CTF-practice-evening:2014-03-24

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

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

Next CTF Competition

• We are having a look at:https://ctftime.org/event/list/upcoming