Difference between revisions of "CTF-practice-evening:2014-08-04"
| Line 43: | Line 43: | ||
** We're following along w/ the protocol here | ** We're following along w/ the protocol here | ||
** It crashed - a child exited with Signal 6 (SIGABT) | ** It crashed - a child exited with Signal 6 (SIGABT) | ||
| + | ** You can use gdb to find the segfault | ||
| + | ** Now we send the payload - nothing crashed, so the stopped child must have the connection | ||
| + | ** We continue it, and we see the segfault | ||
| + | ** You can use 'info reg' to see the register contents | ||
| + | ** eax now has the value of 0x41414141! | ||
| + | ** Also the instruction point (eip), we can look at what it's pointing at | ||
| + | ** You can use: x/i $eip = call *0x8(%eax) | ||
| + | ** We have control over eax. So we can get it to call code that we enter! | ||
Revision as of 19:15, 4 August 2014
| CTF-practice-evening:2014-08-04 | |
|---|---|
| Date | 2014/08/04 |
| Time | |
| Location | ACTA |
| Type | Workshop |
| Contact | Melanie |
Contents
Capture The Flag evening - Part 23
- 4 August, 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
Binary Exploitation
- Brainsmoke is talking about binary exploitation today
Examining the challenge
- objdump -d: see the disassembly, sometimes you can see symbols
- This example has mangled C++ symbols
- From running it, the program appears to be a daemon of some kind - a Socks proxy
- This is a proxy for TCP - we can look at the protocol details w/ Google
- netstat -uplanet (we can see which ports are used)
- What was added between Socks4 and Socks5? (there might be a bug)
- Authentication and connecting directly to a domain
- Most of the fields are fixed length
- But the domain name is a string - it could have a buffer overflow
- There's a 1 byte name length - if you use a 1 byte length, you might end up w/ a negative number
- If you try to read a negative number, you will try to read a lot of bytes
- We want to find out what happens when you tell the program to read and send 255 bytes
- We want to establish a connection
- We need to specify Socks5
- Use xxd to construct the command (converts hex code to binary, and vice-versa)
- Example: xxd -r -p <<< 41414141410a = AAA
- xxd -r -p <<< 050100 | nc localhost 1080 | dd bs=1 > file (authentication)
- hexdump -C file
- xxd -r -p <<< 05010005010003ff:python<<<'print "A"*2000') | nc localhost 1080 | dd bs=1 > file (connection request)
- We're following along w/ the protocol here
- It crashed - a child exited with Signal 6 (SIGABT)
- You can use gdb to find the segfault
- Now we send the payload - nothing crashed, so the stopped child must have the connection
- We continue it, and we see the segfault
- You can use 'info reg' to see the register contents
- eax now has the value of 0x41414141!
- Also the instruction point (eip), we can look at what it's pointing at
- You can use: x/i $eip = call *0x8(%eax)
- We have control over eax. So we can get it to call code that we enter!
