Reverse 100 - Open-Source

Download file code.c.
Simply read the source...

% ./code `python2 -c 'print 0xcafe'` 25 h4cky0u
Brr wrrr grr
Get your key: c0ffee

Flag: c0ffee

Reverse 200 - LoseYou

Download file

Extract the archive to obtain task2.bin and task2.exe.
I decided to study task2.bin.
The routine to reverse is sub_80483DC.
We quicky notice this:


That's very basic, eax will contain our guessed number, ecx will contain the randomly generated number.
All we need to do is to break on the "cmp eax, ecx" at 08048503 and set eax to ecx.

(gdb) b *0x08048503
Breakpoint 1 at 0x8048503
(gdb) r
Starting program: /tmp/task2.bin 
warning: Could not load shared library symbols for
Do you need "set solib-search-path" or "set sysroot"?
Welcome to the LoseYou lottery!
Generating random.....
Make your guess (number 0 or 1): tg
Breakpoint 1, 0x08048503 in ?? ()
(gdb) set $eax=$ecx
(gdb) c
You... you... win??? so lucky! Grab the flag:
::: oh_you_cheat3r :::
[Inferior 1 (process 27619) exited normally]

Flag: oh_you_cheat3r

Reverse 300 - ashtree

Download file
Once again I decided to study the ELF version: task3.bin.

Step 1 - Unpack

The binary seems to be packed by a modified UPX (at least, UPX string is replaced by LOL...).
Let's trace execution:

% strace ./task3.bin 
execve("./task3.bin", ["./task3.bin"], [/* 34 vars */]) = 0
[ Process PID=5640 runs in 32 bit mode. ]
getpid()                                = 5640
gettimeofday({1350215641, 677903}, NULL) = 0
unlink("/tmp/upxCRBOGQOAFQI")           = -1 ENOENT (No such file or directory)
open("/tmp/upxCRBOGQOAFQI", O_RDWR|O_CREAT|O_EXCL, 0700) = 3
ftruncate(3, 9036)                      = 0
old_mmap(NULL, 9036, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = 0xf775a000
old_mmap(0xf775d000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xf775d000
munmap(0xfffffffff775a000, 9036)        = 0
close(3)                                = 0
open("/tmp/upxCRBOGQOAFQI", O_RDONLY)   = 3
getpid()                                = 5640
access("/proc/5640/fd/3", R_OK|X_OK)    = 0
unlink("/tmp/upxCRBOGQOAFQI")           = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
execve("/proc/5640/fd/3", ["./task3.bin"], [/* 41 vars */]) = 0

The binary is unpacking itself into a file "/tmp/upxCRBOGQOAFQI", which is a randomly generated name. Then it is unlinked and executed by execve().

The unlink() function is guaranteed to unlink the file from the file system hierarchy but keep the file on disk until all open instances of the file are closed.

Once the execution is over, the file is deleted, so we need to find the file name, and copy the file elsewhere before it's unlinked. Let's toggle a breakpoint before execve() then.

(gdb) b *0x004024dd
Breakpoint 1 at 0x4024dd
(gdb) r
Starting program: /tmp/task3.bin 
Breakpoint 1, 0x004024dd in ?? ()
(gdb) x/s $ebx
0xffffd494:	"/proc/12767/fd/7"
% file /proc/12767/fd/7
/proc/12767/fd/7: broken symbolic link to `/tmp/upxC5RF3NOAMO5 (deleted)'

Ok! Now we can work on the unpacked binary.

Step 2 - Keygen

Our routine is sub_8048617.
Several conditions must be met:

  • - argv[1] = username
  • - argv[2] = password
  • - username != 'hackyou'
  • - len(password) == 14
  • - password[4] == '-' and password[8] == '-'
  • - sub_804838C(username, password) == True
  • - sub_804844B(username, password[5:]) == True
  • - sub_804850A(username, password[10:]) == True

Although our username must not be 'hackyou', if you take a look at sub_80485F0, the goodboy message, you'll notice it prints "Great! Now submit the license key for 'hackyou'".
sub_804838C, sub_804844B and sub_804850A all proceed in the same way, which finally compares individually 4 bytes of the given password parameter:


Our password character is in edx, the expected one is in eax. Lazy as i am, i didn't go much deeper :)

All we need now is to break on:

.text:0804842B                 cmp     eax, edx

... dump eax, and fix edx so that it will be equal to eax. Same for the two last routines. Let's create a basic pythonGDB script to do the work for us.

import gdb
passwd = ""
passwd_len = 0
def callback_username_condition():
    gdb.execute("set $eax=1")
def callback_compare_password():
    global passwd, passwd_len
    gdb.execute("set $edx=$eax")
    if passwd_len in (4, 9):
        passwd += '-'
        passwd_len += 1
    passwd += chr(gdb.parse_and_eval("$eax"))
    passwd_len += 1
    print "[+]", passwd
class HitBreakpoint(gdb.Breakpoint):
    def __init__(self, loc, callback):
        super(HitBreakpoint, self).__init__(
            loc, gdb.BP_BREAKPOINT, internal=False
        self.callback = callback
    def stop(self):
        return False
HitBreakpoint("*0x08048665", callback_username_condition)
HitBreakpoint("*0x0804842B", callback_compare_password)
HitBreakpoint("*0x080484EA", callback_compare_password)
HitBreakpoint("*0x080485A9", callback_compare_password)

And run it...

(gdb) source 
Breakpoint 1 at 0x8048665
Breakpoint 2 at 0x804842b
Breakpoint 3 at 0x80484ea
Breakpoint 4 at 0x80485a9
(gdb) r hackyou 0123-4567-8910
Starting program: /tmp/unpacked_rev300 hackyou 0123-4567-8910
warning: Could not load shared library symbols for
Do you need "set solib-search-path" or "set sysroot"?
[+] k
[+] ke
[+] kec
[+] kecc
[+] kecc-h
[+] kecc-ha
[+] kecc-hac
[+] kecc-hack
[+] kecc-hack-y
[+] kecc-hack-yo
[+] kecc-hack-yo0
[+] kecc-hack-yo0u
Great! Now submit the license key for 'hackyou'
[Inferior 1 (process 14524) exited with code 01]

Flag: kecc-hack-yo0u