Didier Stevens

Monday 23 March 2020

Quickpost: User-Agent: Microsoft Office Excel 2014

Filed under: Networking,Quickpost — Didier Stevens @ 14:25

To start: there is no version 2014 of Microsoft Office.

That’s why I was intrigued when I saw User Agent String “Microsoft Office Excel 2014” appearing in Wireshark when I did some tests with Excel’s data importing features.

With Excel 2019, when I get data from a CSV file and provide an URL (in stead of a local filename) like this:

Excel will issue several OPTIONS and HEAD requests, with different User Agent Strings:

And finally, a GET request to download the file:

Xavier Mertens has mentioned User Agent String “Microsoft Office Excel 2014” in another context: “Microsoft Apps Diverted from Their Main Use“.


Quickpost info


Tuesday 28 January 2020

etl2pcapng: Support For Process IDs

Filed under: Forensics,Networking — Didier Stevens @ 0:00

You can start a packet capture on a vanilla Windows machine with command “netsh trace start capture=yes” (and end it with “netsh trace stop”).

This packet capture file, with extension .etl, can not be opened with Wireshark. Until recently, I used Microsoft’s Message Analyzer, but this tool is no longer supported and installation files have been removed from Microsoft’s site.

In comes etl2pcapng, a new open-source utility from Microsoft that converts an .etl file to .pcapng format:

Utility that converts an .etl file containing a Windows network packet capture into .pcapng format“.

I contributed to version 1.3.0 of etl2pcapng, by adding a comment containing the Process ID to each packet. etl files contain metadata (like the PID of the process associated with the network traffic) that got lost when translating to pcapng format. As the pcapng format has no option to store the PID for each packet, but it supports packet comments, I stored the PID inside packet comments:

Notice this warning by Microsoft:

The output pcapng file will have a comment on each packet indicating the PID of the current process when the packet was logged. WARNING: this is frequently not the same as the actual PID of the process which caused the packet to be sent or to which the packet was delivered, since the packet capture provider often runs in a DPC (which runs in an arbitrary process). The user should keep this in mind when using the PID information.

Monday 18 November 2019

Update: tcp-honeypot.py Version 0.0.7

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

This new version of tcp-honeypot.py, a simple TCP honeypot and listener, brings TCP_ECHO and option -f as new features.

TCP_ECHO can be used to send back any incoming data (echo). Like this:

dListeners = {4444: {THP_LOOP: 10,THP_ECHO: None,},}

TCP_ECHO also takes a function, which’s goal is to transform the incoming data and return it. Here is an example with a lambda function that converts all lowercase letters to uppercase:

dListeners = {4444: {THP_LOOP: 10,THP_ECHO: lambda x: x.upper(),},}

If persistence is required across function calls, a custom class can also be provide. This class has to implement a method with name Process (input: incoming data, output: transformed data). Consult the man page (option -m) for more details.

And option -f (format) can be used to change the output format of data.
Possible values are: repr, x, X, a, A, b, B
The default value (repr) output’s data on a single line using Python’s repr function.
a is an ASCII/HEX dump over several lines, A is an ASCII/HEX dump too, but with duplicate lines removed.
x is an HEX dump over several lines, X is an HEX dump without whitespace.
b is a BASE64 dump over several lines, B is a BASE64 without whitespace.

 

 

Sunday 20 October 2019

New Tool: simple_tcp_stats.py

Filed under: My Software,Networking — Didier Stevens @ 10:25

My new tool simple_tcp_stats.py is a Python program that reads pcap files and produces simple statistics for each TCP connection.

For the moment, it calculates the entropy of the data (without packet reassembling) of each TCP connection (both directions) and reports this with a CSV file:

ConnectionID;head;Size;Entropy
192.168.10.10:50236-96.126.103.196:80;’GET ‘;364;5.42858024035
192.168.10.10:50235-96.126.103.196:80;’GET ‘;426;5.46464090792
96.126.103.196:80-192.168.10.10:50235;’HTTP’;3308;6.06151478505
96.126.103.196:80-192.168.10.10:50236;’HTTP’;493;6.73520107812

 

simple_tcp_stats_V0_0_1.zip (https)
MD5: 606DB4208BBC5908D9F32A68DDF90AC6
SHA256: 68B275C58736AE450D23BEA82CC1592936E541E00726D8ED95F5CA8ACB02B7CE

Wednesday 2 October 2019

Shark Jack Capture File

Filed under: Hardware,Networking — Didier Stevens @ 0:00

I have a new toy: a “Shark Jack“. It’s a small device sold by Hak5 that performs a nmap scan (-sP) when plugged into a network port (that’s the default “payload”).

In this blog post, I’m sharing the network capture of a scan performed in this “test environment”:

The device (small black box, almost square) between the Shark Jack (SJ) and the router is my “Packet Squirrel”: a simple network capture device.

A couple of observations:

  1. The SJ was tested with its original firmware (1.0.0)
  2. The SJ will randomize its MAC address
  3. The SJ performs 2 full DHCP handshakes prior to the nmap scan
  4. The SJ listens on port 53 (tcp and udp) using dnsmasq (observed while scanning)

Example of different MAC addresses after before and after reboot:

root@shark:~# ifconfig
eth0 Link encap:Ethernet HWaddr 2E:AF:43:F2:3E:22
inet addr:172.16.24.1 Bcast:172.16.24.255 Mask:255.255.255.0

 

root@shark:~# ifconfig
eth0 Link encap:Ethernet HWaddr 86:72:96:71:C3:3C
inet addr:172.16.24.1 Bcast:172.16.24.255 Mask:255.255.255.0

 

And it can get quite hot while charging, as can be observed in this thermal image:

shark_jack_capture.zip (https)
MD5: 9E5C1187D64A6EC7284C06464E791F01
SHA256: 5153F5C7B559BEC1539B0395F97C5852064D7ED9309B837F11A9381EA6ED4C88

Wednesday 7 August 2019

Downloading Executables Over DNS: Capture Files

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

In my BruCON training “Malicious Documents For Red Teams” (October 2019), we will cover downloading of files over DNS. I Tweeted about downloading Mimikatz via DNS-over-HTTPS with an Excel sheet.

I’m not releasing the Python code to serve files via DNS, nor the VBA code to download files over DNS/DoH: this is reserved for the attendees of my training.

But here I am sharing capture files of the downloads via DNS, so that you can understand how traffic looks like, and how to detect it.

Capture files inside the ZIP container (password is infected):

  1. 1-dns-txt.pcap: downloading of files via DNS TXT records, EICAR file (binary, hexadecimal and BASE64 encoded) and Mimikatz.exe (BASE64 encoded)
  2. 2-DoH-txt.pcap: downloading of Mimikatz.exe via DNS TXT records via dns.google.com (Google’s DNS over HTTPS)
  3. 3-DoH-txt-domain-fronting.pcap: same as 2, but with domain fronting (www.google.com)
  4. 4-DoH-txt.pcapng: same as 2, but in a PCAPNG file with decryption keys
  5. 5-DoH-txt.pcapng: same as 4, but with shorter DNS TXT records (to help with decryption)

DNS_TXT_captures.zip (https)
MD5: 5DB5091B9B641E9B8DA0E29CE9870981
SHA256: 49858B8BBA851B86EAB2DB6C5F329C5B587A3B1C7EB1A1E6028BCFBCDF445ECC

Tuesday 30 July 2019

Quickpost: tcp-honeypot.py & Browser Tests

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

tcp-honeypot.py is a Python program that allows you to define listeners using dictionaries: they listen on a given TCP port and process connections according to their configuration.

It started as a simple TCP honeypot, but now I use it too if I need a small network server.

For my quickpost “Quickpost: Browsers & Content-Disposition“, I needed a simple web server that would serve a page that I could fully control (headers & body).

I did this with tcp-honeypot. Dictionary dListeners (used by tcp-honeypot) defines the listeners: the keys are the TCP port numbers to listen on, and the values are dictionaries with configuration entries.

As I wanted to serve 3 different pages, I resorted to listen on 3 different ports (8080, 8081, 8082), each would serve a different page. Each dictionary for these listeners contains one entry with key THP_REPLY. Because each listener is very simple it listens for a connection and reads incoming data, discards it, and then sends its reply (regardless of input).

Here is the code to do this (file content-disposition-test.py):

#!/usr/bin/env python

__description__ = 'TCP honeypot configuration for Content-Disposition tests'
__author__ = 'Didier Stevens'
__version__ = '0.0.1'
__date__ = '2019/04/03'

"""
Source code put in public domain by Didier Stevens, no Copyright
https://DidierStevens.com
Use at your own risk

History:
  2019/04/03: start

Todo:
"""

dListeners = {
    8080:    {THP_REPLY: TW_CRLF(['HTTP/1.1 200 OK', 'Content-Disposition: inline', '', 'Line 1', 'Line 2', 'Line 3'])},
    8081:    {THP_REPLY: TW_CRLF(['HTTP/1.1 200 OK', 'Content-Disposition: attachment', '', 'Line 1', 'Line 2', 'Line 3'])},
    8082:    {THP_REPLY: TW_CRLF(['HTTP/1.1 200 OK', 'Content-Disposition: attachment; filename="test.js"', '', 'Line 1', 'Line 2', 'Line 3'])},
}

THP_REPLY configures a listener to read incoming data when a TCP connection is established, then send a reply (the value of dictionary entry THP_ENTRY) and then close the connection. This value is a string: the HTTP message (start-line, headers and body) to be send to the browser. In stead of defining one long string with start-line, headers and body, separated with carriage return & newline (CR NL), I use convenience function TW_CRNL. When you call convenience function TW_CRNL (Terminate With CR NL) with a list of strings, it terminates each string with CR NL (\r\n) and concatenates all strings into one string, that is returned by the function.

To start the server with this configuration, I just have to run tcp-honeypot.py with content-disposition-test.py as argument.

There are other methods to do this, for example using a single port. I’ll describe these methods in an upcoming blog post.


Quickpost info


Wednesday 3 July 2019

Quickpost: nslookup Types

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

A reminder to myself, how to set a nslookup type via the command-line:

The label of the root domain is an empty string, hence a FQDN with root domain ends with a dot (.), like google.com. :


Quickpost info


Monday 20 May 2019

WebDAV, NTLM & Responder

Filed under: Encryption,Networking — Didier Stevens @ 0:00

I was trying to create a capture file with NTLM authenticated WebDAV traffic, using Responder: I couldn’t get it to work. There was WebDAV traffic, but no NTLMSSP headers.

Long story short: there’s a bug in Responder version 2.3.3.9. It manifests itself when the WebDAV client sends a request with just headers, and “Content-Length: 0”, like this:

The code in Responder “sees” just “Content-Length” and waits for more packets:

I made a quick & dirty fix: break out of the loop when we see “Content-Length: 0” (servers/HTTP.py):

And now I have NTLMSSP headers:

I just start my modified version of Responder:

Generate WebDAV traffic from a Windows 7 client:

And Responder participates in the challenge:

This can of course be cracked (if the password is not too complex), with John The Ripper for example:

I also have a blog post with more details about WebDAV traffic from Windows clients.

Once I got Responder to work, I searched on Laurent’s Responder repository, and found a pull-request to fix issues with “Content-Length: 0” requests (this PR has not been merged yet). Hence I’m not going to do my own PR.

You can find the capture file here:

webdav-ntlm-responder.zip (https)
MD5: A427DDBDAF090E93BB75B7A8DE696826
SHA256: 2F92CDD7382DD3622AC1F8769CF9D065C60C235DEF764E6709C32E2C4A7554A8

Sunday 19 May 2019

Quickpost: Retrieving an SSL Certificate with nmap

Filed under: Encryption,Networking,Quickpost — Didier Stevens @ 8:28

One of my first quickposts, more than 10 years ago, was an howto: using openssl to retrieve the certificate of a web site.

Since then, nmap has a scripting engine, and there is a script to check a certificate with nmap: ssl-cert.nse.

You just have to scan the site and port for which you want to check the certificate, like this: nmap -p 443 –script ssl-cert didierstevens.com

If you want the certificate too, increase verbosity with option -v:

Checking a certificate will not work if you scan a port that is not known to provide SSL/TLS:

In that case, you have to use service discovery (-sV):

 


Quickpost info


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