What security issues should I look out for in PHP

Question

I just starting out learning PHP, I've been developing web apps in ASP.Net for a long while. I was wondering if there are any PHP specific security mistakes that I should be looking out for.

So, my question is what are the top security tips that every PHP developer should know.

Please keep it to one tip per answer so people can vote up down effectively.

Answer 1

here is a link of good PHP security programming practices.

http://phpsec.org/

Most of the security issues revolve around user input (naturally) and making sure they don't screw you over. Always make sure you validate your input.

http://htmlfixit.com/cgi-tutes/tutorial_PHP_Security_Issues.php

Answer 2

Avoid using register_globals

Answer 3

Most of the security issues related to PHP come from using unparsed "outside" (GET/POST/COOKIE) variables. People put that kind of data directly into file paths or sql queries, resulting in file leakage or sql injections.

Answer 4

If you're using a mysql database make sure you call mysql_real_escape_string when sending data to the database

Answer 5

Have a look at the Suhosin Hardening Patch, and check out the security vulnerabilities that it addresses.

Answer 6

Often introductory tutorials don't talk at all about checking data from users. Like all programming environments, never trust the data you get from users. Learn to use functions like is_numeric(), isset(), and mysql_real_escape_string() to protect your system.

There are also features that allow you to access remote files, and other creative things. I'd avoid those until you have a good understand of how and when they work (often they are disabled for security reasons).

Answer 7

OWASP provides a lot of insight into security issues that are the biggest problems in applications today. It is nice to see that they have a PHP dedicated page available

http://www.owasp.org/index.php/PHP_Top_5

Answer 8

don't use "Register Global Variables" and filter user input for xss and injections

Answer 9

Language Vs Programmer. You can write the most serious vulnerability and you won't get a warning or error message. Vulnerabilities can be as simple as adding or removing 2 characters in your code. There are hundreds of different types of vulnerabilities that affect PHP applications. Most people think of XSS and Sql Injection because they are the most popular.

Read the OWASP top 10.

Answer 10

1. Always sanitize and validate data passed from the page
2. In conjunction with #1, always properly escape your output
3. Always turn display_errors off in production
4. If using a DB backend use a driver that supports/emulates prepared statements and use without prejudice :-)

Answer 11

• Cross Site Scripting (XSS) Wiki, Google
• Cross Site Request Forgery (XSRF/CSRF) Wiki, Google (thanks Rook)
  • Session Fixation Wiki, Google
• SQL Injection (SQLi) Wiki, Google
• Turn off error messages in Production environments
• Keep any "include" code in a directory that is not web-accessible (either deny access or keep it outside of the webroot)
• Here's an article I wrote about storing passwords in a secure way, and if you don't feel like taking my word for it, check the links at the bottom.
• Also linked in my article, but given its own separate link here, is a paper published by M.I.T. called The DOs and DON'Ts of Client Authentication on the Web ^[PDF]. While some of its info (recommendation to use MD5 hash, for one) is somewhat out of date simply because of what-we-know-now versus what-we-knew-then, the overall principles are very strong and should be considered.
• One of Rooks' links reminded me of another important set of restrictions
  • Turn off Register Globals (This is the default now, so I hadn't mentioned it before)
  • When dealing with file uploads, be sure to use is_uploaded_file() to validate that a file was uploaded and move_uploaded_file() instead of copy() or rename().
    • Read this section of the PHP Manual if you need to know why (and you do).
• Since I've now mentioned him twice, check out Rooks's Answer (http://stackoverflow.com/questions/2275771/what-are-the-most-important-safety-precautions-that-a-php-developer-needs-to-know#2275788) as it includes a link to a document which contains (Non-PHP-Specific) information on the most important security concerns (and this therefore probably the right answer).

Answer 12

There are tons of safety precautions. I can recommend a book Chris Shiflett: PHP and Web Application Security.

http://phpsecurity.org/

Answer 13

The PHPSec Guide gives a good overview.

Answer 14

(In no particular order)

1. Always check that register globals are OFF
2. Always check that magic quotes are OFF
3. Make sure you understand SQL injection attacks
4. Turn OFF error reporting in production

EDIT: For the "newbies" out there this is a basic why (and since I have time to explain this):

1. Register globals is an aberration. It's the ultimate security hole ever. For example, if register_globals is on, the url http://www.yourdomain.com/foo.php?isAdmin=1 will declare $isAdmin as a global variable with no code required. I don't know why this "feature" has made it's way to PHP, but the people behing this should have the following tattooed on their forehead: "I invented PHP Register Globals" so we can flee them like pest when we see them!

2. Magic quotes is another dumb idea that has made it's way to PHP. Basically, when ON PHP will escape quotes automatically (' become \' and " become \") to help with SQL injection attacks. The concept is not bad (help avoid injection attacks), but escaping all GET, POST and COOKIE values make your code so much complex (for example, have to unescape everytime when displaying and data). Plus if one day you switch this setting OFF without doing any change to your code, all your code and/or data is broken and (even more) vulnerable to injection attacks (yes even when ON you are vulnerable).

3. Your databse data is your most valuable thing on your site. You don't want people to mess with it, so protect yourself and read things about it and code with this in mind.

4. Again this can lead to security concerns. The error message can give hints to hackes on how your code works. Also these messages don't mean anything to your visitors, so why sow them?

Answer 15

1. Always Close you SQL Connection.
2. Always Release SQL results.
3. Always Scrub all variables your putting into a database.
4. When deleteing or dropping from sql use limit 1 just in case.
5. When developing make sure you have a lock on things to keep the undesirable out. If its open and you know not to load the page right now because it could break something, doesn't mean other people do.
6. Never use Admin or Root as your server log in name.

Answer 16

Whenever possible, use prepared statements (tutorial. It's almost a must whenever dealing with user input (I say "almost" because there are a few use cases where they don't work), and even when not dealing with input, they keep you in the habit. Not to mention they can lead to better performance, and are a LOT easier, once you get into the swing of things, than piecemeal sanitizing.