Didier Stevens

Monday 7 December 2020

Quickpost: finger.exe

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

Windows 10 comes with the finger command, an ancient computer network tool.

You can still use it to lookup weather information, for example 🙂

It establishes a TCP connection to the hostname/IP address after the @ character, using destination port 79. And then it sends the text before the @ characters in ASCII, terminated with carriage return & line feed.

After that, it reads the reply, displays it, and closes the TCP connection.

finger.exe is not proxy-aware.

Port 79 is not hardcoded as an integer in finger.exe: the port is identified by service name “finger” (UNICODE), which is defined in the services list (%SystemRoot%\system32\drivers\etc\services). GetAddrInfo uses this list.

If you replace “finger” with “http\x00\x00” (UNICODE) in finger.exe (via binary patching, a shim, …), the finger command will connect to port 80:

As noted by many, finger.exe can be (ab)used to exchange information and files. Here I had my own go at it with finger.exe & Excel:


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Monday 2 November 2020

Quickpost: Portable Power

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

I did some tests to generate electricity (230V AC) with a portable 12V battery (well, it’s 10 Kg).

I have a 12V VRLA battery with a capacity of 35,000 mAh. That’s 12V times 35 Ah = 420 Wh. Or equivalent to a 116,667 mAh (420,000 mWh / 3.6 V) USB powerbank.

Charging this 12V battery with a 12V battery charger connected to a 230V power outlet takes almost 7 hours (6:57) and requires 0.49 kWh. That is measured with a plug-in electricity meter with a .00 kWh precision. And I’m working under the assumption that the power requirement of the electricity meter is so small that it can be neglected.

Then I use this fully charged battery to power a 230V 150W halogen lamp via a 12V DC to 230V AC power inverter (modified sine wave).

It runs for 2 hours (2 tests: 2:01 and 2:03) and consumes 0.30 kWh.

Of the 0.49 kWh energy I put into my system, I get 0.30 kWh out of the system. That’s 61%, or a bit better than half of the energy I put into the system.

The main phases where I expect the energy losses are occurring, is in 230V AC to 12V DC conversion and electrical to chemical energy conversion (charging); and chemical to electrical conversion and 12V DC to 230V AC conversion (discharging). I believe the highest energy loss to occur in the power inverter.

And with energy loss, I mean energy that is converted into forms that are not directly useful to me, like heat.

Remark that the halogen lamp test stopped after 2 hours, because the power inverter stopped converting. The battery voltage was 11.5 V then, and I could still draw 1 A at 11.5 V for an hour (I stopped that test after 1 hour).

Next I’m going to try out a 12V to 5V adapter and power some USB devices.

Saturday 31 October 2020

Quickpost: VMware OS Version Snapshots

Filed under: Quickpost — Didier Stevens @ 0:00

Whenever I upgrade the operating system of my virtual machines, I take a snaphot right after the upgrade.

This gives me a tree of different OS versions:

I give each snapshot a small descriptive name, that starts with the date of the snapshot (YYYYMMDD).

This allows me to revert to older versions to experiment with patched vulnerabilities, like this one.

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Saturday 10 October 2020

Quickpost: 4 Bytes To Crash Excel

Filed under: Hacking,Quickpost,Reverse Engineering — Didier Stevens @ 0:00

A couple of years ago, while experimenting with SYLK files, I created a .slk file that caused Excel to crash.

When you create a text file with content “ID;;”, save it with extension .slk, then open it with Excel, Excel will crash.

Microsoft Security Response Center looked at my DoS PoC last year: the issue will not be fixed. It is a “Safe Crash”, Excel detects the invalid input and calls MsoForceAppExitIf to terminate the Excel process.

If you have Excel crashing with .slk files, then look at the first line. If you see something like “ID;;…”, know that the absence of characters between the semi-colons causes the crash. Add a letter, or remove a semi-colon, and that should fix the issue.

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Monday 28 September 2020

Quickpost: USB Passive Load

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

I just received a USB passive load. It’s basically 2 resistors connected to the USB power wires in parallel, each with a switch in series:

It can draw approximately 1, 2 or 3 amps (depending on switch positions) from a 5 volt USB source.

The resistors can dissipate 10 Watts, and will become very hot.

The resistor for 1 amp (4,7 ohms, tolerance 5%) maxed-out my FLIR One thermal camera (> 150 °C), but I could measure around 220°C (that’s close to 451°F) with another thermal imaging camera.

The second resistor (2 amps: 2,2 ohms, tolerance 5%) maxed-out that other thermal camera too: this one got hotter than 280°C.

I’m referring to 451°F, because presumably, that’s the temperature to ignite paper. Something I’ll have to test out in safe conditions.

I also measured the resistors, and they are well within tolerance:

Here is a short thermal imaging video of the first resistor heating up:

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Sunday 27 September 2020

Quickpost: Ext2explore

Filed under: Quickpost — Didier Stevens @ 17:17

I was looking for a solution to read my Wifi Pineapple’s recon.db file from the SD card (ext2 formatted) on my Windows 10 machine.

The solution I went with is Ext2explore, a tool that can access ext2 volumes.


You have to run it as administrator, otherwise the tool will not be able to get raw access to the ext2 volume:


When you run the tool as administrator, you see your volumes. Mine is an SD card:

I can then explore the content and save file recon.db to a folder on my Windows 10 machine:

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Thursday 10 September 2020

Quickpost: dig On Windows

Filed under: Quickpost — Didier Stevens @ 12:40

I found out there’s a dig command for Windows.

I group small tools like this inside a bin folder. But dig relies on a set of DLLs, that should also be in the PATH, so I put them in the same bin folder.

These are the DLLs dig.exe needs:

  • libbind9.dll
  • libcrypto-1_1-x64.dll
  • libdns.dll
  • libirs.dll
  • libisc.dll
  • libisccfg.dll
  • libuv.dll
  • libxml2.dll

I used procmon on my Win10 machine to figure out which DLLs are needed, as you get no error message (there’s probably a registry setting for that).

I do have a Windows 7 VM, that I can also use to figure out which DLLs are missing because it displays an error message:

And you might also need to install the Visual C redistribuable that is included with the downloaded ZIP:

And now I can run dig from my bin folder:

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Wednesday 9 September 2020

Quickpost: Downloading Files With Windows Defender & User Agent String

Filed under: Quickpost — Didier Stevens @ 7:29

@mohammadaskar2 found out you can use Windows Defender to download arbitrary files. Like this:

"c:\ProgramData\Microsoft\Windows Defender\Platform\4.18.2008.9-0\mpcmdrun.exe" -DownloadFile -url http://didierstevens.com/index.html -path test.html

This command uses MpCommunication as User Agent String:

Update: this download feature has been disabled.

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Sunday 12 July 2020

Quickpost: curl

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

Since I learned that Windows 10 has curl pre-installed now, I notice I use it more often.

Here are some quick notes, mainly for myself:


Using Tor:

curl --socks5-hostname http://didierstevens.com

Option socks5-hostname uses SOCKS5 protocol and does name resolution of the hostname via the SOCKS5 protocol (and not local DNS)


Removing the User-Agent header:

curl --header "User-Agent:" http://didierstevens.com

Option –header (-H) can also be used to remove a header: provide the header name with colon, provide no header value.


Using a custom User-Agent header (-A –user-agent):

curl --user-agent "Mozilla/5.0 DidierStevens" http://didierstevens.com


Saving received data:

curl --dump-header 01.headers --output 01.bin.vir --trace 01.trace --trace-time http://didierstevens.com

Option —dump-header (-D) saves the headers, option –output (-o) saves the body, –trace creates a trace file and –trace-time adds timestamps to the trace file.


Option to ignore certificate errors: -k –insecure


Putting it all together:

curl --socks5-hostname --user-agent "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.116 Safari/537.36" --insecure --dump-header 01.headers --output 01.data --trace 01.trace --trace-time https://didierstevens.com


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Monday 18 May 2020

Quickpost: curl And SSPI Proxy Authentication

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

curl with SSPI feature supports integrated authentication to a proxy: you don’t need to provide credentials.

The command is the following:

curl –proxy proxyname:8080 –proxy-ntlm -U : https://www.didierstevens.com/index.html

This curl command uses a proxy (–proxy) and authenticates to the proxy (–proxy-ntlm) without providing explicit credentials (-U :).

curl will use an SSPI to perform integrated authentication to the proxy. This is explained on curl’s man page:

If you use a Windows SSPI-enabled curl binary and do either Negotiate or NTLM authentication then you can tell curl to select the user name and password from your environment by specifying a single colon with this option: “-U :”.

curl’s SSPI feature can also be used to authenticate to an internal IIS server.

Windows’ built-in curl version supports SSPI. You can use the version option to check if your version of curl supports SSPI:


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