ...
      return;
    }





OWASP PHP Filters

OWASP PHP Filters project allow programmers an easy way to perform data validation. Even if project is quite old and not well mantained it's still well working and defines a valid approach to performa Data Validation.

Logging Errors

Malicious users typicaly attempt to explot SQL Injection Vulnerabilities by looking at some Error Codes on dynamic pages. When PHP fails to query Backend Database an error message will be returned to users if error are not handled on a safe way. WEB Developers uncorrectly debug SQL Errors by displaying some kind of error message on WEB Page when query fails. This approach should not be considered safe since Errors should never be displayed to users.

Users should also never deduce that something wrong happens otherwise it could be considered a flaw to further more exploits a vulnerability. To this aim we're going to how to safely handle errors.

 
  function unavailable_resource_handler() {
     /* Handle an 'Unavailable Resource' event without supplying further information to user 
      *
      * Example:
      *    die('Resource not available');
      *      
      *
      */
     ..
     ..

  }

  function sql_error_handler ($sQuery, $sMsg) {
     /* Log failed SQL Query statement */
     error_log ($sQuery, 3, "/var/log/site/sqlquery_error.log");
     
     /* Log error message */
     error_log ($sMsg, 3, "/var/log/site/site_error.log");

     /* Notify user that resource is unavailable */
     unavailable_resource_handler();
  }


PHP preventing LDAP Injection

LDAP Authentication

As previously states LDAP Authentication in PHP should be handled with ldap_bind() functin in such a way:

     
function authLdap($sUsername, $sPassword) {
  $ldap_ch=ldap_connect("ldap://localhost"); 
  
  if ($ldap_ch) {

    $ldap_user = "uid=$sUsername, ou=Utenti, dc=domain, dc= com";
    $ldap_password = $sPassword;

    $bind = @ldap_bind($ldap_ch, $ldap_user, $ldap_password);
    
    return $bind;
    
  }

  return FALSE;

}

$sUsername = $_GET['username'];
$sPassword = $_GET['password'];

$bIsAuth=authLdap($sUsername, $sPassword);

if (! $bIsAuth ) {
   /* Unauthorized access, handle exception */
   ...
   ...
} 

/* User has been succesfull authenticated */



LDAP Authentication Credentials

Whenever a Backend Directory Server shall not be used to authenticate user is good practice not to query with an anonymous bind. Doind so you need a safe way to store LDAP Bind access credentials. The same techniques of previous section shall be used as well.


Data Validation

Data Validation occours on the same way as in PHP Preventing SQL Injection. Here follows previous LDAP Login Form example with data validation, where both username and password are allowed to contains only alfa numeric values.

 
$sUsername = $_GET['username'];
$sPassword = $_GET['password'];

if (! preg_match("/[a-zA-Z0-9]+$/", $username) {
   /* username doesn't contains valid characters, handle exception */
   ..
   ..
   return;
}

if (! preg_match("/[a-zA-Z0-9]+$/", $password) {
   /* password doesn't contains valid characters, handle exception */
   ..
   ..
   return;
}




$bIsAuth=authLdap($sUsername, $sPassword);

if (! $bIsAuth ) {
   /* Unauthorized access, handle exception */
   ...
   ...
} 

/* User has been succesfull authenticated */



Logging Errors

As in PHP Preventing SQL Injection we're going to define a safe way to handle LDAP Errors. An previously stated incorrect error handling may raise Information Disclousure vulnerbality and a malicious user may furthermore exploit this vulnerability to perform some kind of a attack.


 
  function unavailable_resource_handler() {
     /* Handle an 'Unavailable Resource' event without supplying further information to user 
      *
      * Example:
      *    die('Resource not available');
      *      
      *
      */
     ..
     ..

  }

  function ldap_error_handler ($sQuery, $sMsg) {
     /* Log failed LDAP Query statement */
     error_log ($sQuery, 3, "/var/log/site/ldapquery_error.log");
     
     /* Log error message */
     error_log ($sMsg, 3, "/var/log/site/site_error.log");

     /* Notify user that resource is unavailable */
     unavailable_resource_handler();
  }


Detecting Intrusions from WEBAPP

PHPIDS (PHP-Intrusion Detection System) is a well mantained and fast security layer for PHP based web application. It allows to embed a WEB Application IDS into a PHP WEB Application. It performs a transparent Input Data Validation on each HTTP Request to look forward for Malicious Input. It means that Developer shouldn't explicitly analyze and thrust Input Data. PHP IDS will automatically look for exploit attempts. PHPIDS simply recognizes when an attacker tries to break your WEB Application and let you decide how to reacts.

To install PHPIDS:

  • download and unpack in your application or include folder.
  • open phpids/lib/IDS/Config/Config.ini file and edit the following element to reflect PHPIDS complete installation path:
    filter_path
    tmp_path
    path in both Logging and Caching section


Here is a typical usage of PHPIDS:


init_ids.php

 
<?php
    
set_include_path(get_include_path() . PATH_SEPARATOR . './lib/');
require_once 'IDS/Init.php';

function phpIDSInit() {
    $request = array( 
   	              'REQUEST' => $_REQUEST,   
                      'GET'     => $_GET,
                      'POST'    => $_POST, 
                      'COOKIE'  => $_COOKIE
                    );

  $init = IDS_Init::init('./lib/IDS/Config/Config.ini');
  $ids = new IDS_Monitor($request, $init);
  
  return $ids->run();

}

?>

main.php

 
<?php
  
   require_once './init_ids.php';
   $sIDSAlert = phpIDSInit();

   if ($sIDSAlert) {
       /* PHPIDS raise an alert, handle exception */
       ....
       ....
       die()
   }

/* PHPIDS determined that request is safe */
...
...


?>


References