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A Hitchhiker's Guide to Cross-Site Scripting (XSS) in PHP (Part 1): How Not To Use Htmlspecialchars() For Output Escaping

Note: This article was originally published at Planet PHP on 17 April 5340.
Planet PHP


(Photo credit: bertboerland)

In recent weeks, I consulted with the second most intelligent species on the planet: Dolphins. Dolphins are renowned across the known Universe for their awesome programming skills. After all, it was they who developed such insightful works as aoEvolution By Examplea, aoDude! We Wrote The Laws Of Physics!a, and aoHow Many Humans Does It Take To Screw Up A Planet?a. The answer to the last will be published on 01/01/2013 after the experiment is shut down and sent to a landfill site assuming the Supreme Spaghetti Monster signs off on the permit.

Dolphins think we are really dumb and theorise that this level of stupidity has one obvious cause: self-imposed ignorance. We are, after all, only the third most intelligent species on Earth and appear to have aspirations to lower our IQ just a bit more.

While it's no harm poking fun at ourselves, in PHP we do have a serious problem. Cross-Site Scripting (XSS) remains one of the most significant classes of security problems afflicting PHP applications. Despite years of education, community awareness and the development of frameworks which can offer a huge boost in consistent practices - things are not getting any better.

So, I finally figured out what the core problem is: PHP programmers are completely clueless about XSS. It's that simple. Instead of going out and studying the topic, we blindly follow some preferred herd of people offering advice with heartfelt conviction despite the fact that they are probably just as ignorant as the rest of us. Does that sound like the behaviour of something which allegedly evolved into an intelligent species? The result is a mix of ignorance and stagnant knowledge that leaves PHP in an unenviable position beset by wrongheaded zealots.

To get the ball rolling, this two-part article series is a tour of how NOT to use the htmlspecialchars() function that is typically pressed ganged into service as PHP's universal output escaper. By offering an example based guide, I hope it will illustrate just how many ways a prospective attacker using XSS can exploit this function's misuse to pull off a successful attack. The examples were written for PHP 5.3, so 5.4 users may need to imagine they still have 5.3 installed and/or lodge an official complaint with somebody who looks like they keep a complaints box handy (your local fast food restaurant is a good start).

This example led approach has another motive. Simple examples can be translated into unit tests. Ideally, many of the current crop of frameworks can use this article as a guide to what their unit tests should be looking for. This also makes it far easier for everyday programmers to consume the article and run around the place, drunk with ungodly power, identifying issues in the libraries, frameworks and other projects that they rely on.

To help us on the path of enlightenment before it's too late (I'd lodge an appeal with the Supreme Spaghetti Monster but apparently the Mayans already tried and failed), I also invite other PHP programmers to blog about a security topic over the next month or two. Give programmers one last chance to get it right before the Planet is demolished by the Vogon destructor fleet. Just pick a topic that drives you up the walls in defiance of gravity and spend an hour writing something useful and (optionally) expletive filled. Every little bit helps.

What Is Htmlspecialchars()?

According to

Truncated by Planet PHP, read more at the original (another 75296 bytes)