feat(smtp): fix DATA state machine; add SMTP_OPEN_RELAY mode
- Buffer DATA body until CRLF.CRLF terminator — fixes 502-on-every-body-line bug - SMTP_OPEN_RELAY=1: AUTH accepted (235), RCPT TO accepted for any domain, full DATA pipeline with queued-as message ID - Default (SMTP_OPEN_RELAY=0): credential harvester — AUTH rejected (535) but connection stays open, RCPT TO returns 554 relay denied - SASL PLAIN and LOGIN multi-step AUTH both decoded and logged - RSET clears all per-transaction state - Add development/SMTP_RELAY.md, IMAP_BAIT.md, ICS_SCADA.md, BUG_FIXES.md (live-tested service realism plans)
This commit is contained in:
@@ -1,22 +1,39 @@
|
||||
#!/usr/bin/env python3
|
||||
"""
|
||||
SMTP server — emulates a realistic ESMTP server (Postfix-style).
|
||||
Logs EHLO/AUTH/MAIL FROM/RCPT TO attempts as JSON, then denies auth.
|
||||
|
||||
Two modes of operation, controlled by SMTP_OPEN_RELAY:
|
||||
|
||||
SMTP_OPEN_RELAY=0 (default) — credential harvester
|
||||
AUTH attempts are logged and rejected (535).
|
||||
RCPT TO is rejected with 554 (relay denied) for all recipients.
|
||||
This captures credential stuffing and scanning activity.
|
||||
|
||||
SMTP_OPEN_RELAY=1 — open relay bait
|
||||
AUTH is accepted for any credentials (235).
|
||||
RCPT TO is accepted for any domain (250).
|
||||
DATA is fully buffered until CRLF.CRLF and acknowledged with a
|
||||
queued-as message ID. Attractive to spam relay operators.
|
||||
|
||||
The DATA state machine (and the 502-per-line bug) is fixed in both modes.
|
||||
"""
|
||||
|
||||
import asyncio
|
||||
import base64
|
||||
import os
|
||||
from decnet_logging import syslog_line, write_syslog_file, forward_syslog
|
||||
import random
|
||||
import string
|
||||
from decnet_logging import SEVERITY_WARNING, syslog_line, write_syslog_file, forward_syslog
|
||||
|
||||
NODE_NAME = os.environ.get("NODE_NAME", "mailserver")
|
||||
SERVICE_NAME = "smtp"
|
||||
SERVICE_NAME = "smtp"
|
||||
LOG_TARGET = os.environ.get("LOG_TARGET", "")
|
||||
OPEN_RELAY = os.environ.get("SMTP_OPEN_RELAY", "0").strip() == "1"
|
||||
|
||||
_SMTP_BANNER = os.environ.get("SMTP_BANNER", f"220 {NODE_NAME} ESMTP Postfix (Debian/GNU)")
|
||||
_SMTP_MTA = os.environ.get("SMTP_MTA", NODE_NAME)
|
||||
|
||||
|
||||
|
||||
|
||||
def _log(event_type: str, severity: int = 6, **kwargs) -> None:
|
||||
line = syslog_line(SERVICE_NAME, NODE_NAME, event_type, severity, **kwargs)
|
||||
print(line, flush=True)
|
||||
@@ -24,11 +41,42 @@ def _log(event_type: str, severity: int = 6, **kwargs) -> None:
|
||||
forward_syslog(line, LOG_TARGET)
|
||||
|
||||
|
||||
def _rand_msg_id() -> str:
|
||||
"""Return a Postfix-style 12-char alphanumeric queue ID."""
|
||||
chars = string.ascii_uppercase + string.digits
|
||||
return "".join(random.choices(chars, k=12)) # noqa: S311
|
||||
|
||||
|
||||
def _decode_auth_plain(blob: str) -> tuple[str, str]:
|
||||
"""Decode SASL PLAIN: base64(authzid\0authcid\0passwd) → (user, pass)."""
|
||||
try:
|
||||
decoded = base64.b64decode(blob + "==").decode(errors="replace")
|
||||
parts = decoded.split("\x00")
|
||||
if len(parts) >= 3:
|
||||
return parts[1], parts[2]
|
||||
if len(parts) == 2:
|
||||
return parts[0], parts[1]
|
||||
except Exception:
|
||||
pass
|
||||
return blob, ""
|
||||
|
||||
|
||||
class SMTPProtocol(asyncio.Protocol):
|
||||
def __init__(self):
|
||||
self._transport = None
|
||||
self._peer = ("?", 0)
|
||||
self._buf = b""
|
||||
self._peer = ("?", 0)
|
||||
self._buf = b""
|
||||
# per-transaction state
|
||||
self._mail_from = ""
|
||||
self._rcpt_to: list[str] = []
|
||||
# DATA accumulation
|
||||
self._in_data = False
|
||||
self._data_buf: list[str] = []
|
||||
# AUTH multi-step state (LOGIN mechanism sends user/pass in separate lines)
|
||||
self._auth_state = "" # "" | "await_user" | "await_pass"
|
||||
self._auth_user = ""
|
||||
|
||||
# ── asyncio.Protocol ──────────────────────────────────────────────────────
|
||||
|
||||
def connection_made(self, transport):
|
||||
self._transport = transport
|
||||
@@ -40,14 +88,55 @@ class SMTPProtocol(asyncio.Protocol):
|
||||
self._buf += data
|
||||
while b"\r\n" in self._buf:
|
||||
line, self._buf = self._buf.split(b"\r\n", 1)
|
||||
self._handle_line(line.decode(errors="replace").strip())
|
||||
self._handle_line(line.decode(errors="replace"))
|
||||
|
||||
def connection_lost(self, exc):
|
||||
_log("disconnect", src=self._peer[0] if self._peer else "?")
|
||||
|
||||
# ── Command dispatch ──────────────────────────────────────────────────────
|
||||
|
||||
def _handle_line(self, line: str) -> None:
|
||||
cmd = line.split()[0].upper() if line.split() else ""
|
||||
# ── DATA body accumulation ────────────────────────────────────────────
|
||||
if self._in_data:
|
||||
if line == ".":
|
||||
body = "\r\n".join(self._data_buf)
|
||||
msg_id = _rand_msg_id()
|
||||
_log("message_accepted",
|
||||
src=self._peer[0],
|
||||
mail_from=self._mail_from,
|
||||
rcpt_to=",".join(self._rcpt_to),
|
||||
body_bytes=len(body),
|
||||
msg_id=msg_id)
|
||||
self._transport.write(f"250 2.0.0 Ok: queued as {msg_id}\r\n".encode())
|
||||
self._in_data = False
|
||||
self._data_buf = []
|
||||
self._mail_from = ""
|
||||
self._rcpt_to = []
|
||||
else:
|
||||
# RFC 5321 dot-stuffing: strip leading dot
|
||||
self._data_buf.append(line[1:] if line.startswith(".") else line)
|
||||
return
|
||||
|
||||
# ── AUTH multi-step (LOGIN mechanism) ─────────────────────────────────
|
||||
if self._auth_state == "await_user":
|
||||
self._auth_user = base64.b64decode(line + "==").decode(errors="replace")
|
||||
self._auth_state = "await_pass"
|
||||
self._transport.write(b"334 UGFzc3dvcmQ6\r\n") # "Password:"
|
||||
return
|
||||
if self._auth_state == "await_pass":
|
||||
password = base64.b64decode(line + "==").decode(errors="replace")
|
||||
self._finish_auth(self._auth_user, password)
|
||||
self._auth_state = ""
|
||||
self._auth_user = ""
|
||||
return
|
||||
|
||||
# ── Normal command dispatch ───────────────────────────────────────────
|
||||
parts = line.split(None, 1)
|
||||
cmd = parts[0].upper() if parts else ""
|
||||
args = parts[1] if len(parts) > 1 else ""
|
||||
|
||||
if cmd in ("EHLO", "HELO"):
|
||||
domain = line.split(None, 1)[1] if " " in line else ""
|
||||
_log("ehlo", src=self._peer[0], domain=domain)
|
||||
_log("ehlo", src=self._peer[0], domain=args)
|
||||
self._transport.write(
|
||||
f"250-{_SMTP_MTA}\r\n"
|
||||
f"250-PIPELINING\r\n"
|
||||
@@ -59,41 +148,102 @@ class SMTPProtocol(asyncio.Protocol):
|
||||
f"250-8BITMIME\r\n"
|
||||
f"250 DSN\r\n".encode()
|
||||
)
|
||||
|
||||
elif cmd == "AUTH":
|
||||
_log("auth_attempt", src=self._peer[0], command=line)
|
||||
self._transport.write(b"535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6\r\n")
|
||||
self._transport.close()
|
||||
self._handle_auth(args)
|
||||
|
||||
elif cmd == "MAIL":
|
||||
_log("mail_from", src=self._peer[0], value=line)
|
||||
addr = args.split(":", 1)[1].strip() if ":" in args else args
|
||||
self._mail_from = addr
|
||||
_log("mail_from", src=self._peer[0], value=addr)
|
||||
self._transport.write(b"250 2.1.0 Ok\r\n")
|
||||
|
||||
elif cmd == "RCPT":
|
||||
_log("rcpt_to", src=self._peer[0], value=line)
|
||||
self._transport.write(b"250 2.1.5 Ok\r\n")
|
||||
addr = args.split(":", 1)[1].strip() if ":" in args else args
|
||||
if OPEN_RELAY:
|
||||
self._rcpt_to.append(addr)
|
||||
_log("rcpt_to", src=self._peer[0], value=addr)
|
||||
self._transport.write(b"250 2.1.5 Ok\r\n")
|
||||
else:
|
||||
_log("rcpt_denied", src=self._peer[0], value=addr,
|
||||
severity=SEVERITY_WARNING)
|
||||
self._transport.write(
|
||||
b"554 5.7.1 <" + addr.encode() + b">: Relay access denied\r\n"
|
||||
)
|
||||
|
||||
elif cmd == "DATA":
|
||||
self._transport.write(b"354 End data with <CR><LF>.<CR><LF>\r\n")
|
||||
if not self._rcpt_to:
|
||||
self._transport.write(b"503 5.5.1 Error: need RCPT command\r\n")
|
||||
else:
|
||||
self._in_data = True
|
||||
self._transport.write(b"354 End data with <CR><LF>.<CR><LF>\r\n")
|
||||
|
||||
elif cmd == "RSET":
|
||||
self._mail_from = ""
|
||||
self._rcpt_to = []
|
||||
self._in_data = False
|
||||
self._data_buf = []
|
||||
self._auth_state = ""
|
||||
self._auth_user = ""
|
||||
self._transport.write(b"250 2.0.0 Ok\r\n")
|
||||
|
||||
elif cmd == "VRFY":
|
||||
_log("vrfy", src=self._peer[0], value=line)
|
||||
_log("vrfy", src=self._peer[0], value=args)
|
||||
self._transport.write(b"252 2.0.0 Cannot VRFY user\r\n")
|
||||
|
||||
elif cmd == "NOOP":
|
||||
self._transport.write(b"250 2.0.0 Ok\r\n")
|
||||
|
||||
elif cmd == "STARTTLS":
|
||||
self._transport.write(b"454 4.7.0 TLS not available due to local problem\r\n")
|
||||
|
||||
elif cmd == "QUIT":
|
||||
self._transport.write(b"221 2.0.0 Bye\r\n")
|
||||
self._transport.close()
|
||||
elif cmd == "NOOP":
|
||||
self._transport.write(b"250 2.0.0 Ok\r\n")
|
||||
elif cmd == "RSET":
|
||||
self._transport.write(b"250 2.0.0 Ok\r\n")
|
||||
elif cmd == "STARTTLS":
|
||||
# Pretend we don't support upgrading mid-session
|
||||
self._transport.write(b"454 4.7.0 TLS not available due to local problem\r\n")
|
||||
|
||||
else:
|
||||
_log("unknown_command", src=self._peer[0], command=line)
|
||||
_log("unknown_command", src=self._peer[0], command=line[:128])
|
||||
self._transport.write(b"502 5.5.2 Error: command not recognized\r\n")
|
||||
|
||||
def connection_lost(self, exc):
|
||||
_log("disconnect", src=self._peer[0] if self._peer else "?")
|
||||
# ── AUTH helpers ──────────────────────────────────────────────────────────
|
||||
|
||||
def _handle_auth(self, args: str) -> None:
|
||||
parts = args.split(None, 1)
|
||||
mech = parts[0].upper() if parts else ""
|
||||
initial = parts[1] if len(parts) > 1 else ""
|
||||
|
||||
if mech == "PLAIN":
|
||||
if initial:
|
||||
user, password = _decode_auth_plain(initial)
|
||||
self._finish_auth(user, password)
|
||||
else:
|
||||
# Client will send credentials on next line
|
||||
self._auth_state = "await_plain"
|
||||
self._transport.write(b"334 \r\n")
|
||||
elif mech == "LOGIN":
|
||||
if initial:
|
||||
self._auth_user = base64.b64decode(initial + "==").decode(errors="replace")
|
||||
self._auth_state = "await_pass"
|
||||
self._transport.write(b"334 UGFzc3dvcmQ6\r\n") # "Password:"
|
||||
else:
|
||||
self._auth_state = "await_user"
|
||||
self._transport.write(b"334 VXNlcm5hbWU6\r\n") # "Username:"
|
||||
else:
|
||||
self._transport.write(b"504 5.5.4 Unrecognized authentication mechanism\r\n")
|
||||
|
||||
def _finish_auth(self, username: str, password: str) -> None:
|
||||
_log("auth_attempt", src=self._peer[0],
|
||||
username=username, password=password,
|
||||
severity=SEVERITY_WARNING)
|
||||
if OPEN_RELAY:
|
||||
self._transport.write(b"235 2.7.0 Authentication successful\r\n")
|
||||
else:
|
||||
self._transport.write(b"535 5.7.8 Error: authentication failed\r\n")
|
||||
|
||||
|
||||
async def main():
|
||||
_log("startup", msg=f"SMTP server starting as {NODE_NAME}")
|
||||
mode = "open-relay" if OPEN_RELAY else "credential-harvester"
|
||||
_log("startup", msg=f"SMTP server starting as {NODE_NAME} ({mode})")
|
||||
loop = asyncio.get_running_loop()
|
||||
server = await loop.create_server(SMTPProtocol, "0.0.0.0", 25) # nosec B104
|
||||
async with server:
|
||||
|
||||
Reference in New Issue
Block a user