Didier Stevens

Monday 29 July 2013

OHM2013

Filed under: Announcement — Didier Stevens @ 0:00

I’m attending OHM2013. To mark the occasion of this outdoor hacker conference taking place every 2 years, I’m doing a 20% promo on my workshop videos.

In case you missed it, I posted this during the weekend: MSI: The Case Of The Invalid Signature.

Friday 26 July 2013

MSI: The Case Of The Invalid Signature

Filed under: Forensics,Malware,Windows 7 — Didier Stevens @ 22:01

I found a suspicious file on a Windows XP machine. I was able to trace its origin back to a Windows Installer package (.msi). This package in c:\windows\installer had an invalid digital signature. Like this:

20130726-233848

Very suspicious.

A bit later I found another msi package containing the same suspicious file. But this time, the package had a valid digital signature. What’s going on?

After a deep dive into the internals of msi packages, I found the answer.

When an msi package is installed, it is cached inside the Windows Installer directory (%windir%\Installer). Prior to Windows Installer 5.0 (released with Windows 7), cached packages were stripped of their embedded cab files. But with digitally signed msi files, the signature remained inside the file: the digitally signed file was modified, hence the signature was invalidated. This behavior changed with Windows Installer 5.0: cached packages are no longer stripped, hence the signature remains valid.

This blogpost by Heath Stewart explains this change in more detail. Unfortunately, my Google-skills were not good enough to find this blogpost prior to my deep dive into msi files. Hindsight Googling FTW! 😉

Thursday 25 July 2013

Update: Lookup Tools

Filed under: My Software,Networking,Update — Didier Stevens @ 20:11

It looks like I didn’t release this update to my lookup tools.

lookup-hosts.py has a new argument: -R. This does a reverse lookup of the IP addresses (thus after it resolved the hostname).

20130725-220124

And now you can also use letters as a counter: test-[a-z].com

lookup-tools_V0_0_2.zip (https)
MD5: 310904722F900FA34C567FC38634124E
SHA256: 85626574A99BF4D2AB786D8C2FF5B8F6649F1FC7410F1786A24EF0201AAF64AA

Thursday 18 July 2013

Update: js-unicode-unescape.1sc

Filed under: 010 Editor,My Software,Update — Didier Stevens @ 18:36

Because I had to use a workaround in my js-unicode-unescape.1sc script to copy an array of bytes to the clipboard, I asked the 010 Editor developers if they could add a function that does exactly this.

They included this new function, CopyBytesToClipboard, in their new version 5.0.

Here is a new version that uses this function:
js-unicode-unescape_v0_0_2.zip (https)
MD5: 6200C4F235CA527E8C0DCD5076CB1C09
SHA256: 2CACC9EE1BB1D1BC4C9FABC6EC3B3440CFF304AA560966B0B531279C369549BB

Wednesday 10 July 2013

The Art Of Defuzzing

Filed under: My Software — Didier Stevens @ 21:05

I had something of a puzzle to solve. A friend asked me to look at a set of files, all of the same size, but with some differences.

After some analysis, it dawned on me that these files were the result of a simple fuzzer applied to a single file. So I quickly wrote a program that took these files as input and reconstituted the original file. Later I wrote a more generic defuzzer. Here is an example:

defuzzer.py result.png a*.png

Number of defuzzed bytes: 171
Number of defuzzed sequences: 33
Length of shortest defuzzed sequence: 1
Length of longest defuzzed sequence: 10
Fuzz bytes:
'A': 171

From the result you can see that the program was able to reconstitute the original file, and that the fuzzer that was used to produce the different a*.png files, overwrote 33 byte-sequences with the character A. The longest sequence was 10 bytes long, the shortest only 1 byte. In total, 171 bytes were overwritten.

defuzzer_v0_0_2.zip (https)
MD5: 75188EF950625B78937C3473D825C582
SHA256: 056AB8BA7F3B2B52F8C7BFC2959D7F1AE3FEAC4BE90C675B2DFF6B521225D93E

Wednesday 3 July 2013

Update: virustotal-search.py

Filed under: My Software,Update — Didier Stevens @ 20:09

Mark Woan reported an issue with virustotal-search.py: sometimes VirusTotal returns a JSON object that the json parser can’t parse.

That’s something I didn’t expect. I’ve added error handling for this case.

virustotal-search_V0_0_9.zip (https)
MD5: FECD02796889CDFE9FA67287F2DE567C
SHA256: 0CE06CBAFC6341835EB8A62377F5C4EB067747EE28E7ED8BB25FD69A4B99FA97

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