InCTF Jr Qualifiers 2021
Starts 6PM, 28th Dec


Easy Pwn

Author: Srijiith

Initial Analysis

This is the main function taken from source code. We have 2 variables, username which is a char buffer of size 8, and auth of type int. auth is initialised with the value 0xcafebabe. User input is read using scanf with the format specifier %s. Our goal is to overwrite auth to have value 0xdeadbeef which will result in us getting a shell, which we can use to obtain the flag.


The vulnerability here is caused by using the format specifier %s with scanf. The issue with this particular format specifier is that it reads input of any size and stores it in a buffer of fixed size. This may cause a buffer overflow, if input size exceeds the size of the buffer.

In this particular challenge this vulnerability allows us to write past the memory allocated for username and into auth. This allows us to modify the value of auth. We can test the overflow by debugging the binary with gdb and giving an input such as AAAAAAAABBBB. The first 8 "A"s will fill the username buffer. The remaining "B"s will overwrite 0xcafebabe stored in auth.

Before reading input:

After reading input:

0x41 is the hexadecimal ASCII value of "A" and 0x42 is the hexadecimal ASCII value of "B". Hence it can be seen that we overwrote auth with our "B"s. Since data is stored in little endian (reverse order for every 4 bytes), in order to get 0xdeadbeef in memory, we have to give input "\xef\xbe\xad\xde".

Final exploit:


In summary,

  • we have 2 variables, username which is a char buffer and auth which is an int with value 0xcafebabe.
  • use buffer overflow caused by scanf to fill username buffer and overwrite auth to 0xdeadbeef.
  • check is passed and we are given a shell with which flag can be obtained.