Vega (Launched 1999)

Username: vega
Click to reveal password:
whyishould
Points: 5 for checkpoint, 5 for code, 5 for writeup

Relevant lectures: 3 - Memory Safety Vulnerabilities

Vega was a spacecraft developed in a joint mission between Caltopia and the Gobian Union. However, since Caltopia used all uppercase in its software, and the Gobian Union used all lowercase, a utility was needed to convert between uppercase and lowercase. Hack into Vega to learn the truth about EvanBot.

This question has a flaw more subtle than the previous questions. Can you find it? Can you find a way to exploit this seemingly minor vulnerability?

The exploit script in this question is slightly different. The output of egg is used as an environment variable, which means its value is placed at the top of the stack. The output of arg is used as the input to the program, passed as an argument on the command line (in the argv array to main).

Tips

It might help to read Section 10 of “ASLR Smack & Laugh Reference” by Tilo Müller. (ASLR is disabled for this question, but the idea of the exploit is similar.)
It might also help to read Section 3.5 (off-by-one vulnerabilities) of the memory safety textbook page.
Environment variables are stored at the special pointer variable environ. To see the addresses and values of environment variables in gdb, you can set a breakpoint anywhere in the program, run the program, and run these commands to print out each environment variable as a string:
```
(gdb) p environ[0]
(gdb) p environ[1]
(gdb) p environ[2]
...
```
If you want to view each environment variable as a sequence of bytes (in hex) instead, you can run these commands:
```
(gdb) x/16bx environ[0]
(gdb) x/16bx environ[1]
(gdb) x/16bx environ[2]
...
```
It may take some trial-and-error to find the output of egg among the environment variables. One way to confirm you have the right address is to run x/2wx [your address] and check that gdb displays what you put in egg.
Do not attempt to manually bitwise XOR anything longer than 4 bytes (in particular, shellcode). It is too easy to mess up. Instead, consider putting shellcode somewhere else, or use this Python snippet (replace ZZ with a constant):
```
output = ''.join((chr(0xZZ^ord(c)) for c in input))
```
There is a slight chance (1 in 256) that your VM customization causes the value of the SFP to end in \x00, which makes this question much harder to solve. You can resolve this by printing out extra garbage bytes in your egg script (after whatever you were printing before), which pushes the rest of the stack to different addresses.

Deliverables

Two scripts, egg and arg
A writeup.