TJCTF 2018 - Validator
This challenge is from an old TJCTF competition. The file they gave us displays the message Valid flag if it recieves the correct flag.
The file command tells us that this is a 32 bit elf file.
When we run strings we can see something that looks like the flag
To confirm that this is actually the correct flag let’s disassemble the file.
When we disassemble the main function in radare2 we are able to see values being moved into different vars. These values look like the characters we saw from the strings command earlier.
To confirm that this is infact the correct flag we can enter a dummy string and compare our input with var_38h. We need to make sure that our string is 43 characters so that we can pass the string length comparison at 0x0804858a.
To generate the string quickly we can use this oneliner ''.join(random.choices(string.ascii_uppercase + string.digits, k=43))
We will also need to set a breakpoint at 0x080485ba. This is where eax is being pushed to the string compare after receiving the address of the flag string.
When we hit our breakpoint, eax contains the address 0xffda8060.
When we view the contents stored at the address we get the flag.
When we enter in the flag as a solution we get Valid flag.
