CTF-practice-evening:2014-03-24

CTF-practice-evening:2014-03-24
Date	2014/03/24
Time
Location	ACTA
Type	Workshop
Contact	Melanie

Capture The Flag evening - Part 11

• 24 March, 2014 - 7 PM
• Please bring along a laptop with you!!!

General CTF Info

• See the page for the Ctf-evenings
• Link to the Tech Inc Challenge Website Scoreboard

Walkthrough: Minibomb

• Brainsmoke is explaining how he solved the challenge 'Minibomb' during the Codegate CTF
• Minibomb is a small setuid binary
  • This is probably a handmade binary written in assembler, Linux ELF, 32 bit
  • You can see the ELF header if you use file or hexdump
    • For more information about the ELF header (including the binary entry point, memory pages being loaded, executable text, etc..), you can use readelf
  • Objdump allows us to disassemble the binary
  • It's a static binary - there's no dynamic loader
    • Dynamic binaries have an interpreter section, with more LD-* things that need to be resolved
    • The kernel needs to tell where the binary starts
  • You could also use IDA, but that's overkill for this binary
• If you run it with strace, you see a list of signals and system calls
  • It starts, does an old_mmap call (you can get lots of information from the arguments, including the starting address), an unman (looks like a stack address - bfxx if usually on the stack in 32 bits)
  • It does a write and read
  • If you send lots of A's, you get a segfault - this gives away that you have a bug here
  • You can do this in gdb to get more information
    • You can see that a fault happens on the address 0x41414141 - our input!
    • It's easy to get arbitrary code execution here
• Because it's a small file, we can take a look at the disassembled code
  • We can see the memory map
  • You can get system call information by typing 'syscall mmap' - we can see 0x5a, which is the syscall instruction in the disassembly!
  • We should read up to understand the meanings of: %eax, %ebx, etc…
    • We can give 6 arguments with a system call
  • Next command: %ebp is the frame pointer for the function call frames - this is also for the old_mmap system call
  • The one argument is an array of six arguments - that is a pointer to that argument
  • Next command: int 0x80 is the system call command on x86

• readelf shows us that we have both data and text - the binary executable is loaded into 2 pages, starting from the start of the binary until 4096 bits later
  • The kernel loads it into the virtual address
  • You can also see another offset, used for page alignment with memory (in chunks of 4096 bytes)
  • The address we see in the disassembly is the same as the address in readelf
  • We can also visualise this by looking again in hex dump
    • We can see the protection bits (1-read, 2-write, 3-both)
    • This correlates to the arguments that we see in strafe

• We can do the same with all of the other system calls
  • We can see the mmap, memunmap, write, read, etc…

• This binary is so small that we can decode the whole thing

• We can see a function call that allocated 16 bytes on the stack, by subtracting it from the stack pointer
  • It moves syscall 4 (write) to another address (look in hex-dump)
  • This writes passcode to the output
  • It prints 10 bytes to stdout - (0x1)
  • It does a write and a system call
  • It does a read and a system call
  • But then does something strange - it uses the stack pointer as the buffer it reads to
    • This gives a stack buffer overflow - you can write a page full of data to the stack - but there's only 16 bytes allocated to this purpose

• If we run it again with strace, without having it crash, it also calls close
  • You can also see this in the disassembly
  • It closes stdin (this is a problem if you want to do shellcode, since you can't send data through it anymore)
  • It then does a write again, and then says BOOM!!, and then returns
  • (The binary doesn't really have any use)

• The 4 bytes can be rewritten
  • We can test this by sending a bunch of A's again
  • i.e. echo -n 'AAAADDDDCCCCEEEEF' | strace ./minibomb
  • The read in strace now looks weird, because it's saying EFAULT (bad address)
  • We can look at this more carefully in gdb
    • We can print the stack pointer: x/40x $sp
    • (This behaves differently inside and outside of GDB since aslr is turned off in gdb)
    • We can write until the end of the page, and then it will give a fault
    • A complication: The address space is randomised, so we don't know exactly where the stack is
  • You have arbitrary code execution for free, but the problem is that you can't inject your shellcode directly and run it, since there's a special section that determines if the writeable address space is being protected as non-executable
    • This is tells the kernel if the stack should be non-executable - in this case, the stack is both readable and writeable
    • Since it's RW, (not-executable) you'll need to use Return Oriented Programming (ROP)
    • This explains why the binary is so tiny - in this case, the amount of addresses to return to is very small (actually impossible)
    • This makes it much more difficult
  • Run this in gdb with lots of A's, and look in the registers: info reg
    • eax is 0xc = the number of bytes written
    • ecx is the buffer
    • edx is the number of bytes that it wanted to write
    • ebx the first argument
    • esp is the stack pointer
  • Do 'info proc map' in gdb to see the memory mapping
    • You can see the text, stack segments and the Virtual Dynamic Shared Object [vdso]

Next CTF Competition

• PlaidCTF: https://ctftime.org/event/119
  • April 11, 2014, 7 p.m. — April 13, 2014, 7 p.m. (Pittsburgh time)
  • We could meet on Saturday April 12 (all day) to play!
    • Alex is volunteering to host our CTF game at his apartment!