Didier Stevens

Tuesday 11 June 2019

Quickpost: C Random Functions in Other Languages

Filed under: Quickpost — Didier Stevens @ 0:00

Some time ago, I had to implement a particular C-runtime random number generator in Python. That’s not difficult to do, you just need a variable that maintains the state (seed) of the random number generator, and then you use a simple algebraic expression: a linear congruential generator.

What’s more difficult to figure out, is knowing which multiplier (a) and increment (c) you need to reproduce the particular C-runtime random number generator.

Fortunately, I discovered that Wikipedia has a table with a and c values for many C compilers and other languages: parameters in common use.


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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):

 


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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.


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Saturday 23 March 2019

Quickpost: PDF Tools Download Feature

Filed under: My Software,PDF,Quickpost — Didier Stevens @ 9:34

When I’m asked to perform a quick check of an online PDF document, that I expect to be benign, I will just point my PDF tools to the online document. When you provide an URL argument to pdf-parser, it will download the document and perform the analysis (without writing it to disk).


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Monday 3 December 2018

Quickpost: Developing for ESP32 with the Arduino IDE

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

I have a couple of ESP32’s that can also be programmed with the Arduino IDE, provided the necessary board manager is installed:

After starting the IDE

I open the preferences:

And add the board manager URL for the ESP32 (https://dl.espressif.com/dl/package_esp32_index.json):

And via the Tools menu I launch the Boards Manager:

And install the ESP32 board manager:

And then I can select the right board (ESP32 Dev Module):

Then I can connect my ESP32 board to my Windows machine, and it will complain about missing drivers:

I install the CP210x drivers:

Then I can select the right port in the Tools menu:

And now everything is ready to program my ESP32. I will start with the WiFiScan example:

Which can then be compiled and uploaded to the ESP32 board:

Once it is uploaded and running, I can connect to the ESP32 board via the serial monitor:

 

 

Monday 26 November 2018

Quickpost: Compiling with Build Tools for Visual Studio 2017

Filed under: Quickpost — Didier Stevens @ 0:00

Compiling C/C++ programs with Microsoft’s command-line compilers is possible, even if you don’t have Visual Studio installed. You can do this with the Build Tools for Visual Studio 2017 (a free download).

Go to https://visualstudio.microsoft.com/downloads/ and download the Build Tools:

The downloaded file does not include the build tools, but it’s a stager that will download the necessary build tools. It requires .NET, you might get an error if the proper version is not installed:

Installing the correct .NET framework will fix this problem:

Once this download is completed, you can get to the actual installer where you choose the tools you want:

I selected the Visual C++ build tools, a download of about 1 GB:

Once the build tools are installed, you can open a shell via the start menu:

The C/C++ compiler is invoked with command cl:

As an example, I’m compiling the following program:


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Monday 19 November 2018

Quickpost: Compiling 32-bit Static ELF Files on Kali

Filed under: Quickpost — Didier Stevens @ 0:00

Here I compile EICARgen on Kali Linux to a 32-bit, statically linked Linux executable.

gcc’s option -m32 creates a 32-bit executable on 64-bit Linux.

If you get this error:

then one way to solve it is by installing libc6-dev-i386 (apt install libc6-dev-i386):

Then option -m32 can be used to create a 32-bit executable:

This executable will not run on 64-bit system that don’t have the libraries we just installed. A work-around is to statically link the ELF file with option -static:

 


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Monday 5 November 2018

Quickpost: Using pcapy with Npcap on Windows

Filed under: Quickpost — Didier Stevens @ 0:00

I installed pcapy on a Windows machine, but importing in Python failed due to a missing DLL.

Process Monitor showed me what was missing: wpcap.dll, a WinPcap DLL:

The DLL was missing because I had installed Npcap (an alternative for WinPcap, that provides loopback packet capture).

This problem can be fixed by setting a toggle to install a WinPcap compatible API (e.g. wpcap.dll) during installation:


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Monday 24 September 2018

Quickpost: Signing Windows Executables on Kali

Filed under: Quickpost — Didier Stevens @ 0:00

Windows executables (PE files) can be signed on Kali using osslsigncode.

osslsigncode needs to be installed:

apt install osslsigncode

Then you need a certificate. For this demo, I’m using a self-signed cert.

The command to sign file demo-x64.exe with the demo certificate using SHA1 and timestamping, is:

osslsigncode sign -certs cert-20180729-110705.crt -key key-20180729-110705.pem -t http://timestamp.globalsign.com/scripts/timestamp.dll -in demo-x64.exe -out demo-x64-signed.exe

The signed file is demo-x64-signed.exe

To dual sign this executable (add SHA256 signature), use this command:

osslsigncode sign -certs cert-20180729-110705.crt -key key-20180729-110705.pem -t http://timestamp.globalsign.com/?signature=sha2 -h sha256 -nest -in demo-x64-signed.exe -out demo-x64-dual-signed.exe

The signed file is demo-x64-dual-signed.exe

Of course, Windows reports the signatures as invalid, because we used a self-signed certificate. For a valid signature, you can add your certificate to the trusted root certificates store, buy a code-signing certificate, …

For single SHA256 signing, use the second osslsigncode command without option -nest.

 


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Monday 17 September 2018

Quickpost: Compiling EXEs and Resources with MinGW on Kali

Filed under: Quickpost — Didier Stevens @ 0:00

To compile a Windows executable with version information and an icon on Kali, we use MinGW again.

The version information and icon (demo.ico) we want to use are defined in a resource file (demo.rc):

#include "winver.h"


#define IDI_ICON1                       101

/////////////////////////////////////////////////////////////////////////////
//
// Version
//

#define VER_FILEVERSION             0,0,0,1
#define VER_FILEVERSION_STR         "0.0.0.1\0"

#define VER_PRODUCTVERSION          0,0,0,1
#define VER_PRODUCTVERSION_STR      "0.0.0.1\0"

#ifndef DEBUG
#define VER_DEBUG                   0
#else
#define VER_DEBUG                   VS_FF_DEBUG
#endif

VS_VERSION_INFO VERSIONINFO
FILEVERSION     VER_FILEVERSION
PRODUCTVERSION  VER_PRODUCTVERSION
FILEFLAGSMASK   VS_FFI_FILEFLAGSMASK
FILEFLAGS       VER_DEBUG
FILEOS          VOS__WINDOWS32
FILETYPE        VFT_APP
FILESUBTYPE     VFT2_UNKNOWN
BEGIN
    BLOCK "StringFileInfo"
    BEGIN
        BLOCK "040904E4"
        BEGIN
            VALUE "CompanyName", "example.com"
            VALUE "FileDescription", "demo"
            VALUE "FileVersion", VER_FILEVERSION_STR
            VALUE "InternalName", "demo.exe"
            VALUE "LegalCopyright", "Public domain"
            VALUE "OriginalFilename", "demo.exe"
            VALUE "ProductName", "demo"
            VALUE "ProductVersion", VER_PRODUCTVERSION_STR
        END
    END
    BLOCK "VarFileInfo"
    BEGIN
        VALUE "Translation", 0x409, 1252
    END
END


/////////////////////////////////////////////////////////////////////////////
//
// Icon
//

// Icon with lowest ID value placed first to ensure application icon
// remains consistent on all systems.
IDI_ICON1               ICON                    "demo.ico"
/////////////////////////////////////////////////////////////////////////////

More info on the VERSIONINFO resource can be found here.
We use the resource compiler windres, and then the gcc compiler.

Compile for 64-bit:

x86_64-w64-mingw32-windres demo.rc demo-resource-x64.o
x86_64-w64-mingw32-gcc -o demo-x64.exe demo-resource-x64.o demo.c

Compile for 32-bit:

i686-w64-mingw32-windres demo.rc demo-resource-x86.o
i686-w64-mingw32-gcc -o demo-x86.exe demo-resource-x86.o demo.c

 

DemoResource_V_0_0_0_1.zip (https)
MD5: 9104DDC70264A9C2397258F292CC8FE4
SHA256: 722B3B52BAE6C675852A4AC728C08DBEEF4EC9C96F81229EF36E30FB54DC49DE


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