Php Security By Mugdha And Anish

OSScamp, Impetus Noida, Sept,’07
Anish & Mugdha
Value One InfoTech

Topics of Discussion

 Importance of PHP Security

 Concerns of PHP Security
 Input Validation
 Cross-Site Scripting
 SQL Injection
 Code Injection
 Session Security
 Shared Hosting

Importance of PHP Security

 PHP is widely used language for web applications

 PHP is making headway into enterprise as well as corporate
markets.

 Most effective & often overlooked measure to prevent malicious
users

 PHP applications often end up working with sensitive data.

Input Validation

 All user inputs are unreliable and can’t be trusted.

 Need for validating any user input before use :

 Unexpected Modification by the user
 Intentional attempt to gain unauthorized access to the
application
 Attempt to crash the application by the malicious users

Register Globals

 Most common source of vulnerabilities in PHP applications.

 Any input parameters are translated to variables :-
?foo=bar >> $foo = “bar”;

 No way to determine the input source.
 Prioritized sources like cookies can overwrite GET values.

 When register global is set ON, un-initialized variables can be “injected” via user
inputs.

Solutions To Register Globals

 Disable register_globals in PHP.ini (Disabled by-default as of PHP 4.2.0)

 Alternative to Register Global : SUPER GLOBALS
 $_GET – data from get requests.
 $_POST – post request data.
 $_COOKIE – cookie information.
 $_FILES – uploaded file data.
 $_SERVER – server data
 $_ENV – environment variables
 $_REQUEST – mix of GET, POST, COOKIE

Contd…

 Type sensitive validation conditions.
 Because input is always a string, type sensitive compare to a Boolean or
an integer will always fail.

 Example
if ($authorized === TRUE)
{
// LOGIN SUCCESS
}

Contd…

 Code with error_reporting set to E_ALL.
 Allows you to see warnings about the use of un-initialized
variables.

 Use of constants
 Created via define() function
 Once set, remains defined until end of request
 Can be made case-insensitive to avoid accidental access to a
different datum caused by case variance.

Cons of $ REQUEST

 Suffers from the loss of data problem, caused when the same parameter is
provided by multiple input sources.

 PHP.ini: variables_order = GPCS (Last data source has highest priority)

 Example
echo $_GET['id']; // 1
echo $_COOKIE['id']; // 2
echo $_REQUEST['id']; // 2

 Use the input method-specific superglobals intead of $_REQUEST

Numeric Data Validation

 All data passed to PHP (GET/POST/COOKIE) ends up being a string. Using
strings where integers are needed is not only inefficient but also dangerous.

 Casting is a simple and very efficient way to ensure that variables contain
numeric values.

 Example of floating point number validation
if (!empty($_GET['price'])) {
$price = (float) $_GET['price'];
} else $price = 0;

String Validation

 PHP comes with a ctype, extension that offers a very quick mechanism for
validating string content.

if (!ctype_alnum($_GET['login'])) {
echo "Only A-Za-z0-9 are allowed.";
}
if (!ctype_alpha($_GET['captcha'])) {
echo "Only A-Za-z are allowed.";
}
if (!ctype_xdigit($_GET['color'])) {
echo "Only hexadecimal values are allowed";
}

Using Magic Quotes

 What are Magic Quotes ??

 Problems associated with it !!

 How to deal with it ??

Cross Site Scripting (XSS)

 Cross Site Scripting (XSS) is a situation where by attacker injects HTML
code, which is then displayed on the page without further validation.

 Can lead to embarrassment
 Session take-over
 Password theft
 User tracking by 3rd parties

Preventing XSS

 Prevention of XSS is as simple as filtering input data via one of
the following:

 htmlspecialchars()
Encodes ‘, “, <, >, &
 htmlentities()
Convert anything that there is HTML entity for.
 strip_tags()
Strips anything that resembles HTML tag.

 Tag allowances in strip_tags() are dangerous, because attributes
of those tags are not being validated in any way.

Preventing XSS
 $str = strip_tags($_POST['message']);

 // encode any foreign & special chars
$str = htmlentities($str);

 // strip tags can be told to "keep" certain tags
$str = strip_tags($_POST['message'], '');

 // tag allowance problems

Lot's of text

SQL Injection
 SQL injection is similar to XSS, in the fact that not validated data
is being used. But in this case this data is passed to the database.

 Arbitrary query execution
 Removal of data.
 Modification of existing values.
 Denial of service.
 Arbitrary data injection.

 // consider this query, it will delete all records from users

$name = “mugdha’; DELETE FROM users;”;
mysql_query(“SELECT * FROM users WHERE name =’{$name}’”);

SQL Escaping

 If your database extension offers a specific escaping function then
always use it; instead of other methods

 MySQL
 mysql_escape_string()
 mysql_real_escape_string()
 PostgreSQL
 pg_escape_string()
 pg_escape_bytea()
 SQLite
 sqlite_escape_string()

SQL Escaping in Practice

 // undo magic_quotes_gpc to avoid double escaping
if (get_magic_quotes_gpc()) {
$_GET['name'] = stripslashes($_GET['name'];
$_POST['binary'] = stripslashes($_GET['binary']);
}

$name = pg_escape_string($_GET['name']);
$binary = pg_escape_bytea($_POST['binary']);

pg_query($db, "INSERT INTO tbl (name,image)
VALUES('{$name}', '{$image}')");

Escaping Shortfall

 When un-quoted integers are passed to SQL queries, escaping functions
won’t save you, since there are no special chars to escape.

 http://example.com/db.php?id=0;DELETE%20FROM%20users

 <?php
$id = sqlite_escape_string($_GET['id']);
// $id is still 0;DELETE FROM users
sqlite_query($db,"SELECT * FROM users WHERE id={$id}");
// Bye Bye user data...
?>

Prepared Statements
 Prepared statements are a mechanism to secure and optimize execution
of repeated queries.

 Works by making SQL “compile” the query and then substitute in the
changing values for each execution.
 Increased performance, one compile vs one per query.
 Better security, data is “type set” will never be evaluated as
separate query.
 Supported by most database systems.

 MySQL users will need to use version 4.1 or higher.
 SQLite extension does not support this either.

Prepared Statements
 <?php
$data = "Here is some text to index";
pg_query($db, "PREPARE my_stmt (text) AS
INSERT INTO search_idx (word) VALUES($1)");
foreach (explode(" ", $data) as $word) {// no is escaping needed
pg_query($db, "EXECUTE my_stmt({$word})");
}
// de-allocte the prepared statement
pg_query($db, "DEALLOCATE my_stmt");
?>
Unless explicitly removed, prepared statements “stay alive”
between persistent connections.

Code Injection

 Code Injection is the execution of arbitrary local or remote code.

 The two of the most common sources of code injection are:
 Dynamic paths/files used in require/include statements
 eval(): A major source of code injection is the improper validation of
eval().

Code Injection Prevention
 Avoid using dynamic or relative paths/files in your code. Although somewhat less
convenient; always use full paths, defined by constants, which will prevent attacks
like these:
 <?php
//dynamic path
$_GET['path'] = ‘http://bad_site.org’;
include "{$_GET['path']}/header.inc";
//dynamic file
$_GET[‘interface’] = ‘../../../../../etc/passwd’;
require‘home/mbr/profile/templates_c/interfaces/’.$_GET[‘interface’];
?>
 There are some other ways to secure include or require calls...

Code Injection Prevention
 work with a white-list of acceptable values.
//create an array of acceptable file names
$tmpl = array();

foreach(glob("templates/*.tmpl") as $v) {
$tmpl[md5($v)] = $v;
}

if (isset($tmpl[$_GET['path']])) {
$fp = fopen($tmpl[$_GET['path']], "r");
}

Session Security

 Sessions are a common tool for user tracking across a web site.

 For the duration of a visit, the session is effectively the user’s identity.

 If an active session can be obtained by 3rd party, it can assume the identity of
the user who’s session was compromised.

Securing Session ID
 To prevent session id theft, the id can be altered on every request,
invalidating old values.
<?php
session_start();
if (!empty($_SESSION)) { // not a new session
session_regenerate_id(TRUE); // make new session id
}
?>
 Because the session changes on every request, the “back” button
in a browser will no longer work, as it will make a request with
the old session id.

Session Validation
 Another session security technique is to compare the browser signature
headers.
session_start();
$chk = @md5(
$_SERVER['HTTP_ACCEPT_CHARSET'] .
$_SERVER['HTTP_ACCEPT_ENCODING'] .
$_SERVER['HTTP_ACCEPT_LANGUAGE'] .
$_SERVER['HTTP_USER_AGENT']);
if (empty($_SESSION))
$_SESSION['key'] = $chk;
else if ($_SESSION['key'] != $chk)
session_destroy();

Safer Session Storage

 By default PHP sessions are stored as files inside the common /
tmp directory.

 This often means any user on the system could see active sessions and
“acquire” them or even modify their content.

 Solutions?
 Separate session storage directory via
session.save_path
 Database storage mechanism, mysql, pgsql, oci, sqlite.
 Custom session handler allowing data storage anywhere.

Shared Hosting

 Most PHP applications run in shared environments where
all users “share” the same web server instances.

 This means that all files that are involved in serving content
must be accessible to the web server (world readable).

 Consequently it means that any user could read the content
of files of all other users.

The PHP Solution

 PHP’s solution to this problem are 2 php.ini directives.

 open_basedir – limits file access to one or more specified directories.
 Relatively Efficient.
 Uncomplicated.

 safe_mode – limits file access based on uid/gid of running script
and file to be accessed.
 Slow and complex approach.
 Can be bypassed with little effort.

References

 php|architect’s Guide to PHP Security
 By Ilia Alshanetsky

 Essential PHP Security
 By Chris Shiflett

T hank you!

For more information, contact us:
OSSCube
India: +91 995 809 0987
USA: +1 919 791 5427
Web: www.osscube.com
Mail: info@osscube.com

Php Security By Mugdha And Anish

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