Malware | Didier Stevens

Wednesday 12 July 2017

Analyzing ClamAV Signatures

Filed under: Malware — Didier Stevens @ 0:00

While updating my Petya/Notpetya notes, I saw that ClamAV now detects resources 1 and 2 (zlib compressed PE files) as Mimikatz. Curious about how they detect Mimikatz, I wanted to take a look at the signature. I’ve done this before, but I forgot exactly how. So here is a blog post to remind me next time.

First I update the signatures (yup, that’s ClamAV on Windows):

This is a standard scan:

The signature is Win.Trojan.Mimikatz-6331391-0.

Then I do a scan with option –debug, this will print out the signature:

The signature is: 2813d34f6197eb4df42c886ec7f234a1:47616:Win.Trojan.Mimikatz-6331391-0

I hoped for something more interesting: this is an MD5 hash-based signature. 2813d34f6197eb4df42c886ec7f234a1 is the MD5 hash of the file, 47616 is its file size, and Win.Trojan.Mimikatz-6331391-0 is the signature name.

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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):

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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:

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Tuesday 23 May 2017

WannaCry Simple File Analysis

Filed under: Malware,My Software,Reverse Engineering — Didier Stevens @ 7:32

In this video, I show how to get started with my tools and a WannaCry sample.

Tools: pecheck.py, zipdump.py, strings.py

Sample: 84c82835a5d21bbcf75a61706d8ab549

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Sunday 14 May 2017

Quickpost: WannaCry’s Mutex Is MsWinZonesCacheCounterMutexA0 (Digit Zero At The End)

Filed under: Malware,Quickpost — Didier Stevens @ 11:23

I’ve seen reports that WannaCry uses a mutex with name Global\MsWinZonesCacheCounterMutexA.

The samples I analyzed all use another mutex: Global\MsWinZonesCacheCounterMutexA0. That’s a digit zero at the end.

I have not found a sample that uses mutex Global\MsWinZonesCacheCounterMutexA (e.g. without digit zero at the end).

Update 1: I got confirmation from Costin Raiu from Kaspersky that the mutex is Global\MsWinZonesCacheCounterMutexA0.

Update 2: dynamic analysis with sample 84c82835a5d21bbcf75a61706d8ab549 shows that there are 2 mutexes that can prevent the ransoming of files: MsWinZonesCacheCounterMutexA and Global\MsWinZonesCacheCounterMutexA0. Remark that the Global namespace must be used with mutex MsWinZonesCacheCounterMutexA0, while it may not be used with mutex MsWinZonesCacheCounterMutexA.

Remark that the code above contains string “Global\\MsWinZonesCacheCounterMutexA”, but that is not the actual string used for OpenMutexA.

The actual string used for OpenMutexA is created by a sprintf “%s%d” call, and results in “Global\\MsWinZonesCacheCounterMutexA0“, that is “Global\\MsWinZonesCacheCounterMutexA” with a digit 0 (zero) appended.

Mutexes have long been used by malware authors to prevent more than one instance of the malware running on the same machine. An old anti-malware trick consists in the creation of a specific mutex, to prevent the execution of a specific malware.

I’ve seen tools and scripts published to create mutex Global\MsWinZonesCacheCounterMutexA to prevent WannaCry from infecting machines. This will not work for the samples I analyzed.

Samples I disassembled:

7c465ea7bcccf4f94147add808f24629644be11c0ba4823f16e8c19e0090f0ff (contained as a resource in 5ad4efd90dcde01d26cc6f32f7ce3ce0b4d4951d4b94a19aa097341aff2acaec).

86721e64ffbd69aa6944b9672bcabb6d (contained as a resource in 5bef35496fcbdbe841c82f4d1ab8b7c2).

Samples I searched for containing the mutex and sprintf code:

509c41ec97bb81b0567b059aa2f50fe8
5bef35496fcbdbe841c82f4d1ab8b7c2
638f9235d038a0a001d5ea7f5c5dc4ae
7f7ccaa16fb15eb1c7399d422f8363e8
84c82835a5d21bbcf75a61706d8ab549
86721e64ffbd69aa6944b9672bcabb6d
d6114ba5f10ad67a4131ab72531f02da
db349b97c37d22f5ea1d1841e3c89eb4
f107a717f76f4f910ae9cb4dc5290594

If you have a sample that actually uses mutex Global\\MsWinZonesCacheCounterMutexA and not mutex Global\\MsWinZonesCacheCounterMutexA0 (e.g. with digit zero appended), please post a comment with the hash of your sample.

Quickpost info

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Saturday 13 May 2017

Quickpost: WannaCry Killswitch Check Is Not Proxy Aware

Filed under: Malware,Quickpost — Didier Stevens @ 11:54

It looks like #WannaCry’s killswitch check (www[.]iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea[.]com) is not proxy aware:

Organizations that use proxies will not benefit from the killswitch.

Sample: 5ad4efd90dcde01d26cc6f32f7ce3ce0b4d4951d4b94a19aa097341aff2acaec

~~I have not tested this in a VM. If someone has, please post a comment with your findings.~~

Update: I did test the sample, it is not proxy aware. In an environment with an HTTP proxy and no direct connections to the Internet, the sample can not connect to www[.]iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea[.]com, and it will infect the host.

If I patch the sample to make it proxy aware, it can connect to the site through the proxy, and it does not infect the host.

Quickpost info

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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.

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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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Wednesday 15 February 2017

Quickpost: ClamAV and ZIP File Decryption

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

While reading-up on ClamAV and YARA, I came across something I wanted to try for some time: have ClamAV decrypt and scan a password protected ZIP file.

It can be done by creating a .pwdb password signature file, as explained in section 3.12 of Creating signatures for ClamAV.

I created one signature for password “infected”:

ZipPasswordInfected;Engine:81-255;0;infected

ZipPasswordInfected is the name I gave to the signature.

Engine:81-255 defines the required functionality level of the ClamAV engine. If I’m not mistaken, 81 is version 0.99.

0 indicates that the password is in ASCII.

infected is the password to attempt ZIP decryption.

And then I can pass the password signature file to clamscan with option -d. Or I can put the password signature file in the database directory.

In this example, notepad.exe is stored in a password protected ZIP file (password infected), and is_pe_file.yara is a YARA rule to detect PE files.

clamscan.exe -d is_pe_file.yara -d passwords.pwdb notepad.exe.zip
notepad.exe.zip: YARA.is_PE_File.UNOFFICIAL FOUND

----------- SCAN SUMMARY -----------
Known viruses: 1
Engine version: 0.99.2
Scanned directories: 0
Scanned files: 1
Infected files: 1
Data scanned: 0.21 MB
Data read: 0.14 MB (ratio 1.50:1)
Time: 0.063 sec (0 m 0 s)

Quickpost info

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