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Be Watchful: PHP And WordPress Functions That Can Make Your Site Insecure

Before deploying a new plugin in WordPress, it’s a good idea to keep a list of easy-to-misuse functions by your side. Let’s take a closer look at some functions which you can and should use as part of a broader security strategy.

Security of a WordPress (or any) website is a multi-faceted problem. The most important step anyone can take to make sure that a site is secure is to keep in mind that no single process or method is sufficient to ensure nothing bad happens. But there are things you can do to help. One of them is to be on the watch, in the code you write and the code from others you deploy, for functions that can have negative consequences. In this article, we’ll cover exactly those: functions that a WordPress developer should think clearly before using.

WordPress itself provides a sizable library of functions, some of which can be dangerous. Beyond that, there are lots of PHP functions that a WordPress (PHP) developer will use with some frequency that can be dangerous when used.

All of these functions have legitimate and safe uses, but they’re also functions that can make it easy for your code to be abused for bad ends. We’ll cover the stuff most likely to be the cause of a security vulnerability, and why you must watch for them. We’ll first run down PHP functions in your code that bad actors can use for evil, and then talk about WordPress-specific PHP functions that can also cause headaches.

PHP Functions To Watch Out For

As we said, PHP contains a lot of easy-to-use functions. Some of those functions are notorious for the ease with which people can do bad things with them. All of these functions have a proper use, but be careful about how you use them and how other code you pull into a project does.

We’re going to assume you’ve already got magic quotes and register globals turned off if your version of PHP supports them. They were off by default in PHP 5 and up, but could be turned on for versions below 5.4. Few hosts allowed it, but I don’t think a PHP security article is truly complete without including them.

Or a mention that PHP 5.6 is the last 5.x version still with ongoing security support. You should be on it, at least. And you should have a plan to move onto PHP 7 before 5.6 support ends at the end of 2018.

After that, you’ll just have some risky functions to deal with. Starting with…

extract Is Bad News, Especially On $_POST Or Similar

Fortunately, use of PHP’s extract function has largely fallen out of favor. The core of its use was that you could run it on an array of data, and its key-value pairs would become living variables in your code. So

$arr = array(
    'red' => 5
);
extract($arr);
echo $red; // 5

would work. This is cool, but it’s also really dangerous if you’re extracting $_GET, $_POST, etc. In those cases, you’re essentially recreating the register_globals problem yourself: an outside attacker can change your variable values easily by adding a query string or form field. The best solution is simple: don’t use extract.

If you create the array you extract yourself, it’s not specifically a security problem to use extract, and in some cases, it can be useful. But all uses of it have the problem of confusing future readers. Creating an array and calling extract is more confusing than just declaring your variables. So I encourage you to do manual declarations instead, unless it’s completely unfeasible.

If you must use extract on user input, you should always use the EXTR_SKIP flag. This still has the confusion problem, but it gets rid of the possibility that your preset values can be changed by a malicious outsider via a simple query string or web form modification. (Because it “skips” already set values.)

eval Is Evil” Because Arbitrary Code Is Scary

eval in PHP and just about any other language that carries it is always high up on lists like this. And for good reason. PHP’s official documentation on PHP.net says it pretty frankly:

CAUTION: The eval() language construct is very dangerous because it allows execution of arbitrary PHP code. Its use thus is discouraged. If you have carefully verified that there is no other option than to use this construct, pay special attention not to pass any user provided data into it without properly validating it beforehand.

eval lets any arbitrary string in your program be run as if it’s PHP code. That means that it’s useful for “meta-programming” where you’re building a program that can itself build a program. It’s also really dangerous because if you ever allow arbitrary sources (like a text box on a web page) to be passed immediately to your eval string evaluator, you’ve suddenly made it trivially easy for a malicious attacker to do pretty much anything PHP can do on your server. This includes, obviously, connecting to the database, deleting files, and just about anything else someone can do when SSHed into a machine. This is bad.

If you must use eval in your program, you should go out of your way to make sure that you don’t allow arbitrary user input to be passed into it. And if you must allow arbitrary users access to the inputs to eval, confine what they can do via a blacklist of commands you don’t let them run, or (better, but far harder to implement) a whitelist which is only the commands you consider safe. Better still, only allow a small number of specific parameter changes, like only validated integers.

But always keep in mind this line from Rasmus Lerdorf, the founder of PHP:

"If `eval()` is the answer, you're almost certainly asking the wrong question."

Variations On eval

There are, in addition to the well-known eval, a variety of other ways that PHP has historically supported strings-evaluated-as code. The two that are most relevant for WordPress developers are preg_replace with the /e modifier, and create_function. Each works a little differently, and preg_replace after PHP 5.5.0 doesn’t work at all. (PHP 5.5 and below are no longer getting official security updates, and are thus best not used.)

/e Modifiers In Regex Are Also “Evil”

If you’re running PHP 5.4.x or below, you’ll want to keep an eye out for PHP preg_replace calls that end with an e. That can look like:

$html = preg_replace(
    '((.*?))e',
    '"" . strtoupper("$2") . ""',
    $html
);)
// or
$html = preg_replace(
    '~(.*?)~e',
    '"" . strtoupper("$2") . ""',
    $html
);)

PHP offers various ways to “fence” in your regular expression, but the core thing you want to be on the lookout for is “e” as the last character to the first argument of preg_replace. If it’s there, you’re actually passing the whole found segment as the argument to your inline PHP, and then evaling it. This has the exact same issues as eval if you’re letting user-input go into your function. The example code can be replaced by instead using preg_replace_callback. The advantage of this is that you’ve written out your function, so it’s harder for an attacker to change what gets evaluated. So you’d write the above as:

// uppercase headings
$html = preg_replace_callback(
    '((.*?))',
    function ($m) {
        return "" . strtoupper($m[2]) . "";
    },
    $html
);

Letting User Input create_functions Is Also Bad…

PHP also has the create_function function. This is now deprecated in PHP 7.2, but it was very similar to eval and had the same basic drawbacks: it allows a string (the second argument) to be turned into executable PHP. And it had the same risks: it’s too easy for you to accidentally give a smart cracker the ability to do anything on your server if you weren’t careful.

This one, if you’re on PHP above 5.3, is even easier to fix than preg_replace though. You can just create your own anonymous function without using strings as intermediaries. This is both more secure and more readable, at least to my eyes.

assert Is Also eval-Like

assert isn’t a function I see many PHP developers use, in WordPress or outside of it. Its intent is for very-lightweight assertions about preconditions for your code. But it does, also, support an eval-type operation. For that reason, you should be as wary of it as you are of eval. String-based assertions (the heart of why this is bad) are also deprecated in PHP 7.2, which means it should be less of concern in the future.

Including Variable Files Is A Way Of Possibly Allowing Uncontrolled PHP Execution

We’ve pretty well explored why eval is bad, but something like include or require($filename'.php') can, especially where $filename is set from a user-controllable value, be similarly bad news. The reason is subtly different from eval. Variable file names are often used for things like simple URL-to-file routing in non-WordPress PHP apps. But you might see them used in WordPress as well.

The heart of the issue that when you include or require (or include_once or require_once) you’re making your script execute the included file. It’s, more or less, conceptually evaling that file, though we rarely think about it that way.

If you’ve written all the files that your variable include might pull in, and have considered what would happen when it does, you’re fine. But it’s bad news if you’ve not considered what includeing password.php or wp-config.php will do. It’s also bad news if someone could add a malicious file and then execute your include (although at that point you probably have bigger problems).

The solution to this isn’t too hard though: hard-code includes when you can. When you can’t, have either a whitelist (better) or a blacklist of files that can be included. If the files in the whitelist (that is: you’ve audited what it does when added), you’ll know you’re safe. If it’s not on your whitelist, your script won’t include it. With that simple tweak, you’re pretty secure. A whitelist would look something like this:

$white_list = [‘db.php’, filter.php’, ‘condense.php’]
If (in_array($white_list, $file_to_include)) {
include($file_to_include);
}

Never Pass User Input To shell_exec And Variants

This is a big one. shell_exec, system, exec, and backticks in PHP all allow the code you’re running to talk to the underlying (usually Unix) shell. This is similar to what makes eval dangerous but doubled. Doubled because if you let user-input through here carelessly, the attacker isn’t even bound by the constraints of PHP.

The ability to run shell commands from PHP can be very useful as a developer. But if you ever let a user’s input in there, they have the ability to surreptitiously gain lots of dangerous powers. So I’d go so far as to say that user input should never be passed to shell_exec-type functions.

The best way I could think to handle this type of situation, if you were tempted to implement it, would be to provide users access to a small set of known-safe predefined shell commands. That might be possible to secure. But even then I’d warn you to be very careful.

Watch For unserialize; It Runs Code Automatically

The core action of calling serialize on a living PHP object, storing that data off somewhere, and then using that stored value later to unserialize that object back to life is cool. It’s also reasonably common, but it can be risky. Why risky? If the input to that unserialize call is not completely secure (say it is stored as a cookie rather than in your database…), an attacker can change the internal state of your object in a way that makes the unserialize call do something bad.

This exploit is more esoteric and less likely to be noticed than an eval issue. But if you’re using a cookie as a storage mechanism for serialized data, don’t use serialize for that data. Use something like json_encode and json_decode. With those two PHP will never auto-execute any code.

The core vulnerability here is that when PHP unserializes a string into a class, it calls the magic __wakeup method on that class. If unvalidated user input is allowed to be unserialized, something like a database call or file deletion in the __wakeup method could potentially be pointed at a dangerous or undesirable location.

unserialize is distinct from the eval vulnerabilities because it requires the use of magic methods on objects. Rather than creating their own code, the attacker is forced to abuse your already-written methods on an object. Both the __destruct and __toString magic methods on objects are also risky, as this OWASP Wiki page explains.

In general, you’re OK if you don’t use the __wakeup, __destruct, or __toString methods in your classes. But because you may later see someone add them to a class, it’s a good idea to never let a user near your calls to serialize and unserialize and pass all public data for that sort of use though something like JSON (json_encode and json_decode) where there is never automatic code execution.

Fetching URLs With file_get_contents Is Risky

A common practice when quickly writing some PHP code that has to call an external URL is to reach for file_get_contents. It’s quick, it’s easy, but it’s not super secure.

The issue with file_get_contents is subtle, but it’s common enough that hosts will sometimes configure PHP not to even allow you to access external URLs. This is meant to protect you.

The issue here is that file_get_contents will fetch remote pages for you. But when it does that, it doesn’t check on the integrity of the HTTPS protocol connection. What that means is that your script could potentially be a victim of a man-in-the-middle attack which would allow an attacker to put pretty much whatever they want into your file_get_contents page result.

This is a more esoteric attack. But to protect against it when I write modern (Composer-based) PHP, I almost always use Guzzle to wrap the more-secure cURL API. In WordPress, it’s even easier: use wp_remote_get. It works much more consistently than file_get_contents, and it’ll default to verifying SSL connections. (You can toggle that off, but, um, maybe don’t…) Better still, but slightly more annoying to talk about, are wp_safe_remote_get, etc. These work identically to the functions without safe_ in their name, but they’ll make sure that unsafe redirects and forwards do not happen along the way.

Don’t Blindly Trust URL Validation From filter_var

So this one’s a little obscure, so props to Chris Weigman for explaining it in this WordCamp talk. In general, PHP’s filter_var is a great way to either validate or sanitize data. (Although, don’t get confused about which you’re trying to do…)

The issue here is pretty specific: if you’re trying to use it to ensure that a URL is safe, filter_var doesn’t validate the protocol. That isn’t often an issue, but if you’re passing user input to this method for validation, and are using FILTER_VALIDATE_URL, something like javascript://comment%0aalert(1) will get through. That is, this can be a very good vector for a basic XSS attack in a place you won’t have expected.

For validating a URL, WordPress’s esc_url function will have a similar impact, but only lets through allowed protocols. javascript is not in the default list, so it would keep you safe. However, unlike filter_var, it’ll return an empty string (not a false) for a disallowed protocol that is passed to it.

WordPress-specific Functions To Keep An Eye On

In addition to core-PHP potentially-vulnerable functions, there are some WordPress-specific functions that can be a bit of a gotcha. Some of these are very similar to the variety of dangerous functions listed above, some a little different.

WordPress Unserializes With maybe_unserialize

This one’s probably obvious if you read the above. In WordPress there’s a function called maybe_unserialize and, as you’d guess, it unserializes what’s passed to it if need be.

There’s not any new vulnerability that this introduces, the issue is simply that just like the core unserialize function, this one can cause a vulnerable object to be exploited when it’s unserialized.

is_admin Doesn’t Answer If A User Is An Administrator!

This one’s pretty simple, but the function is ambiguous in name, and so it’s prone to confuse people or be missed if you’re in a hurry. You should always check that a user who’s trying to perform an action in WordPress has the rights and privileges necessary to perform that action. In order to that, you should use the current_user_can function.

But you might, mistakenly, think that is_admin will tell if you if the current user is an Administrator-level account and thus should be able to set an option your plugin uses. This is a mistake. What is_admin does in WordPress is instead telling you if the current page load is on the administration side of the site (vs. the front-side). So every user who can access an administration page (like the “Dashboard”) will be able to potentially pass this check. As long as you remember that is_admin is about the type of page, not the current user, you’ll be fine.

add_query_arg() Doesn’t Sanitize URLs

This isn’t so common, but there was a big wave of updates in the WordPress ecosystem a few years ago because the public documentation on this functions wasn’t correct. The core issue is that the add_query_arg function (and its inverse remove_query_arg) doesn’t automatically sanitize the site URL if a URL wasn’t passed to it, and people had thought that it did. Many plugins had been misled by the Codex, and were using this unsafely as a result.

The core thing they had to do different: sanitize the result of a call to this function before using it. If you do that, you’re really safe from the XSS attacks that you thought they were. So that looks something like:

echo esc_url( add_query_arg( 'foo', 'bar' ) );

$wpdb->query() Is Open To SQL Injection Attack

If you know about SQL injection, this may seem silly, even superfluous to list. Because the thing is that any way you access a database (say using PHP’s mysqli or PDO database drivers) for creating database queries that allow for SQL injection attacks.

The reason I’m specifically calling out $wpdb->query is that some other methods (like insert, delete, etc.) on $wpdb take care of injection attacks for you. Additionally, if you’re used to making basic WordPress database queries with WP_Query or similar, you won’t have needed to consider SQL injection. This is why I’m calling it out: to make sure you understand an injection attack on the database is possible when you first try to use $wpdb to make your own query.

What to do? Use $wpdb->prepare() and then $wpdb->query(). You’ll also want to make sure you prepare before other “getting” methods of $wpdb like, get_row() and get_var(). Otherwise Bobby Tables might get you.

esc_sql Doesn’t Secure You From SQL Injection Either

For most WordPress developers, I’d say that esc_sql doesn’t register with any meaning, but it should. Ase we just mentioned, you should be using wpdb->prepare() before you make any database query. This’ll keep you safe. But it’s tempting and understandable that a developer may reach for esc_sql instead. And they might expect it’d be safe.

The issue is that esc_sql doesn’t have as robust protections against SQL injection. It’s really a glorified version of PHP’s add_slashes function, which you’ve been discouraged from using to protect your database for years.

There’s More To Do, But This Is A Big Start

There’s a lot more to security than just being on the lookout for functions in your code that attackers can misuse. We didn’t, for example, discuss in much depth the need to validate and sanitize all data you receive from users and escape it before you put it into a webpage (though I did recently publish an article on that topic, “Protecting Your WordPress Site Against A Cross-Site Scripting Attack”). But you can and should use this as part of a broader security strategy.

Keeping this list of easy-to-misuse functions by your side as you do a final inspection before deploying a new plugin in WordPress is a great idea. But you’ll also want to make sure you trust the security of your hosting, make sure your users have good passwords, and much more.

The core thing to remember about security is that your weakest link is the one that matters. Good practices in one area bolster possible weak-points somewhere else, but they can never totally correct it. But be watchful of these functions, and you’ll have a leg-up on most.

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