# 6.1.8 pwn DCTF2017 Flex - [题目解析](#题目解析) - [参考资料](#参考资料) [下载文件](../src/writeup/6.1.8_pwn_dctf2017_flex) ## 题目解析 ``` $ file flex flex: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=30a1acbc98ccf9e8f4b3d1fc06b6ba6f0cbe7c9e, stripped $ checksec -f flex RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE Partial RELRO Canary found NX enabled No PIE No RPATH No RUNPATH Yes 0 4 flex ``` 随便玩一下，了解程序的基本功能： ``` $ ./flex 1.start flexmd5 2.start flexsha256 3.start flexsha1 4.test security 0 quit option: 1 FlexMD5 bruteforce tool V0.1 custom md5 state (yes/No) No custom charset (yes/No) yes charset length: 10 charset: a bruteforce message pattern: aaaa ``` 第四个选项很吸引人，但似乎没有发现什么突破点，而第一个选项可以输入的东西较多，问题应该在这里，查看该函数 `sub.bruteforcing_start:_85f`： ``` 0x00401500 55 push rbp | 0x00401501 4889e5 mov rbp, rsp | 0x00401504 4883ec10 sub rsp, 0x10 | 0x00401508 e83bfcffff call sub.FlexMD5_bruteforce_tool_V0.1_148 | 0x0040150d e87dfaffff call fcn.00400f8f | 0x00401512 bf4f464000 mov edi, str.bruteforcing_start: ; 0x40464f ; "bruteforcing start:" | 0x00401517 e8b4f6ffff call sym.imp.puts ; int puts(const char *s) | ; JMP XREF from 0x00401534 (sub.bruteforcing_start:_500) | .-> 0x0040151c e88cfeffff call sub.strlen_3ad ; size_t strlen(const char *s) | : 0x00401521 85c0 test eax, eax | : 0x00401523 0f94c0 sete al | : 0x00401526 84c0 test al, al | ,==< 0x00401528 740c je 0x401536 | |: 0x0040152a bf01000000 mov edi, 1 | |: 0x0040152f e83cf7ffff call sym.imp.sleep ; int sleep(int s) | |`=< 0x00401534 ebe6 jmp 0x40151c | | ; JMP XREF from 0x00401528 (sub.bruteforcing_start:_500) | | ; JMP XREF from 0x0040155d (sub.bruteforcing_start:_500 + 93) | `.-> 0x00401536 b800000000 mov eax, 0 | ,==< 0x0040153b eb22 jmp 0x40155f |: 0x0040153d 4883fa01 cmp rdx, 1 ; 1 ,===< 0x00401541 7408 je 0x40154b ||: 0x00401543 4889c7 mov rdi, rax ||: 0x00401546 e8f5f7ffff call sym.imp._Unwind_Resume ||: ; JMP XREF from 0x00401541 (sub.bruteforcing_start:_500 + 65) `---> 0x0040154b 4889c7 mov rdi, rax |: 0x0040154e e8bdf7ffff call sym.imp.__cxa_begin_catch |: 0x00401553 8b00 mov eax, dword [rax] |: 0x00401555 8945fc mov dword [rbp - 4], eax |: 0x00401558 e8a3f7ffff call sym.imp.__cxa_end_catch |`=< 0x0040155d ebd7 jmp 0x401536 ; sub.bruteforcing_start:_500+0x36 | | ; JMP XREF from 0x0040153b (sub.bruteforcing_start:_500) | `--> 0x0040155f c9 leave \ 0x00401560 c3 ret ``` 函数 `sub.FlexMD5_bruteforce_tool_V0.1_148`： ``` [0x00400d80]> pdf @ sub.FlexMD5_bruteforce_tool_V0.1_148 / (fcn) sub.FlexMD5_bruteforce_tool_V0.1_148 613 | sub.FlexMD5_bruteforce_tool_V0.1_148 (); | ; var int local_124h @ rbp-0x124 | ; var int local_120h @ rbp-0x120 | ; var int local_18h @ rbp-0x18 | ; CALL XREF from 0x00401508 (sub.bruteforcing_start:_500) | 0x00401148 55 push rbp | 0x00401149 4889e5 mov rbp, rsp | 0x0040114c 53 push rbx | 0x0040114d 4881ec280100. sub rsp, 0x128 | 0x00401154 64488b042528. mov rax, qword fs:[0x28] ; [0x28:8]=-1 ; '(' ; 40 | 0x0040115d 488945e8 mov qword [local_18h], rax | 0x00401161 31c0 xor eax, eax | 0x00401163 bf47454000 mov edi, str.FlexMD5_bruteforce_tool_V0.1 ; 0x404547 ; "FlexMD5 bruteforce tool V0.1" | 0x00401168 e863faffff call sym.imp.puts ; int puts(const char *s) | 0x0040116d bf64454000 mov edi, str.custom_md5_state__yes_No_ ; 0x404564 ; "custom md5 state (yes/No)" | 0x00401172 e859faffff call sym.imp.puts ; int puts(const char *s) | 0x00401177 488d85e0feff. lea rax, [local_120h] | 0x0040117e be04000000 mov esi, 4 | 0x00401183 4889c7 mov rdi, rax | 0x00401186 e8ebfcffff call sub.read_e76 ; ssize_t read(int fildes, void *buf, size_t nbyte) | 0x0040118b 488d85e0feff. lea rax, [local_120h] | 0x00401192 ba03000000 mov edx, 3 | 0x00401197 be7e454000 mov esi, 0x40457e ; "yes" | 0x0040119c 4889c7 mov rdi, rax | 0x0040119f e85cfaffff call sym.imp.strncmp ; int strncmp(const char *s1, const char *s2, size_t n) | 0x004011a4 85c0 test eax, eax | ,=< 0x004011a6 755e jne 0x401206 | | 0x004011a8 c705f24f2000. mov dword [0x006061a4], 1 ; [0x6061a4:4]=0 | | 0x004011b2 bf82454000 mov edi, str.initial_state_0_: ; 0x404582 ; "initial state[0]:" | | 0x004011b7 e814faffff call sym.imp.puts ; int puts(const char *s) | | 0x004011bc e884fdffff call sub.atoi_f45 ; int atoi(const char *str) | | 0x004011c1 8905e94f2000 mov dword [0x006061b0], eax ; [0x6061b0:4]=0 | | 0x004011c7 bf94454000 mov edi, str.initial_state_1_: ; 0x404594 ; "initial state[1]:" | | 0x004011cc e8fff9ffff call sym.imp.puts ; int puts(const char *s) | | 0x004011d1 e86ffdffff call sub.atoi_f45 ; int atoi(const char *str) | | 0x004011d6 8905d84f2000 mov dword [0x006061b4], eax ; [0x6061b4:4]=0 | | 0x004011dc bfa6454000 mov edi, str.initial_state_2_: ; 0x4045a6 ; "initial state[2]:" | | 0x004011e1 e8eaf9ffff call sym.imp.puts ; int puts(const char *s) | | 0x004011e6 e85afdffff call sub.atoi_f45 ; int atoi(const char *str) | | 0x004011eb 8905c74f2000 mov dword [0x006061b8], eax ; [0x6061b8:4]=0 | | 0x004011f1 bfb8454000 mov edi, str.initial_state_3_: ; 0x4045b8 ; "initial state[3]:" | | 0x004011f6 e8d5f9ffff call sym.imp.puts ; int puts(const char *s) | | 0x004011fb e845fdffff call sub.atoi_f45 ; int atoi(const char *str) | | 0x00401200 8905b64f2000 mov dword [0x006061bc], eax ; [0x6061bc:4]=0 | | ; JMP XREF from 0x004011a6 (sub.FlexMD5_bruteforce_tool_V0.1_148) | `-> 0x00401206 bfca454000 mov edi, str.custom_charset__yes_No_ ; 0x4045ca ; "custom charset (yes/No)" | 0x0040120b e8c0f9ffff call sym.imp.puts ; int puts(const char *s) | 0x00401210 488d85e0feff. lea rax, [local_120h] | 0x00401217 be04000000 mov esi, 4 | 0x0040121c 4889c7 mov rdi, rax | 0x0040121f e852fcffff call sub.read_e76 ; ssize_t read(int fildes, void *buf, size_t nbyte) | 0x00401224 488d85e0feff. lea rax, [local_120h] | 0x0040122b ba03000000 mov edx, 3 | 0x00401230 be7e454000 mov esi, 0x40457e ; "yes" | 0x00401235 4889c7 mov rdi, rax | 0x00401238 e8c3f9ffff call sym.imp.strncmp ; int strncmp(const char *s1, const char *s2, size_t n) | 0x0040123d 85c0 test eax, eax | ,=< 0x0040123f 0f858a000000 jne 0x4012cf | | 0x00401245 c705554f2000. mov dword [0x006061a4], 1 ; [0x6061a4:4]=0 | | 0x0040124f bfe2454000 mov edi, str.charset_length: ; 0x4045e2 ; "charset length:" | | 0x00401254 e877f9ffff call sym.imp.puts ; int puts(const char *s) | | 0x00401259 e8e7fcffff call sub.atoi_f45 ; int atoi(const char *str) | | 0x0040125e 8905ac4e2000 mov dword [0x00606110], eax ; [0x606110:4]=62 | | 0x00401264 8b05a64e2000 mov eax, dword [0x00606110] ; [0x606110:4]=62 | | 0x0040126a 3d00010000 cmp eax, 0x100 ; 256 | ,==< 0x0040126f 7e22 jle 0x401293 | || 0x00401271 bf04000000 mov edi, 4 | || 0x00401276 e855faffff call sym.imp.__cxa_allocate_exception | || 0x0040127b c70002000000 mov dword [rax], 2 | || 0x00401281 ba00000000 mov edx, 0 | || 0x00401286 be70616000 mov esi, obj.typeinfoforint ; 0x606170 | || 0x0040128b 4889c7 mov rdi, rax | || 0x0040128e e85dfaffff call sym.imp.__cxa_throw | || ; JMP XREF from 0x0040126f (sub.FlexMD5_bruteforce_tool_V0.1_148) | `--> 0x00401293 bff2454000 mov edi, str.charset: ; 0x4045f2 ; "charset:" | | 0x00401298 e833f9ffff call sym.imp.puts ; int puts(const char *s) | | 0x0040129d 8b056d4e2000 mov eax, dword [0x00606110] ; [0x606110:4]=62 | | 0x004012a3 83c001 add eax, 1 | | 0x004012a6 89c2 mov edx, eax | | 0x004012a8 488d85e0feff. lea rax, [local_120h] | | 0x004012af 89d6 mov esi, edx | | 0x004012b1 4889c7 mov rdi, rax | | 0x004012b4 e8bdfbffff call sub.read_e76 ; ssize_t read(int fildes, void *buf, size_t nbyte) | | 0x004012b9 488d85e0feff. lea rax, [local_120h] | | 0x004012c0 4889c7 mov rdi, rax | | 0x004012c3 e8e8f9ffff call sym.imp.strdup ; char *strdup(const char *src) | | 0x004012c8 488905494e20. mov qword str.ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789, rax ; [0x606118:8]=0x404508 str.ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 | | ; JMP XREF from 0x0040123f (sub.FlexMD5_bruteforce_tool_V0.1_148) | `-> 0x004012cf bffb454000 mov edi, str.bruteforce_message_pattern: ; 0x4045fb ; "bruteforce message pattern:" | 0x004012d4 e8f7f8ffff call sym.imp.puts ; int puts(const char *s) | 0x004012d9 be00040000 mov esi, 0x400 ; 1024 | 0x004012de bfc0616000 mov edi, 0x6061c0 | 0x004012e3 e836fcffff call sub.read_f1e ; ssize_t read(int fildes, void *buf, size_t nbyte) | 0x004012e8 bfc0616000 mov edi, 0x6061c0 | 0x004012ed e85ef9ffff call sym.imp.strlen ; size_t strlen(const char *s) | 0x004012f2 8905a84e2000 mov dword [0x006061a0], eax ; [0x6061a0:4]=0 | 0x004012f8 c785dcfeffff. mov dword [local_124h], 0 | ; JMP XREF from 0x00401334 (sub.FlexMD5_bruteforce_tool_V0.1_148) | .-> 0x00401302 8b85dcfeffff mov eax, dword [local_124h] | : 0x00401308 4863d8 movsxd rbx, eax | : 0x0040130b bfc0616000 mov edi, 0x6061c0 | : 0x00401310 e83bf9ffff call sym.imp.strlen ; size_t strlen(const char *s) | : 0x00401315 4839c3 cmp rbx, rax | ,==< 0x00401318 731d jae 0x401337 | |: 0x0040131a 8b85dcfeffff mov eax, dword [local_124h] | |: 0x00401320 4898 cdqe | |: 0x00401322 0fb680c06160. movzx eax, byte [rax + 0x6061c0] ; [0x6061c0:1]=0 | |: 0x00401329 3c2e cmp al, 0x2e ; '.' ; 46 | ,===< 0x0040132b 7409 je 0x401336 | ||: 0x0040132d 8385dcfeffff. add dword [local_124h], 1 | ||`=< 0x00401334 ebcc jmp 0x401302 | || ; JMP XREF from 0x0040132b (sub.FlexMD5_bruteforce_tool_V0.1_148) | `---> 0x00401336 90 nop | | ; JMP XREF from 0x00401318 (sub.FlexMD5_bruteforce_tool_V0.1_148) | `--> 0x00401337 8b85dcfeffff mov eax, dword [local_124h] | 0x0040133d 4863d8 movsxd rbx, eax | 0x00401340 bfc0616000 mov edi, 0x6061c0 | 0x00401345 e806f9ffff call sym.imp.strlen ; size_t strlen(const char *s) | 0x0040134a 4839c3 cmp rbx, rax | ,=< 0x0040134d 7522 jne 0x401371 | | 0x0040134f bf04000000 mov edi, 4 | | 0x00401354 e877f9ffff call sym.imp.__cxa_allocate_exception | | 0x00401359 c70000000000 mov dword [rax], 0 | | 0x0040135f ba00000000 mov edx, 0 | | 0x00401364 be70616000 mov esi, obj.typeinfoforint ; 0x606170 | | 0x00401369 4889c7 mov rdi, rax | | 0x0040136c e87ff9ffff call sym.imp.__cxa_throw | | ; JMP XREF from 0x0040134d (sub.FlexMD5_bruteforce_tool_V0.1_148) | `-> 0x00401371 bf17464000 mov edi, str.md5_pattern: ; 0x404617 ; "md5 pattern:" | 0x00401376 e855f8ffff call sym.imp.puts ; int puts(const char *s) | 0x0040137b be21000000 mov esi, 0x21 ; '!' ; 33 | 0x00401380 bfc0656000 mov edi, 0x6065c0 | 0x00401385 e8ecfaffff call sub.read_e76 ; ssize_t read(int fildes, void *buf, size_t nbyte) | 0x0040138a b800000000 mov eax, 0 | 0x0040138f 488b4de8 mov rcx, qword [local_18h] | 0x00401393 6448330c2528. xor rcx, qword fs:[0x28] | ,=< 0x0040139c 7405 je 0x4013a3 | | 0x0040139e e81df9ffff call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void) | | ; JMP XREF from 0x0040139c (sub.FlexMD5_bruteforce_tool_V0.1_148) | `-> 0x004013a3 4881c4280100. add rsp, 0x128 | 0x004013aa 5b pop rbx | 0x004013ab 5d pop rbp \ 0x004013ac c3 ret ``` 在 `0x004012b4` 下断点，以检查溢出点： ``` gdb-peda$ x/s $rbp 0x7fffffffe3f0: "5A%KA%gA%6A%" gdb-peda$ pattern_offset 5A%KA%gA%6A% 5A%KA%gA%6A% found at offset: 288 ``` 所以 rbp 的地址为 `288 / 8 = 36`。 找到 libc 的 `do_system` 函数里的 one\_gadget\_rce，地址为 `0x00041ee7`： ``` | 0x00041ee7 488b056aff36. mov rax, qword [0x003b1e58] ; [0x3b1e58:8]=0 | 0x00041eee 488d3d409313. lea rdi, str._bin_sh ; 0x17b235 ; "/bin/sh" | 0x00041ef5 c70521253700. mov dword [obj.lock_4], 0 ; [0x3b4420:4]=0 | 0x00041eff c7051b253700. mov dword [obj.sa_refcntr], 0 ; [0x3b4424:4]=0 | 0x00041f09 488d742430 lea rsi, [local_30h] ; sym.lm_cache ; 0x30 | 0x00041f0e 488b10 mov rdx, qword [rax] | 0x00041f11 67e8c9260800 call sym.execve ``` #### Exploit 完整的 exp 如下： ```python ``` ## 参考资料 - [Shanghai-DCTF-2017 线下攻防Pwn题](https://www.anquanke.com/post/id/89855)