De-obfuscating a botnet infection

Last night I was working on a PHP website for a client when I stumbled upon this line of code whitespaced way off the screen on the first line of a few of our files:

That looked pretty suspicious to me, so I googled “$GLOBALS[‘QQOO’]” and the only result that came up was this Pastebin:

Definitely didn’t like the look of that, so I grepped the rest of our files and found the same code embedded in the same way on four more files. I removed it from all of them, re-uploaded the cleaned files, and got to work figuring out what this code was doing.

I copied the code, commented out the call to “$Ill11I1lI” at the end of “function Q0QQOOQO,” and echoed out $Ill11I1lI. This gave me “preg_replace.” A little research revealed that the “/e” flag in preg_replace makes preg_replace automatically eval the result. (This seems like a spectacularly bad design decision to me, but that’s just my opinion.)

So I echoed “Q0QQOOQO(710, 2563)” to see what code they were trying to make preg_replace run. I got back:

More preg_replace eval'ing, so I commented out the call to “$Q00Q0QOQQ” and echoed IlI11lll(721, 2563). I got back a pretty sizable string of code, which I formatted and then took the liberty of renaming what was pretty obviously a “get string” function to “get_string.” That gave me this code:

I then did a search for all calls to “get_string” and isolated them to one line each, then used a little regex-fu to generate this PHP file:

And then I used the output from that PHP script to create a Python script (I realize I could have just used PHP, but my Python-fu is stronger) to find and replace all the calls to “get_string” with their actual returned values:

That got me a file I could actually read somewhat. I started working my way down the file, renaming variables like “$QO0OQO” and “$IIl11I” to things like “$curl” and “$curl_init.” After working at it like a crossword puzzle (“What’s a name for the third parameter in fsockopen?”) for around half an hour, I was finally treated to its final, naked, legible form (which I then filled with helpful comments):

Basically this code allows the botnet owners to execute arbitrary PHP code on the infected server. Not a very friendly piece of code!

TL;DR: Found obfuscated botnet code on a client’s server and spent about an hour and a half making it readable: