Didier Stevens

Thursday 25 April 2019

Update: python-per-line.py Version 0.0.6

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

In this new version of python-per-line, I introduce libraries.

Custom Python code can be stored in a “library file”, i.e. a text file with name python-per-line.library. This file is loaded automatically upon execution when it is found in the current directory or in the same directory as the script (or both).

Currently, the distributed library file contains a small Python function to defang URLs: Defang.

It can be used like this:

If you just want to apply a function to each line, you don’t have to type a full expression like in the example above (Defang(line)).

You can also use option -n and just type the function name, like this:

python-per-line_V0_0_6.zip (https)
MD5: FDA3365E2DC54EF65B2E8F6EE8D0DB9E
SHA256: E7496229BF64B2772AF5C49E4BC065281F06043192453E96A783808F6F3E61D1

Sunday 21 April 2019

Update: translate.py Version 2.5.6

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

This is just a small update to the man page.

translate_v2_5_6.zip (https)
MD5: 9615167810202129C0CFC3D5125CC354
SHA256: F926E474B966790A1077B76C029F912100128C4F1CE848781C14DF4B628395D7

Saturday 20 April 2019

Extracting “Stack Strings” from Shellcode

Filed under: Malware,My Software,Reverse Engineering — Didier Stevens @ 0:00

A couple of years ago, I wrote a Python script to enhance Radare2 listings: the script extract strings from stack frame instructions.

Recently, I combined my tools to achieve the same without a 32-bit disassembler: I extract the strings directly from the binary shellcode.

What I’m looking for is sequences of instructions like this: mov dword [ebp – 0x10], 0x61626364. In 32-bit code, that’s C7 45 followed by one byte (offset operand) and 4 bytes (value operand).

Or: C7 45 10 64 63 62 61. I can write a regular expression for this instruction, and use my tool re-search.py to extract it from the binary shellcode. I want at least 2 consecutive mov … instructions: {2,}.

I’m using option -f because I want to process a binary file (re-search.py expects text files by default).

And I’m using option -x to produce hexadecimal output (to simplify further processing).

I want to get rid of the bytes for the instruction and the offset operand. I do this with sed:

I could convert this back to text with my tool hex-to-bin.py:

But that’s not ideal, because now all characters are merged into a single line.

My tool python-per-line.py gives a better result by processing this hexadecimal input line per line:

Remark that I also use function repr to escape unprintable characters like 00.

This output provides a good overview of all API functions called by this shellcode.

If you take a close look, you’ll notice that the last strings are incomplete: that’s because they are missing one or two characters, and these are put on the stack with another mov instruction for single or double bytes. I can accommodate my regular expression to take these instructions into account:

This is the complete command:

re-search.py -x -f "(?:\xC7\x45.....){2,}(?:(?:\xC6\x45..)|(?:\x66\xC7\x45...))?" shellcode.bin.vir | sed "s/66c745..//g" | sed "s/c[67]45..//g" | python-per-line.py -e "import binascii" "repr(binascii.a2b_hex(line))"

Thursday 4 April 2019

Quickpost: Browsers & Content-Disposition

Filed under: Quickpost — Didier Stevens @ 0:00

A quick check confirmed that response header Content-Disposition can direct browsers to display or save a file.

I used my tcp-honeypot.py to serve 3 HTTP responses:

HTTP/1.1 200 OK
Content-Disposition: inline

Line 1
Line 2
Line 3

 

HTTP/1.1 200 OK
Content-Disposition: attachment

Line 1
Line 2
Line 3

 

HTTP/1.1 200 OK
Content-Disposition: attachment; filename=”test.js”

Line 1
Line 2
Line 3

 

Only the Content-Disposition response header changes between these 3 responses.

With Content-Disposition response header “inline”, Internet Explorer displays the content inside the browser window:

With Content-Disposition response header “attachment”, Internet Explorer proposes to save the content to disk using a generated filename:

With Content-Disposition response header “attachment; filename=”test.js””, Internet Explorer proposes to open or save the content to disk using the provided filename test.js:

When option Open is selected, file test.js will be opened with the Windows scripting host (after warnings are clicked away).

The behavior of Edge is quite similar:

Google Chrome saves the file to disk without prompting the user (attachment):

And Firefox prompts the user (attachment):

Tests were conducted on a fully patched Windows 10 1809 machine, with default configurations for Internet Explorer and Edge.

The latest versions of Chrome and Firefox were installed with default configurations.


Quickpost info


Monday 1 April 2019

list-interfaces.xlsm

Filed under: Entertainment,My Software — Didier Stevens @ 0:01

Inspired by today’s date and ShadowHammer, I created an Excel spreadsheet that will list all the interfaces on your Windows machine (using GetIfTable).

One of the properties that is listed, is the MAC address, and it is compared with a list of MAC addresses found in sheet “List”. As a PoC, I populated that sheet with the initial ShadowHammer list published by @SkylightCyber.

And I got a hit on one of my laptops:

00:50:56:C0:00:08 is a generic MAC address used by VMware for the “VMware Virtual Ethernet Adapter for VMnet8” (VMware Workstation is installed on that machine). So no, that laptop was not targeted by the ShadowHammer actor: it’s a false positive (revised lists were published, one with 2 MAC addresses per line, and that’s where this MAC address appears now).

Enjoy! 😉

list-interfaces.zip (https)
MD5: B7DFF86AA0AFD83EF7796F12CEF46D6C
SHA256: 2AD35C825D1A5D9BCFF75C1374C238415C15BADA3CDB0A5EA7178DE4E1DEF0A2

Overview of Content Published in March

Filed under: Announcement — Didier Stevens @ 0:00

Here is an overview of content I published in March:

Blog posts:

YouTube videos:

Videoblog posts:

SANS ISC Diary entries:

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