BackdoorCTF 2016 - debug
BackdoorCTF is the annual flagship CTF competition conducted by SDSLabs and InfoSecIITR.
Take sha256 of string obtained. http://hack.bckdr.in/DEBUG/debug32
So they gave a 32-bit ELF stripped executable. Simply running the binary
./debug32 didn’t do nothing.
I looked through the assembly in IDA and saw “Printing Flag” being printed somewhere.
So the first and probably the last thing I needed to do was to jump to the function printing it. The address of the function as we can see is at
For this I used gdb. My first instinct was to set a breakpoint at
main, then set the
eip to the address of the required function and continue. This would print out the flag.
But since this was a stripped binary(hence no symbols table), it didn’t recognise
main as a valid breakpoint. So I set the breakpoint at
__libc_start_main() function. This is the function which sets up the environment and then calls the
main() function when the binary is run.
So to carry out the required task, these were the commands I used:
break __libc_start_main set $eip = 0x804849b continue
This as expected printed out the flag!
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BackdoorCTF 2016 - buzybee