PHP apps: security's low-hanging fruit
Why PHP apps accounted for 43% of security issues in 2006
PHP has become the most popular application language on the web, but common security mistakes by developers are giving PHP a bad name. Here's how PHP coding errors have become the new low-hanging fruit for attackers, contributing to the phishing problems on the web.
PHP became one of my favorite languages because of how quickly one can write a highly functional, standards-based web application with a database back-end. Unfortunately, attackers are taking these applications down even faster than they appear.
I'm sure I'll receive my share of flames under this column - but this is unfortunate, as I would hate to see such a nice language start to languish - however, for many folks there's no easier way to compromise a web server than to find a vulnerable application written in PHP.
The great rise of PHP
Let me start by saying that I'm a big fan of PHP and have written a number of web applications with it over the years. It's a great language that is now object-oriented, powerful and easy to learn, has a simple syntax, integrated SQL connectors, and high performance. It's simple to compile, very cross-platform, and has become arguably the dominant language on the web - thousands of commercial and open-source applications are available and in use.
The developers of PHP are doing many things right, from offering excellent support, protecting intellectual property in commercial applications, maintaining both the current release (5.x) and one prior, extending the object oriented approach, working with the community, and all sorts of other things. But not everything is rosy in the PHP world.
The problem is, PHP applications accounted for about 43 per cent of the security issues in 2006, according to the National Institute of Standards and Technology (NIST). Diving deeper into this number, there's the issue of determining who's most responsible for the problem. Do we blame PHP itself for being an insecure language, or do we blame the many inexperienced programers who have created vulnerable PHP applications and released them to the world?
The fact is, the vast majority of vulnerabilities found in PHP applications are due to poor programming practices, and are one step away from the language itself. Poor programming is a security problem in any language, but with PHP in particular the coding errors often lead to common web security problems such as Cross-Site Scripting (XSS), include file injection vulnerabilities, and database injection or manipulation issues.
What strikes me is how the really basic mistakes pop up again and again.
A quick tour through the SecurityFocus vulnerability database and Bugtraq and a website like milw0rm reveals many PHP applications ripe for exploitation. Many require only very simple file inclusion exploits. And that's exactly why some people are exploiting them. It's the low-hanging fruit.
It only takes a few minutes to understand a typical web application's coding errors and then search for vulnerable installations using Google. In just a few minutes, an average attacker with little talent and even less time can compromise a typical server.
While PHP application developers are by far the most responsible, there have also been numerous vulnerabilities in the PHP language itself. These are often harder to exploit, but when found they can affect the entire base of installed PHP applications on the web. So there's really two main issues at work here.
The recent departure of Stephan Esser from the internal PHP security team should also raise some eyebrows, especially after reading the explanation he gave on his blog about leaving the very group he founded. Even as a user of PHP myself and having a passion for security, I wasn't aware of some of the ongoing internal issues. Esser's Suhosin hardening project looks like one that more people should check out.
I'm focusing on the user issues in this article, but the PHP Group can still take some of the blame. They are the ones who design and grow the language, and they've chosen to take a certain route - giving application programmers more than enough rope to hang themselves, in terms of security - just as any language does. Many developers, including some very experienced ones, have indeed hung themselves due to easily made mistakes that lead to poor security.
Anyone who's experienced a server compromise due to a PHP app they didn't write and didn't audit has probably done some soul-searching and become much more wary of all the “free” PHP apps. I really enjoy using PHP, but I'm quite cautious of most applications unless it's one that is a large and successful project with a real focus on security, a dedicated security mailing list for announcements, and so on.
There are many infrastructure tools needed to secure a typical PHP web server from an unknown developer's mistakes. While this is beyond the scope of the article, I'll just list some of them here: web server hardening, CHROOT/JAIL, PHP as a separate user, an IDS with daily updates, a web application firewall, and a file integrity monitor to help detect the inevitable server compromise.
The Windows defense?
PHP shouldn't be blamed for its popularity, so I don't want readers to get the wrong idea. Many lower level languages like C/C++ are even more popular and give developers far more rope to hang themselves than PHP. Therefore, there are other issues at play.
Applications written in every language can, will, and have had a myriad of security vulnerabilities over the years. It doesn't matter if it's C++, Perl, ASP, Visual Basic, Python, or Ruby and Ruby on Rails. Every language or rapid development framework has its strengths and weaknesses. Personally, I've tended to avoid frameworks like Ruby on Rails, for example, because it felt like I was drilling my own teeth – sometimes it's a little scary not knowing exactly what's going on inside, just in case something goes wrong.
PHP has had the greatest appeal among new web programmers eager to build database applications and support XML and Ajax. The language has welcomed thousands of new programmers to the joy of programming. The PHP Group should be proud of that. But it comes with a dark side.
I have no doubt that some PHP Group developers shake their head at the very basic security mistakes that many new programmers make. The problems are all over the web. They're user issues, so they probably don't concern themselves with them. But they should.
The same mistakes are being made over and over again, and people aren't learning. This is an opportunity for the PHP Group to provide something new. How about a new SAFE_MODE that actually makes things safer in the language, and doesn't try to do what can already be done with infrastructure?
I'd like to see new defaults that limit include() and require() to only allow local files, thereby avoiding remote file injection. It would be nice to have a global way for a script to ignore all variables in the URL, avoiding unexpected variable manipulation and XSS forgeries. Maybe there's even a way to force users to filter input and escape output every time, helping to avoid SQL injection and all sorts of other common problems. That last one would go a long way.
It's an awful lot easier to compromise a typical LAMP server (Linux/Apache/MySQL/PHP) than it is to exploit a full-patched modern Windows desktop. It's not the OS, it's the application. Part of it is the popularity of PHP itself that has put so many insecure applications on the web, and part of it is how easy it is to make security mistakes when coding PHP. Without a defense-in-depth infrastructure, which is often lacking on a typical LAMP server, a PHP programming flaw often results in a full web server compromise.
The short of it all is, PHP apps can be made secure, but far too many are not.
The bigger picture
Why should the PHP Group care more about the mistakes of inexperienced programmers? That's an easy one. Many websites being exploited today are turned into phishing sites designed to steal money and identities from people who don't even know what PHP is, and in many cases barely know how to operate their own computer. In other words, the applications are abused by the criminal element. PHP doesn't need to stand for Perfect Haven for Phishing.
There are all sorts of automated scripts out there that search for vulnerable PHP applications, exploit them when found, and then automatically download a set of phishing HTML files and images that make John's Awesome Blog suddenly look like the Bank of America's login page. This also happens with ASP and Perl applications too, as well as those written in other languages, but today PHP is far more popular a target. That website owner, John, might be held responsible too if there weren't dozens of these incidents each day.
I'm not saying the PHP Group is responsible, but they could help. As architects of the language they should consider ways of hardening the language and its defaults against some of the real basic mistakes so many people are making.
I'm also not saying they should make PHP idiot-proof – think of all the wasted cycles and extra code trying to anticipate a thousand common mistakes! But there's an opportunity to continue to grow PHP without being at the expense of the security of the Internet as a whole.
If the PHP Group decides to take a leadership role in this, by adding new features or functionality that help new programmers make fewer common security mistakes, we'll all be better off.
This article originally appeared in Security Focus.
Copyright © 2007, SecurityFocus