overflow

Overflow

Level - Medium

Description:

Please don't enter the Declaration of Independence.

nc byuctf.xyz 40001

[overflow]

Writeup

This is a very simple buffer overflow. The vulnerable() function takes in 64 characters and fits it in a buffer with only 0x30 bytes of space. This means that the extra characters will overflow into the stack. The item just above the buffer is the key variable, which is initially set to 0xc. If you put in 60 garbage characters followed by 1234, then the buffer will be filled and the last four characters will set the value of key to 0x34333231. This allows us to run the win() function and get the flag.

Example:

$ ./overflow
Enter a text please: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa1234
Your flag is - byuctf{s1mpl3_0verfl0w_bug}

Flag - byuctf{s1mpl3_0verfl0w_bug}

Hosting

overflow was compiled with gcc -o overflow overflow.c.

This challenge should be a Docker container that runs the executable overflow every time someone connects on port 40001. All the proper files are included in here. The command to build the docker container is (when located inside of this directory):

sudo docker build -t overflow .

The command to start the challenge is:

sudo docker run -p 40001:40000 --detach --name overflow overflow:latest

The command to stop the challenge (since CTRL+C won't work) is:

sudo docker stop overflow