1. Unvalidated Parameters
Most importantly, turn off register_globals. This
configuration setting defaults to off in PHP 4.2.0 and later. Access
values from URLs, forms, and cookies through the superglobal arrays $_GET, $_POST, and $_COOKIE.
Before you use values from the superglobal arrays, validate them to
make sure they don’t contain unexpected input. If you know what type of
value you are expecting, make sure what you’ve got conforms to an
expected format. For example, if you’re expecting a US ZIP Code, make
sure your value is either five digits or five digits, a hyphen, and four
more digits (ZIP+4). Often, regular expressions are the easiest way to
validate data:
if (preg_match('/^\d{5}(-\d{4})?$/',$_GET['zip'])) {
$zip = $_GET['zip'];
} else {
die('Invalid ZIP Code format.');
}
If you’re expecting to receive data in a cookie or a hidden form
field that you’ve previously sent to a client, make sure it hasn’t been
tampered with by sending a hash of the data and a secret word along with
the data. Put the hash in a hidden form field (or in the cookie) along
with the data. When you receive the data and the hash, re-hash the data
and make sure the new hash matches the old one:
// sending the cookie
$secret_word = 'gargamel';
$id = 123745323;
$hash = md5($secret_word.$id);
setcookie('id',$id.'-'.$hash);
// receiving and verifying the cookie
list($cookie_id,$cookie_hash) = explode('-',$_COOKIE['id']);
if (md5($secret_word.$cookie_id) == $cookie_hash) {
$id = $cookie_id;
} else {
die('Invalid cookie.');
}
If a user has changed the ID value in the cookie, the hashes won’t
match. The success of this method obviously depends on keeping $secret_word
secret, so put it in a file that can’t be read by just anybody and
change it periodically. (But remember, when you change it, old hashes
that might be lying around in cookies will no longer be valid.)
2. Broken Access Control
Instead of rolling your own access control solution, use PEAR modules. Auth does cookie-based authentication for you and Auth_HTTP does browser-based authentication.
3. Broken Account and Session Management
Use PHP’s built-in session management functions for secure,
standardized session management. However, be careful how your server is
configured to store session information. For example, if session
contents are stored as world-readable files in /tmp, then any user that
logs into the server can see the contents of all the sessions. Store the
sessions in a database or in a part of the file system that only
trusted users can access.
To prevent network sniffers from scooping up session IDs,
session-specific traffic should be sent over SSL. You don’t need to do
anything special to PHP when you’re using an SSL connection, but you do
need to specially configure your webserver.
4. Cross-Site Scripting (XSS) Flaws
Never display any information coming from outside your program
without filtering it first. Filter variables before including them in
hidden form fields, in query strings, or just plain page output.
PHP gives you plenty of tools to filter untrusted data:
htmlspecialchars() turns & > " < into their HTML-entity equivalents and can also convert single quotes by passing ENT_QUOTES as a second argument.strtr() filters any characters you’d like. Pass strtr() an array of characters and their replacements. To change ( and ) into their entity equivalents, which is recommended to prevent XSS attacks, do:
$safer = strtr($untrusted, array('(' => '(', ')' => ')'));strip_tags() removes HTML and PHP tags from a string.utf8_decode() converts the ISO-8859-1 characters in a
string encoded with the Unicode UTF-8 encoding to single-byte ASCII
characters. Sometimes cross-site scripting attackers attempt to hide
their attacks in Unicode encoding. You can use utf8_decode() to peel off that encoding.
5. Buffer Overflows
You can’t allocate memory at runtime in PHP and their are no pointers
like in C so your PHP code, however sloppy it may be, won’t have any
buffer overflows. What you do have to watch out for, however, are buffer
overflows in PHP itself (and its extensions.) Subscribe to the
php-announce mailing list to keep abreast of patches and new releases.
6. Command Injection Flaws
Cross-site scripting flaws happen when you display unfiltered,
unescaped malicious content to a user’s browser. Command injection flaws
happen when you pass unfiltered, unescaped malicious commands to an
external process or database. To prevent command injection flaws, in
addition to validating input, always escape user input before passing it
to an external process or database.
If you’re passing user input to a shell (via a command like exec(), system(),
or the backtick operator), first, ask yourself if you really need to.
Most file operations can be performed with native PHP functions. If you
absolutely, positively need to run an external program whose name or
arguments come from untrusted input, escape program names with escapeshellcmd() and arguments with escapeshellarg().
Before executing an external program or opening an external file, you should also canonicalize its pathname with realpath(). This expands all symbolic links, translates . (current directory) ..
(parent directory), and removes duplicate directory separators. Once a
pathname is canonicalized you can test it to make sure it meets certain
criteria, like being beneath the web server document root or in a user’s
home directory.
If you’re passing user input to a SQL query, escape the input with addslashes() before putting it into the query. If you’re using MySQL, escape strings with mysql_real_escape_string() (or mysql_escape_string()
for PHP versions before 4.3.0). If you’re using the PEAR DB database
abstraction layer, you can use the DB::quote() method or use a query
placeholder like ?, which automatically escapes the value that replaces the placeholder.
7. Error Handling Problems
If users (and attackers) can see the raw error messages returned from
PHP, your database, or external programs, they can make educated
guesses about how your system is organized and what software you use.
These educated guesses make it easier for attackers to break into your
system. Error messages shouldn’t contain any descriptive system
information. Tell PHP to put error messages in your server’s error log
instead of displaying them to a user with these configuration
directives:
log_errors = On
display_errors = Off
8. Insecure Use of Cryptography
The mcrypt extension provides a standardized interface to many popular cryptographic algorithms. Use mcrypt
instead of rolling your own encryption scheme. Also, be careful about
where (if anywhere) you store encryption keys. The strongest algorithm
in the world is pointless if an attacker can easily obtain a key for
decryption. If you need to store keys at all, store them apart from
encrypted data. Better yet, don’t store the keys and prompt users to
enter them when something needs to be decrypted. (Of course, if you’re
prompting a user over the web for sensitive information like an
encryption key, that prompt and the user’s reply should be passed over
SSL.)
9. Remote Administration Flaws
When possible, run remote administration tools over an SSL connection
to prevent sniffing of passwords and content. If you’ve installed
third-party software that has a remote administration component, change
the default administrative user names and passwords. Change the default
administrative URL as well, if possible. Running administrative tools on
a different web server than the public web server that the
administrative tool administrates can be a good idea as well.
10. Web and Application Server Misconfiguration
Keep on top of PHP patches and security problems by subscribing to
the php-announce mailing list. Stay away from the automatic PHP source
display handler (AddType application/x-httpd-php-source .phps), since it lets attackers look at your code. Of the two sample php.ini files distributed with PHP ( php.ini-dist and php.ini-recommended), use php.ini-recommended as a base for your site configuration.
