fix(hub): the mail server is not a relay

The previous commit metered the paths that send mail. It was not enough, and
saying it was would have been wrong: a 60-second cooldown per account still
allows one stranger a minute — 1440 a day — and registration is open, so
"per account" is a bound an attacker buys more of. And there was a third door
nobody had counted.

  POST  /v1/users/register            an address nobody has verified
  PATCH /v1/users/me                  an address nobody has verified, signed in
  POST  /v1/users/password/reset      only the address already on file
  POST  /v1/groups/{id}/invite-notify only a registered member's address

The widest was the register *resend* branch: no token, no captcha, and the
username and address are the caller's own from a moment ago — registering a
victim's address once bought the right to mail them at the endpoint's rate
limit for as long as the account stayed pending.

So the bound moves into `mail.py`, where every message passes one function.
`purpose` is keyword-required and checked against a closed list, so a helper
that names anything else does not send and one that names nothing is a
TypeError rather than an unrestricted send. Under it:

  - a bound per **recipient**, across every purpose, account and endpoint —
    what a person being mail-bombed actually experiences, and the only bound
    that describes it. Keyed on a hash, because this would otherwise be the
    one place in the hub holding plaintext addresses in memory (S2)
  - an instance-wide hourly ceiling, which cannot be bought with more accounts
  - a cooldown on the resend branch, a cooldown and a daily ceiling on the
    address change, and the IP-log entry that endpoint never wrote — alone
    among the ones that mail

The ceiling on address changes counts IP-log rows, not EmailVerification: the
handler deletes this account's unverified rows before writing a new one, so
counting those counts one, always. Which is what the first version of it did.

Refusals never carry the address: that line goes to the journal.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T4YmK41VsEURWFdop4EEeT

2 files changed, 160 insertions, 10 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index 05bf075..702d687 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -130,6 +130,10 @@ class RefreshRequest(BaseModel):
 
 # ── Endpoints ─────────────────────────────────────────────────────────────────
 
+# Between two verification mails to one pending account.
+VERIFICATION_RESEND_COOLDOWN = 120
+
+
 @router.post("/register", status_code=201)
 @limiter.limit("5/minute")
 async def register(
@@ -147,9 +151,26 @@ async def register(
         if found.status == "pending" and found.email_hash == eh:
             # Same person retrying before validation — resend a code.
             # No captcha: the initial registration already passed it.
-            await _create_and_send_verification(
-                db, found, body.email, eh, body.recovery_key)
-            await db.commit()
+            #
+            # Which made this the widest of the three mail doors: no token, no
+            # captcha, and the username and address are the caller's own from
+            # a moment ago. Registering a victim's address once bought the
+            # right to mail them at the endpoint's rate limit for ever.
+            # `mail.py` bounds the recipient regardless, and this stops the
+            # door being hammered — the answer is the same either way, which
+            # is the one this endpoint has always given.
+            recent = await db.execute(
+                select(EmailVerification).where(
+                    EmailVerification.user_id == found.id,
+                    EmailVerification.purpose == "registration",
+                    EmailVerification.created_at
+                    > datetime.now(timezone.utc)
+                    - timedelta(seconds=VERIFICATION_RESEND_COOLDOWN),
+                ))
+            if not recent.first():
+                await _create_and_send_verification(
+                    db, found, body.email, eh, body.recovery_key)
+                await db.commit()
             return {"user_id": found.id, "email_verification_required": True}
         raise HTTPException(status_code=409, detail="Username already taken")
 
@@ -640,6 +661,7 @@ class UpdateProfileRequest(BaseModel):
 # the account's, not the caller's IP: the point is the mailbox on the receiving
 # end, and an IP is not what fills it.
 EMAIL_CHANGE_COOLDOWN = 60
+MAX_EMAIL_CHANGES_PER_DAY = 3
 
 
 @router.patch("/me")
@@ -680,6 +702,27 @@ async def update_profile(
                 status_code=429,
                 detail="A code was just sent. Wait a minute before asking again.")
 
+        # And a ceiling per day, because the cooldown alone still allows one
+        # stranger a minute — 1440 a day, from one account, on a hub where
+        # registration is open. This is the only path where a signed-in
+        # account chooses the recipient, so it is the only one that needs it.
+        #
+        # Counted from the IP log, not from EmailVerification: the block below
+        # deletes this account's unverified rows before writing a new one, so
+        # counting those would have counted one, always. (Which is what the
+        # first version of this did.) The log entry is worth having for its
+        # own sake — this endpoint wrote none, alone among the ones that mail.
+        today = await db.execute(
+            select(func.count()).select_from(IPLog).where(
+                IPLog.user_id == current_user.id,
+                IPLog.event == "email_change_request",
+                IPLog.timestamp > datetime.now(timezone.utc) - timedelta(days=1),
+            ))
+        if (today.scalar() or 0) >= MAX_EMAIL_CHANGES_PER_DAY:
+            raise HTTPException(
+                status_code=429,
+                detail="Too many address changes today. Try again tomorrow.")
+
         # Check that no other active/pending account uses this email
         dup = await db.execute(
             select(User).where(
@@ -709,6 +752,8 @@ async def update_profile(
             user_id=current_user.id,
             expires_at=datetime.now(timezone.utc) + timedelta(seconds=VERIFICATION_TTL),
         ))
+        db.add(IPLog(user_id=current_user.id, event="email_change_request",
+                     ip_address=client_ip(request)))
         await db.flush()
         await mail.send_off_loop(mail.send_email_change_code, new_email, code)
         pending_email = new_email
diff --git a/packages/meshbay-hub/src/meshbay_hub/mail.py b/packages/meshbay-hub/src/meshbay_hub/mail.py
index 8373204..5d13b8d 100644
--- a/packages/meshbay-hub/src/meshbay_hub/mail.py
+++ b/packages/meshbay-hub/src/meshbay_hub/mail.py
@@ -3,15 +3,107 @@ MeshBay Hub — email sending via localhost Postfix.
 
 Postfix listens on loopback only (inet_interfaces = loopback-only), so no
 authentication is needed. See docs/MAIL-SERVER.md for the full setup.
+
+**No authentication is needed** is exactly why everything below exists. The
+hub can emit mail from its own domain to anywhere, and three API paths reach
+that ability — two of them at an address the caller types. Unbounded, that is
+an open relay wearing the instance's reputation, so the bounds are here, in
+the one function every send passes through, rather than at the call sites
+where the next one added would forget them (`AV9`–`AV10`, §13.5b).
 """
 
 import asyncio
+import hashlib
 import logging
 import smtplib
+import time
 from email.message import EmailMessage
 
 log = logging.getLogger(__name__)
 
+
+class MailRefused(Exception):
+    """The gate below declined to send. Never carries the address."""
+
+
+# The complete list of reasons this hub will ever send mail. A `send_*`
+# function that names anything else does not send — and one that names nothing
+# is a TypeError, because `purpose` is keyword-required.
+#
+#   registration   confirm an address at sign-up
+#   password_reset a code to the address already on file for that account
+#   invite         tell a registered member they were invited to a group
+#   email_change   confirm a new address before it replaces the old one
+#
+# Only `registration` and `email_change` can reach an address this hub has no
+# prior relationship with. Both are necessary; both are why the per-destination
+# bound below is keyed on the recipient rather than on who asked.
+ALLOWED_PURPOSES = frozenset({
+    "registration", "password_reset", "invite", "email_change"})
+
+# Per recipient, across every purpose, every account and every IP. This is the
+# bound that matters: it is what a person being mail-bombed actually
+# experiences, and no combination of accounts, addresses or endpoints moves it.
+DESTINATION_COOLDOWN_SECONDS = 120
+DESTINATION_DAILY_CAP = 5
+
+# Instance-wide ceiling. Registration is open, so "per account" is a bound an
+# attacker buys more of; this one cannot be bought. Sized far above a real
+# hub's traffic — meshbay.org sends single-digit mails a day — and low enough
+# that being used as a relay is not worth the trouble.
+HOURLY_BUDGET = 200
+
+# Single-process state, like `_connected_nodes` and `_node_groups` next door:
+# the hub serves on one worker (`server.workers` defaults to 1) and the
+# signaling registries already require it. A restart clears these, which costs
+# at most one burst and is not a security decision — unlike the denylist,
+# which persists for exactly that reason (S3).
+_destinations: dict[str, tuple[float, int, float]] = {}   # key → (last, day count, day start)
+_hour: tuple[float, int] = (0.0, 0)                       # (window start, count)
+
+
+def _destination_key(address: str) -> str:
+    """A stable handle for one recipient that is not the address itself.
+
+    Hashed because this dict is the one place in the hub that would otherwise
+    hold a list of plaintext addresses in memory — the rest of the codebase
+    goes to the trouble of encrypting them at rest (S2).
+    """
+    return hashlib.sha256(address.strip().lower().encode()).hexdigest()[:32]
+
+
+def _budget_or_refuse(purpose: str, address: str) -> None:
+    """Raise MailRefused unless this hub may send this, to this person, now."""
+    global _hour
+
+    if purpose not in ALLOWED_PURPOSES:
+        raise MailRefused(f"not a purpose this hub sends mail for: {purpose!r}")
+
+    now = time.monotonic()
+    start, count = _hour
+    if now - start >= 3600:
+        start, count = now, 0
+    if count >= HOURLY_BUDGET:
+        _hour = (start, count)
+        raise MailRefused("the hub's hourly mail budget is spent")
+
+    key = _destination_key(address)
+    last, sent_today, day_start = _destinations.get(key, (0.0, 0, now))
+    if now - day_start >= 86400:
+        sent_today, day_start = 0, now
+    if last and now - last < DESTINATION_COOLDOWN_SECONDS:
+        raise MailRefused("too soon since the last mail to this recipient")
+    if sent_today >= DESTINATION_DAILY_CAP:
+        raise MailRefused("this recipient has had its daily allowance")
+
+    if len(_destinations) > 10_000:
+        for k, (_, _, started) in list(_destinations.items()):
+            if now - started >= 86400:
+                _destinations.pop(k, None)
+
+    _destinations[key] = (now, sent_today + 1, day_start)
+    _hour = (start, count + 1)
+
 _hub_domain: str = "meshbay.org"
 _hub_url: str = "https://meshbay.org"
 
@@ -22,14 +114,27 @@ def configure(hub_id: str) -> None:
     _hub_url = f"https://{hub_id}"
 
 
-def _send(msg: EmailMessage) -> bool:
-    """Blocking. Every caller in an async handler must use `send_off_loop`."""
+def _send(msg: EmailMessage, *, purpose: str) -> bool:
+    """Blocking. Every caller in an async handler must use `send_off_loop`.
+
+    The gate is here rather than in `send_off_loop` so that it cannot be
+    stepped around: a new `send_*` helper, a script, a test — everything that
+    puts a message on the wire comes through this function, and `purpose` is
+    keyword-required so forgetting it is a TypeError rather than an
+    unrestricted send.
+    """
+    try:
+        _budget_or_refuse(purpose, msg["To"] or "")
+    except MailRefused as refusal:
+        # Never the address: this line goes to the journal.
+        log.warning("Mail refused (%s): %s", purpose, refusal)
+        return False
     try:
         with smtplib.SMTP("localhost", 25, timeout=10) as s:
             s.send_message(msg)
         return True
     except Exception:
-        log.exception("Failed to send email to %s", msg["To"])
+        log.exception("Failed to send email to %s", _mask_email(msg["To"] or ""))
         return False
 
 
@@ -86,7 +191,7 @@ def send_verification_code(to: str, code: str, recovery_key: str | None = None)
     msg["To"] = to
     msg["Subject"] = f"MeshBay — Your verification code: {code}"
     msg.set_content(body)
-    _send(msg)
+    _send(msg, purpose="registration")
     log.info("Verification code sent to %s (recovery_key=%s)",
              _mask_email(to), bool(recovery_key))
 
@@ -106,7 +211,7 @@ def send_email_change_code(to: str, code: str) -> None:
         "\n"
         f"{_hub_url}\n"
     )
-    _send(msg)
+    _send(msg, purpose="email_change")
     log.info("Email change code sent to %s", _mask_email(to))
 
 
@@ -132,7 +237,7 @@ def send_password_reset_code(to: str, code: str) -> None:
         "\n"
         f"{_hub_url}\n"
     )
-    _send(msg)
+    _send(msg, purpose="password_reset")
     log.info("Passphrase reset code sent to %s", _mask_email(to))
 
 
@@ -153,7 +258,7 @@ def send_invite_notification(
         "\n"
         f"{_hub_url}\n"
     )
-    _send(msg)
+    _send(msg, purpose="invite")
     log.info("Invite notification sent to %s", _mask_email(to))