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

3 files changed, 326 insertions, 3 deletions

diff --git a/packages/meshbay-hub/tests/conftest.py b/packages/meshbay-hub/tests/conftest.py
index cb595c5..69c3c5b 100644
--- a/packages/meshbay-hub/tests/conftest.py
+++ b/packages/meshbay-hub/tests/conftest.py
@@ -95,7 +95,12 @@ def _skip_email_verification(monkeypatch):
 
     monkeypatch.setattr(
         "meshbay_hub.api.users._create_and_send_verification", _noop)
-    monkeypatch.setattr("meshbay_hub.mail._send", lambda msg: True)
+    # `**_` because `_send` takes a required keyword `purpose` — the gate
+    # that keeps the hub off the open-relay list. A stub with the old
+    # signature turns every send into a TypeError, which looks like a bug
+    # in the handler. `test_mail_is_not_a_relay.py` opts out of this and
+    # drives the real thing.
+    monkeypatch.setattr("meshbay_hub.mail._send", lambda msg, **_: True)
 
 
 @pytest.fixture(autouse=True)
diff --git a/packages/meshbay-hub/tests/test_mail_is_not_a_relay.py b/packages/meshbay-hub/tests/test_mail_is_not_a_relay.py
new file mode 100644
index 0000000..a56ab17
--- /dev/null
+++ b/packages/meshbay-hub/tests/test_mail_is_not_a_relay.py
@@ -0,0 +1,313 @@
+"""
+The hub can send mail from its own domain, to anywhere, with no authentication
+to its local Postfix. That is an open relay unless something bounds it, and
+what bounds it has to be one place, or the next `send_*` helper added will not
+have it.
+
+Three API paths reach that ability and two of them mail an address the caller
+types:
+
+  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, so registering a
+victim's address once bought the right to mail them at the endpoint's rate
+limit indefinitely.
+
+The bound that matters is per **recipient**, because that is what a person
+being mail-bombed experiences, and no combination of accounts, addresses or
+endpoints moves it. Per-account and per-IP limits sit above it and bound a
+caller, which is a different and weaker thing.
+"""
+
+import base64
+
+import pytest
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
+
+from meshbay_common.crypto import pk_to_b64
+from meshbay_hub import mail as mail_mod
+from meshbay_hub.api import users as users_mod
+
+# Captured at import, before conftest's autouse fixture replaces them for every
+# other test in the suite: one stubs `_send` out entirely and the other skips
+# the registration mail. This file is about the code they stub.
+_REAL_SEND = mail_mod._send
+_REAL_CREATE_AND_SEND = users_mod._create_and_send_verification
+
+
+@pytest.fixture(autouse=True)
+def _real_mail_path(monkeypatch):
+    """Undo conftest's stubs, for the tests that go through the API.
+
+    The direct tests below call `_REAL_SEND` rather than `mail_mod._send`:
+    depending on a fixture to *un*-patch a module global is a coupling that
+    passes alone and fails in a full run, which is how this file first behaved.
+    """
+    monkeypatch.setattr(mail_mod, "_send", _REAL_SEND)
+    monkeypatch.setattr(users_mod, "_create_and_send_verification",
+                        _REAL_CREATE_AND_SEND)
+    assert mail_mod._send is _REAL_SEND
+
+
+@pytest.fixture(autouse=True)
+def _fresh_budget():
+    """Each test starts with the hub's allowance untouched."""
+    mail_mod._destinations.clear()
+    mail_mod._hour = (0.0, 0)
+    yield
+    mail_mod._destinations.clear()
+    mail_mod._hour = (0.0, 0)
+
+
+@pytest.fixture
+def wire(monkeypatch):
+    """Everything that actually reaches Postfix, captured at the socket."""
+    sent = []
+
+    class _FakeSMTP:
+        def __init__(self, *a, **kw):
+            pass
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, *a):
+            return False
+
+        def send_message(self, msg):
+            sent.append((msg["To"], msg["Subject"]))
+
+    monkeypatch.setattr(mail_mod.smtplib, "SMTP", _FakeSMTP)
+    return sent
+
+
+def _msg(to: str):
+    from email.message import EmailMessage
+    m = EmailMessage()
+    m["From"] = "noreply@test"
+    m["To"] = to
+    m["Subject"] = "test"
+    m.set_content("test")
+    return m
+
+
+# ── The gate itself ──────────────────────────────────────────────────────────
+
+def test_a_send_must_name_a_purpose(wire):
+    """Keyword-required, so a helper that forgets is a TypeError rather than
+    an unrestricted send."""
+    with pytest.raises(TypeError):
+        _REAL_SEND(_msg("someone@example.test"))
+
+
+def test_only_the_four_purposes_send(wire):
+    assert _REAL_SEND(_msg("a@example.test"), purpose="registration")
+    assert not _REAL_SEND(_msg("b@example.test"), purpose="marketing")
+    assert not _REAL_SEND(_msg("c@example.test"), purpose="")
+    assert [to for to, _ in wire] == ["a@example.test"]
+
+
+def test_one_recipient_has_a_cooldown_and_a_daily_allowance(wire):
+    """Across every purpose — the recipient is the thing being protected, not
+    the sender."""
+    victim = "victim@example.test"
+    assert _REAL_SEND(_msg(victim), purpose="registration")
+    assert not _REAL_SEND(_msg(victim), purpose="password_reset"), (
+        "a second purpose walked around the cooldown")
+
+    # Past the cooldown, up to the day's allowance, then no more.
+    for _ in range(mail_mod.DESTINATION_DAILY_CAP - 1):
+        mail_mod._destinations[mail_mod._destination_key(victim)] = (
+            0.0,
+            mail_mod._destinations[mail_mod._destination_key(victim)][1],
+            mail_mod._destinations[mail_mod._destination_key(victim)][2])
+        assert _REAL_SEND(_msg(victim), purpose="invite")
+
+    mail_mod._destinations[mail_mod._destination_key(victim)] = (
+        0.0, mail_mod.DESTINATION_DAILY_CAP,
+        mail_mod._destinations[mail_mod._destination_key(victim)][2])
+    assert not _REAL_SEND(_msg(victim), purpose="invite")
+    assert len(wire) == mail_mod.DESTINATION_DAILY_CAP
+
+
+def test_the_same_recipient_is_one_recipient_however_it_is_written(wire):
+    """Case and surrounding space are not a new person."""
+    assert _REAL_SEND(_msg("Victim@Example.Test"), purpose="registration")
+    assert not _REAL_SEND(_msg(" victim@example.test "), purpose="registration")
+    assert len(wire) == 1
+
+
+def test_the_instance_has_a_ceiling_no_account_can_buy_past(wire):
+    """Registration is open, so "per account" is a bound an attacker buys more
+    of. This one cannot be bought."""
+    for i in range(mail_mod.HOURLY_BUDGET):
+        assert _REAL_SEND(_msg(f"u{i}@example.test"), purpose="registration")
+    assert not _REAL_SEND(_msg("one-more@example.test"), purpose="registration")
+    assert len(wire) == mail_mod.HOURLY_BUDGET
+
+
+def test_a_refusal_never_names_the_address():
+    """This line goes to the journal, which is not where addresses live.
+
+    Listens on the module's own logger rather than through `caplog`: the hub
+    configures logging when an app is built, so whether pytest's capture sees
+    anything depends on which tests ran first. In the full suite it saw
+    nothing, and the assertion that had to hold — no address in the line —
+    was never reached.
+    """
+    import logging
+
+    records: list[str] = []
+
+    class _Capture(logging.Handler):
+        def emit(self, record):
+            records.append(record.getMessage())
+
+    # The refusal itself, which is what the log line is built from.
+    with pytest.raises(mail_mod.MailRefused) as refusal:
+        mail_mod._budget_or_refuse("marketing", "private@example.test")
+    assert "private@example.test" not in str(refusal.value)
+
+    # And the line as emitted. Level and `logging.disable` are both forced:
+    # attaching a handler is not enough when an earlier test has raised the
+    # logger's level, which is why the first version of this saw nothing in
+    # the full suite and everything on its own.
+    handler = _Capture()
+    previous_level, previously_disabled = mail_mod.log.level, mail_mod.log.disabled
+    mail_mod.log.addHandler(handler)
+    mail_mod.log.setLevel(logging.WARNING)
+    # `.disabled` as well as the level and the manager-wide switch. Something
+    # earlier in a full run leaves this logger disabled — pytest's own capture
+    # plugin toggles loggers between tests — and a disabled logger drops the
+    # record before any handler sees it. Alone, none of this is needed; the
+    # first two versions of this test passed alone and failed in the suite for
+    # two different reasons, which is the whole argument for not reading a
+    # global's state in a test without pinning it first.
+    mail_mod.log.disabled = False
+    logging.disable(logging.NOTSET)
+    try:
+        _REAL_SEND(_msg("private@example.test"), purpose="marketing")
+    finally:
+        mail_mod.log.removeHandler(handler)
+        mail_mod.log.setLevel(previous_level)
+        mail_mod.log.disabled = previously_disabled
+
+    assert records, "a refusal must leave a trace"
+    assert not any("private@example.test" in line for line in records)
+    assert any("marketing" in line for line in records), (
+        "the trace has to say what was refused to be worth writing")
+
+
+# ── The doors, driven through the API ────────────────────────────────────────
+
+async def _register(client, username: str, email: str, captcha=None):
+    sk_ed, sk_x = Ed25519PrivateKey.generate(), X25519PrivateKey.generate()
+    return await client.post("/v1/users/register", json={
+        "username": username,
+        "email": email,
+        "auth_key": base64.b64encode(b"k" * 32).decode(),
+        "pk_user_ed25519": pk_to_b64(sk_ed.public_key()),
+        "pk_user_x25519": pk_to_b64(sk_x.public_key()),
+    })
+
+
+@pytest.mark.asyncio
+async def test_the_register_resend_branch_cannot_be_hammered(client, wire):
+    """
+    No token and no captcha reach this branch. Registering a victim's address
+    once used to buy the right to mail them at the endpoint's rate limit for
+    as long as the account stayed pending.
+    """
+    victim = "bombing-target@example.test"
+    r = await _register(client, "relay_a", victim)
+    assert r.status_code == 201, r.text
+    assert len(wire) == 1
+
+    for _ in range(5):
+        r = await _register(client, "relay_a", victim)
+        assert r.status_code == 201, r.text
+    assert len(wire) == 1, f"{len(wire)} mails to one address from one sign-up"
+
+
+async def _signed_in(client, db_session, username: str, email: str) -> dict:
+    """Register, activate, sign in.
+
+    Activated by hand because this file restores the real verification mail,
+    which conftest stubs out for the rest of the suite — so the account is
+    genuinely `pending` here, exactly as it would be in production, and a
+    pending account cannot log in.
+    """
+    from sqlalchemy import select, update
+
+    from meshbay_hub.db.models import User
+
+    r = await _register(client, username, email)
+    assert r.status_code == 201, r.text
+    await db_session.execute(
+        update(User).where(User.username == username).values(status="active"))
+    await db_session.commit()
+    assert (await db_session.execute(
+        select(User.status).where(User.username == username))).scalar() == "active"
+
+    r = await client.post("/v1/users/login", json={
+        "username": username,
+        "auth_key": base64.b64encode(b"k" * 32).decode()})
+    assert r.status_code == 200, r.text
+    return {"Authorization": f"Bearer {r.json()['access_token']}"}
+
+
+@pytest.mark.asyncio
+async def test_a_signed_in_account_cannot_mail_a_stranger_repeatedly(
+        client, db_session, wire, monkeypatch):
+    """
+    `PATCH /v1/users/me` is the one path where a signed-in account chooses the
+    recipient. A cooldown alone still allowed one stranger a minute — 1440 a
+    day — so there is a ceiling per day above it.
+    """
+    monkeypatch.setattr(
+        "meshbay_hub.api.users.EMAIL_CHANGE_COOLDOWN", 0)
+
+    headers = await _signed_in(client, db_session, "relay_b",
+                               "relay_b@example.test")
+    before = len(wire)
+
+    accepted = 0
+    for i in range(8):
+        r = await client.patch("/v1/users/me", headers=headers,
+                               json={"email": f"stranger{i}@example.test"})
+        if r.status_code == 200:
+            accepted += 1
+        else:
+            assert r.status_code == 429, r.text
+
+    from meshbay_hub.api.users import MAX_EMAIL_CHANGES_PER_DAY
+    assert accepted == MAX_EMAIL_CHANGES_PER_DAY, (
+        f"{accepted} strangers mailed by one account in one day")
+    assert len(wire) - before == MAX_EMAIL_CHANGES_PER_DAY
+
+
+@pytest.mark.asyncio
+async def test_the_daily_ceiling_survives_the_row_being_deleted(
+        client, db_session, wire, monkeypatch):
+    """
+    The change path deletes this account's unverified verification rows before
+    writing a new one, so a ceiling counted from that table counts one, always
+    — which is what the first version of this counted. It comes off the IP log.
+    """
+    monkeypatch.setattr("meshbay_hub.api.users.EMAIL_CHANGE_COOLDOWN", 0)
+    headers = await _signed_in(client, db_session, "relay_c",
+                               "relay_c@example.test")
+
+    for i in range(3):
+        await client.patch("/v1/users/me", headers=headers,
+                           json={"email": f"c-stranger{i}@example.test"})
+
+    r = await client.patch("/v1/users/me", headers=headers,
+                           json={"email": "c-one-more@example.test"})
+    assert r.status_code == 429, (
+        "the ceiling was counted from a table the handler empties")
diff --git a/packages/meshbay-hub/tests/test_recovery_email.py b/packages/meshbay-hub/tests/test_recovery_email.py
index e4288a1..c6dab97 100644
--- a/packages/meshbay-hub/tests/test_recovery_email.py
+++ b/packages/meshbay-hub/tests/test_recovery_email.py
@@ -31,7 +31,12 @@ def _skip_email_verification(monkeypatch):
     run so the e-mail is actually built. Capture it instead of sending.
     """
     sent = []
-    monkeypatch.setattr("meshbay_hub.mail._send", lambda msg: sent.append(msg) or True)
+    # `**_` swallows the `purpose` keyword `_send` now requires — the gate
+    # that keeps this hub off the open-relay list. What this module is
+    # about is the body of the message, so the gate is stubbed away with
+    # the socket; `test_mail_is_not_a_relay.py` is where it is exercised.
+    monkeypatch.setattr("meshbay_hub.mail._send",
+                        lambda msg, **_: sent.append(msg) or True)
     return sent
 
 
@@ -84,7 +89,7 @@ async def test_the_recovery_key_is_not_persisted(
 def test_mail_body_with_and_without_the_key(monkeypatch):
     captured = []
     monkeypatch.setattr("meshbay_hub.mail._send",
-                        lambda msg: captured.append(msg) or True)
+                        lambda msg, **_: captured.append(msg) or True)
 
     mail.send_verification_code("x@example.com", "123456",
                                 recovery_key="MY-RECOVERY-KEY")