Tulsa techfest2010 security

What is Security?
Jason Ragsdale
Sr. Technical Yahoo
Yahoo!
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Friday, November 12, 2010

A good place to start...
php.ini
display_errors = Off
register_globals = Off
open_basedir = ....
What about safe_mode??

Don’t be stupid
Never require/include any ﬁle based on user
input without checking it ﬁrst.
<?php
if (isset($_GET[‘page’])
{
require $_GET[‘page’];
}
?>
URL: script.php?page=/etc/passwd
....
nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false
root:*:0:0:System Administrator:/var/root:/bin/sh

Don’t be stupid... 2
If your solution uses eval().... you are doing it
wrong
<?php
if (isset($_GET[‘input’])
{
eval($_GET[‘input’]);
}
?>
URL: script.php?input=passthru(“cat /etc/passwd”);
....
nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false
root:*:0:0:System Administrator:/var/root:/bin/sh

Input Filtering
What is input?
Anything the user or interacting system
sends to your site i.e. ($_POST, $_GET,
$_REQUEST, $_COOKIE...)
What is a whitelist?
“A list of approved or favored items”
What is a blacklist?
“A list persons who are disapproved of or
are to be punished or boycotted”

Input Validation
Unﬁltered code
Example
<?php
if (isset($_POST[‘username’]))
{
$username = $_POST[‘username’];
}

Input Validation
ctype
Example
<?php
$clean = array();
if (ctype_alnum($_POST[‘username’]))
{
$clean[‘username’] = $_POST[‘username’];
}

Input Validation
Zend_Filter_Input
Example
<?php
{
$filterChain = new Zend_Filter();
$filterChain->addFilter(new Zend_Filter_Alpha())
->addFilter(new Zend_Filter_StringToLower());
$username = $filterChain->filter($_POST[‘username’]);
}

Input Validation
php/ﬁlter
Example
<?php
{
$username = ﬁlter_var(‘username’, FILTER_VALIDATE_REGEXP,
array(
‘options’=>
array(‘regexp’=>’/([a-zA-Z0-9]+)/’)
)
);
}

Output Encoding
What is output?
Anything sent back to the user / sender
of the request (RSS Feed, Form Validate,
User created data...)
htmlentities Example
<?php
$str = “A ‘quote’ is <b>bold</b>”;
//Outputs: A ‘quote’ is <b>bold</b&gt
echo htmlentities($str);
//Outputs: A 'quote' is <b>bold</b&gt
echo htmlentities($str, ENT_QUOTES);

Tim Stiles
At this point mention XmlWriter and all
it’s wonders.... ;)

Database Inputs
(or: How I Learned to Stop Worrying and Love the Users)

How do i deal with it?
A input filter (whitelist) combined with
prepared statements... DONE
$clean = array();
if (ctype_alnum($_POST[‘username’]))
{
$clean[‘username’] = $_POST[‘username’];
}
$sql = “SELECT ùsername` FROM ùsers` WHERE ùsername`
= :username”;
$sth = $dbh->prepare($sql);
$sth->execute(array(‘:username’=> $clean[‘username’]));
$username = $sth->fetchColumn();

XSS
(Cross Site Scripting)
Example
<?php
echo “<p> Welcome back, {$_GET[‘username’]}.</p>”;
?>
------
Let’s exploit this
------
<p> Welcome back, <script> ....do something bad here... </script>.
</p>

XSS
(Cross Site Scripting)
If you do the two items we spoke about
Input Filtering
Output Encoding
You most likely are still vulnerable, but it’ll be a
lot harder to exploit
Almost impossible to completely nullify all
security / XSS stuff (new browsers and plugins all
the time + bad guys keep getting smarter)

CSRF
(Cross Site Request Forgeries)
Somewhere on MyFavoriteForum.com:
<img src=”bank.com/transfermoney.php?
to=me&amount=100.00”>
...if users are logged in, invisible actions can
be taken on their behalf, with their
authority.

CSRF
(Cross Site Request Forgeries)
Solutions
Sign your forms with a token (MD5 hash
with a secret key)
Validate the token before processing the
data
This can be done with Cookie and Session
data as well

Protecting Source Code
Make sure all code ﬁle extensions are
blocked from viewing
You can remove them from the html root
Or block them in the apache conﬁg
<FilesMatch “.inc$”>
order deny, allow
deny from all
</FilesMatch>

Protecting Source Code
Watch for editor backup files too!
.file.php.tmp
file.php~
etc...
Or don’t edit code on production boxes.

Code Auditing
Set a standard for your team (and yes a
team can be a single person)
Input Filtering Methods
Output Encoding Methods
Database Access Methods
Search code security points (echo, print...)
Enforce these methods

Code Auditing
Default to Secure.
Make being unsecure obvious and auditable
YAHOO_GET_RAW( “blah” )

System Security
Your website is only as secure as the
server/network is it hosted on
Perform regular package updates
Make sure you apply any updated PHP or
Apache code as soon as you can, there are
reasons for security releases

Firewalls & Access
Control
Only allow access to ports that you need to
80 - Web
443 - SSL
22 - SSH

Misc...
Signed Data (MD5)
Encrypted passwords in the DB
Config Files outside DOCROOT
Secret keys outside code, in config files
If it’s customer data USE SSL

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