Didier Stevens

Thursday 14 December 2017

Update: plugin_biff.py Version 0.0.2 / oledump.py Version 0.0.31

Filed under: maldoc,My Software,Update — Didier Stevens @ 0:00

This is an update to plugin_biff, the oledump plugin to parse the BIFF format (used in .xls files).

New options allow to search for opcodes (-o) and strings/bytes (-f) inside BIFF records:

 

oledump_V0_0_31.zip (https)
MD5: 63B2B5ECE2BC46B937D33A6494F7F6A0
SHA256: D2CF42662897642DF27C863F6C246CE70019EDF03F275354A7A505DCE27632D1

Monday 13 November 2017

WebDAV Traffic To Malicious Sites

Filed under: maldoc — Didier Stevens @ 0:00

If observed WebDAV traffic to malicious sites in the past (in proxy logs), and recently I took some time to take a closer look.

TL;DR: when files are retrieved remotely with the file:// URI scheme on Windows, Windows will fallback to WebDAV when SMB connections can not be established.

I did my tests with 2 Windows 7 VMs on the same subnet, one Windows 7 machine with IIS/WebDAV, and the other Windows 7 machine with Word 2016 and a .docx document with a remote template (template.dotx) (using the file:// URI scheme). The Windows firewall on the IIS machine was configured to block ports 139 and 445.

When the .docx document is opened, Word will retrieve the template:

Here is the URI:

First we see attempts to connect on ports 445 and 139 on the IIS machine (SYN packets):

These come from the “System process”:

There are no packets coming back from the IIS machine (I blocked port 139 and 445), and after almost 30 seconds we see an HTTP request to port 80 on the IIS machine:

This is a WebDAV request, notice the User Agent string “DavClnt”:

This TCP connection originates from the Word process:

And about 3 seconds after this request, we get another WebDAV request:

For this request, the User Agent string is “Microsoft-WebDAV-MiniRedir/6.1.7601”.

This TCP connection originates from the WebClient service:

This service was not started:

The svchost service host process will load and start the WebClient service:

WebClient (WebClnt.dll) is the WebDAV service:

To summarize, when the file:// URI scheme is used in a Word document and SMB connections can not be established, we will see WebDAV requests from:

  1. Word (DavClnt)
  2. WebClient service (Microsoft-WebDAV-MiniRedir/6.1.7601)

I’ve observed the same behavior with Windows 10 (with a different version number for the WebClient User Agent string).

When the document is opened a second time, there is no WebDAV request from Word (1), only requests from the WebClient service (2).

When I stop the WebClient service and reopen the document, there is first a WebDAV request from Word (1) followed by requests from the WebClient service (2).

When I disable the WebClient service and reopen the document, there are no more WebDAV requests at all.

 

Thursday 2 November 2017

Analyzing Metasploit’s Office Maldoc

Filed under: maldoc — Didier Stevens @ 0:00

Metasploit has a module to create Microsoft Word document with macros (.docm): office_word_macro.

Documents generated with this module are not that hard to analyze and detect, because they always use the same VBA code. As I explain in my workshops and trainings, although the “new” Office file format (OOXML) is a ZIP container for XML files, VBA code is still stored inside a binary file (vbaProject.bin) using the “old” file format (Compound File Binary Format, or ole file as I like to call it). This Metasploit module always uses the same vbaProject.bin file (inside the template file), and I explain how to analyze and detect it in this video:

I show YARA rules and ClamAV signatures in this video to detect documents created with this Metasploit module.

Here are the YARA rules:

/*
  Version 0.0.1 2017/08/20
  Source code put in public domain by Didier Stevens, no Copyright
  https://DidierStevens.com
  Use at your own risk

  History:
    2017/08/20: start
*/

import "hash"

rule metasploit_office_word_macro_ID_GUID {

    meta:
        description = "Detect Metasploit's office_word_macro unique GUID"

    strings:
        $ID = "ID=\"{BB64F33D-3617-FA44-AFC9-63F65314A8A3}\""

    condition:
        $ID
}

rule metasploit_office_word_macro_vbaproject_bin_zipped {
    meta:
        description = "Detect .docm files created with Metasploit's office_word_macro exploit"

    strings:
        $a = {776F72642F76626150726F6A6563742E62696EED3B0D7853D775E75D3D0959B6B1640C7120908B4CB04C2421C9B22D3B98EADF86D860B0032421C1FA79C222B2A44A4FD8E4A795B1D3928435AC5D33CAD20E42DAA62DEB489AB07EE9BA8976FB42F3B5DF48D3ED4BBA7531C99666FDBE0E4AB32F69B6C43BF7BD275BFE2350BA}

    condition:
        $a and hash.md5(@a + 19, 5962) == "e5995aba8551f30cc15c87ee49fb834a"
}

The first rule (metasploit_office_word_macro_ID_GUID) detects the vbaProject.bin file used by this Metasploit module based on the unique ID ({BB64F33D-3617-FA44-AFC9-63F65314A8A3}) stored inside stream PROJECTwm of file vbaProject.bin. This rule must be used with a tool that can scan inside ZIP files, like zipdump.py or ClamAV.

If you can’t use such a tool, you can still use the second rule (metasploit_office_word_macro_vbaproject_bin_zipped) with the standard YARA scanner: this rule looks for the datastream of the compressed vbaProject.bin file inside Office files.

Here are the ClamAV signatures:

Signature to be put inside a .ndb file:
metasploit_office_word_macro_ID_GUID:0:*:49443D227B42423634463333442D333631372D464134342D414643392D3633463635333134413841337D22
Signature to be put inside a .hdb file:
1788454ae206101fa6febf99005ce03b:15872:metasploit_office_word_macro_vbaproject_bin

The first signature (metasploit_office_word_macro_ID_GUID) detects the unique ID (just like the first YARA rule), and the second signature (metasploit_office_word_macro_vbaproject_bin) detects the vbaProject.bin file based on the MD5 hash (1788454ae206101fa6febf99005ce03b).

ClamAV is able to scan inside OOXML/ZIP files.

Tuesday 31 October 2017

Analyzing A Malicious Document Cleaned By Anti-Virus

Filed under: maldoc,Malware — Didier Stevens @ 0:00

@futex90 shared a sample with me detected by many anti-virus programs on VirusTotal but, according to oledump.py, without VBA macros:

I’ve seen this once before: this is a malicious document that has been cleaned by an anti-virus program. The macros have been disabled by orphaning the streams containing macros, just like when a file is deleted from a filesystem, it’s the index that is deleted but not the content. FYI: olevba will find macros.

Using the raw option, it’s possible to extract the macros:

I was able to find back the original malicious document: f52ea8f238e57e49bfae304bd656ad98 (this sample was analyzed by Talos).

The anti-virus that cleaned this file, just changed 13 bytes in total to orphan the macro streams and change the storage names:

This can be clearly seen using oledir:

 

Monday 31 July 2017

Update: translate.py Version 2.5.0

Filed under: maldoc,My Software,Update — Didier Stevens @ 20:17

I analyzed a malicious document send by a reader of the Internet Storm Center, and to decode the payload I wanted to use my tool translate.py.

But an option was lacking: I had to combine 2 byte streams to result in the decoded payload, while translate will only accept one byte stream (file, stdout, …).

I solved my problem with a small custom Python script, but then I updated translate.py to accept a second file/byte stream (option -2).

This is how I use it to decode the payload:

 

translate_v2_5_0.zip (https)
MD5: 768F895537F977EF858B4D82E0E4387C
SHA256: 5451BF8A58A04547BF1D328FC09EE8B5595C1247518115F439FC720A3436519F

Tuesday 18 July 2017

.ISO Files With Zone.Identifier

Filed under: maldoc,Malware — Didier Stevens @ 22:20

An .iso file downloaded from the Internet (thus with a Zone.Identifier ADS) opened in Windows 10 will not propagate this “mark-of-the-web” to the contained files.

Here is an example with file demo.iso, marked as downloaded from the Internet:

When this file is opened (double-clicked), it is mounted as a drive (E: in this example), and we see the content (a Word document: demo.docx):

This file is not marked as downloaded from the Internet:

Word does not open it in Protected View:

Monday 10 July 2017

Select Parent Process from VBA

Filed under: Forensics,Hacking,maldoc,Malware,My Software — Didier Stevens @ 0:00

Years ago I wrote a C program to create a new process with a chosen parent process: selectmyparent. And recently I showed what process monitor and system monitor report when you use this tool.

Starting a new process with a chosen parent process can be done from VBA too, as shown in this video (I’m not sharing the VBA code):

Thursday 6 July 2017

I Will Follow (no, not talking about social media)

Filed under: maldoc,Malware — Didier Stevens @ 20:54

I can’t help feeling some kind of satisfaction when a friend uses my tools to analyze malware, and hacks his way to a solution when my tool falls short 🙂

In this nice blogpost, @bluejay00 analyzes RTF malware with my rtfdump.py tool. But because of obfuscation, rtfdump.py is not able to extract the object. @bluejay00 understands this, deobfuscates the RTF sample with an editor, and is then able to get my tool to work correctly.

I’ll just show how I would have used my translate.py tool to remove the obfuscation:

 

Thursday 20 April 2017

Malicious Documents: The Matryoshka Edition

Filed under: maldoc,Malware,PDF — Didier Stevens @ 0:02

I must admit that I was (patiently) waiting for the type of malicious document I’m about to describe now. First I’m going to analyze this document with my tools, and after that I’m going to show you some of the mitigations put in place by Adobe and Microsoft.

Malicious document 123-148752488-reg-invoice.pdf is a PDF with an embedded file and JavaScript. Here is pdfid’s report:

As we can notice from this report, the PDF document contains /JavaScript and an /OpenAction to launch this JavaScript upon opening of the PDF file, and also an /EmbeddedFile.

pdf-parser.py searching for JavaScript (option -s javascript) reveals that the JavaScript is in object 5:

Object 5 contains JavaScript (option -o 5 to select object 5, and option -f to decompress the stream with JavaScript):

This script (this.exportDataObject) will save the embedded file (996502.docm) to a temporary file and launch the associated application (if MS Office is installed, Word will be launched). A .docm file is a Word document with macros.

So let’s search for this embedded file:

The embedded file is stored in object 3, as a compressed stream (/FlateDecode).

So let’s decompress and extract the file with pdf-parser: option -f to filter (decompress) and option -d to dump the content. Since I expect the embedded file to be a Word document with macros, I’m going to analyze it with oledump. So in stead of writing the embedded file to disk, I’m going to extract it to stdout (-d -) and pipe it into oledump:

oledump‘s report confirms that it is a Word document with macros. I’m not going to spend much time on the analysis of the VBA code, because the intent of the code becomes clear when we extract all the strings found in the VBA code. First we select and extract all VBA code (options -s a -v) and then we pipe this into re-search to produce a list of unique strings (enclosed in double quotes) with these options: -n str -u

One of the extracted strings contains 3 URLs separated by character V. The macros will download an XOR encoded EXE file from these sites, decode it and execute it.

Mitigations

The first mitigation is in Adobe Reader: the embedded .docm file will not be extracted and launched without user interaction. First the user is presented a dialog box:

Only when clicking OK (the default option), will the .docm file be extracted and launched. Remark that the maldoc authors use some weak social engineering to entice the user to click OK: see in 996502.

When opened in Word, macros will be disabled:

This next mitigation is put into place by Microsoft Word: macros are detected, and by default, they are not executed. Here we see a better attempt at social engineering the user into executing the macros.

You might have expected that this document would be opened in Protected View first. After all, the PDF document was e-mailed to the victims, and Outlook will mark the PDF with a mark-of-web when it is saved to disk:

But Adobe Reader will not propagate that mark-of-web of the PDF document to the extracted Word document (at least the version I tested, version XI). Without mark-of-web, Word will open the document without Protected View.

Another simple mitigation for this type of malicious document that you can put into place but that is not enabled by default, is to disable JavaScript in Adobe Reader.

Remark that these documents do not contain exploits: they just use scripting.

Tuesday 18 April 2017

CVE-2017-0199

Filed under: maldoc,Malware,Vulnerabilities — Didier Stevens @ 0:00

I have an analysis of a CVE-2017-0199 maldoc with my tools here, and produced 2 videos:

In the second video, I use nixawk‘s Metasploit module for cve-2017-0199 (not yet merged into the Metasploit GitHub repository at time of writing).

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