fix(hub): a desktop solve reports no hostname at all, not "meshbay"

Registering from the native client failed with `captcha_failed` while the
checkbox was green — a worse symptom than the one being fixed, because the
widget now looked fine and only the hub's own log said otherwise:

    captcha solved on an unexpected host ''; allowed: ['localhost', 'meshbay', 'meshbay.org']

The previous commit assumed Google would report the host component of the
origin, so `app://meshbay` would come back as `meshbay` and could sit in
`allowed_hosts`. It does not. A solve Google cannot attribute to a domain
reports an **empty** hostname, and no allowlist entry can match that. An empty
entry is not the answer either: a blank in a TOML list is a typo far more
often than an intention, and `load_config` drops blanks for that reason —
`captcha.allow_unattributed_host` is a named flag instead, so the trade is
stated where it is made.

What it admits, plainly: every non-web client, not only ours. A file:// page
or somebody else's Electron application look identical from here. That is the
same bar the client's own origin would have been — main.js already records
that `app://meshbay` is not a credential — and it is a bar: the captcha still
has to be solved, per token, in something that can render it. What is given up
is the origin restriction for non-web clients, not the captcha. Off by
default, and a hub without the desktop client should leave it off.

The refusal now names which of the two it is, since they need different
answers: an unexpected host names the host, an unattributed one says to set
the flag.

docs/captcha.md §6 said `meshbay` was the value and told operators to add it;
it now records what was measured and why the guess was wrong. The packaged
example config carries the flag with the same warning.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014UtzVrzM7e2tG9fSpkR9ML

5 files changed, 114 insertions, 12 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index ece98cd..56208a0 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -69,6 +69,7 @@ async def _verify_captcha_or_raise(token: str | None, request: Request) -> None:
         token,
         request.client.host if request.client else None,
         _cfg.captcha.host_check,  # type: ignore[union-attr]
+        _cfg.captcha.allow_unattributed_host,  # type: ignore[union-attr]
     )
     if not ok:
         raise HTTPException(status_code=400, detail="captcha_failed")
diff --git a/packages/meshbay-hub/src/meshbay_hub/captcha.py b/packages/meshbay-hub/src/meshbay_hub/captcha.py
index 74a77b6..cf33d52 100644
--- a/packages/meshbay-hub/src/meshbay_hub/captcha.py
+++ b/packages/meshbay-hub/src/meshbay_hub/captcha.py
@@ -14,6 +14,7 @@ async def verify_captcha(
     token: str,
     remote_ip: str | None = None,
     allowed_hosts: frozenset[str] | set[str] | None = None,
+    allow_unattributed: bool = False,
 ) -> bool:
     """True when Google accepts the token, and it came from a host we expect.
 
@@ -33,6 +34,21 @@ async def verify_captcha(
 
     `None` (the default) skips it, which is what a deployment leaving the
     origin check with Google wants: it is then already done, one layer up.
+
+    `allow_unattributed` covers the case that made the desktop client fail
+    anyway. A solve from a page Google cannot attribute to a domain comes back
+    with an **empty** hostname, not the host part of the origin — measured
+    live: `app://meshbay` reports `''`, never `'meshbay'`. So an allowlist
+    entry can never match it, and an empty string cannot be an allowlist entry
+    either: a blank in a TOML list is a typo far more often than it is an
+    intention, and the config parser drops blanks for that reason.
+
+    What it admits is every non-web client, not only ours — a `file://` page or
+    somebody else's Electron application report the same nothing. That is the
+    same bar the desktop client's own origin would have been (`app://meshbay`
+    is not a credential; any application can claim it), and it is a bar: the
+    captcha still has to be *solved*, per token. What is given up is the origin
+    restriction for non-web clients, not the captcha.
     """
     payload: dict[str, str] = {"secret": secret_key, "response": token}
     if remote_ip:
@@ -48,9 +64,17 @@ async def verify_captcha(
             if allowed_hosts is not None:
                 # Logged at warning with the hostname spelled out: this is also
                 # how an operator finds what to allow after adding a client
-                # whose origin they have not seen before.
+                # whose origin they have not seen before. It is how the empty
+                # one was found.
                 host = result.get("hostname") or ""
-                if host not in allowed_hosts:
+                if not host:
+                    if not allow_unattributed:
+                        log.warning(
+                            "captcha solved on a host Google did not attribute; "
+                            "set captcha.allow_unattributed_host to admit "
+                            "non-web clients such as the desktop application")
+                        return False
+                elif host not in allowed_hosts:
                     log.warning(
                         "captcha solved on an unexpected host %r; allowed: %s",
                         host, sorted(allowed_hosts))
diff --git a/packages/meshbay-hub/src/meshbay_hub/config.py b/packages/meshbay-hub/src/meshbay_hub/config.py
index 39fdc52..e61e69e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/config.py
+++ b/packages/meshbay-hub/src/meshbay_hub/config.py
@@ -74,6 +74,13 @@ class CaptchaConfig:
     # so the hostname Google sees is not the hub's and never can be. See
     # docs/captcha.md §6.
     allowed_hosts: list[str] = field(default_factory=list)
+    # A solve from a page Google cannot attribute to a domain reports an empty
+    # hostname — `app://meshbay` does, measured live, and so does any other
+    # non-web client. No `allowed_hosts` entry can match that, and a blank
+    # entry is not the answer: the parser drops blanks because in a TOML list
+    # a blank is a typo far more often than an intention. Hence a named flag,
+    # which also states the trade at the place it is made.
+    allow_unattributed_host: bool = False
 
     @property
     def enabled(self) -> bool:
@@ -122,6 +129,8 @@ def load_config(path: Path | None = None) -> HubConfig:
                 cfg.captcha.secret_key = cap.get("secret_key", cfg.captcha.secret_key)
                 if hosts := cap.get("allowed_hosts"):
                     cfg.captcha.allowed_hosts = [str(h).strip() for h in hosts if str(h).strip()]
+                if "allow_unattributed_host" in cap:
+                    cfg.captcha.allow_unattributed_host = bool(cap["allow_unattributed_host"])
             break
 
     # Env var overrides
@@ -143,5 +152,7 @@ def load_config(path: Path | None = None) -> HubConfig:
         cfg.captcha.secret_key = captcha_secret
     if captcha_hosts := os.environ.get("MESHBAY_CAPTCHA_ALLOWED_HOSTS"):
         cfg.captcha.allowed_hosts = [h.strip() for h in captcha_hosts.split(",") if h.strip()]
+    if unattributed := os.environ.get("MESHBAY_CAPTCHA_ALLOW_UNATTRIBUTED_HOST"):
+        cfg.captcha.allow_unattributed_host = unattributed.lower() in ("1", "true", "yes")
 
     return cfg
diff --git a/packages/meshbay-hub/tests/test_captcha_host_check.py b/packages/meshbay-hub/tests/test_captcha_host_check.py
index bbd6c6d..68cad34 100644
--- a/packages/meshbay-hub/tests/test_captcha_host_check.py
+++ b/packages/meshbay-hub/tests/test_captcha_host_check.py
@@ -19,6 +19,13 @@ turning the check off would otherwise open.
 `allowed_hosts` empty means "do not check", which is what a deployment that
 left the origin check with Google wants. That is the default, so an existing
 hub upgrades without its captcha changing behaviour.
+
+The desktop client needed one more thing, found only by trying it. Its origin
+is `app://meshbay`, and the guess was that Google would report the host part,
+`meshbay`. It does not: a solve it cannot attribute to a domain comes back
+with an **empty** hostname. `allow_unattributed_host` is what admits those,
+and it is a named flag rather than an allowlist entry because a blank in a
+TOML list is a typo far more often than an intention.
 """
 
 import json
@@ -79,12 +86,44 @@ async def test_a_solve_from_an_expected_host_is_accepted(google):
 
 
 @pytest.mark.asyncio
-async def test_the_desktop_client_s_own_host_is_just_another_entry(google):
-    """`app://meshbay` is what the reported hostname comes from — the reason
-    any of this exists. Nothing about it is special to the check."""
-    google(success=True, hostname="meshbay")
-    allowed = frozenset({"meshbay.org", "localhost", "meshbay"})
-    assert await verify_captcha("s", "t", allowed_hosts=allowed)
+async def test_an_unattributed_solve_is_refused_by_default(google):
+    """What the desktop client actually produces, and what it costs.
+
+    `app://meshbay` does not report `meshbay`. Google attributes the solve to
+    no domain at all and the hostname comes back **empty** — measured live,
+    after the first attempt to allowlist `meshbay` failed. Refusing by default
+    is right: an answer with no hostname must not read as "any hostname".
+    """
+    google(success=True, hostname="")
+    assert not await verify_captcha(
+        "s", "t", allowed_hosts=frozenset({"meshbay.org", "localhost"}))
+
+
+@pytest.mark.asyncio
+async def test_the_flag_admits_it_and_the_allowlist_is_not_how(google):
+    """The flag is the only way in, on purpose.
+
+    An empty allowlist entry would be the obvious alternative and is a bad
+    one: a blank in a TOML list is a typo far more often than an intention,
+    and `load_config` drops blanks for exactly that reason. So this must not
+    work by accident from an allowlist alone.
+    """
+    google(success=True, hostname="")
+    allowed = frozenset({"meshbay.org", "localhost"})
+    assert await verify_captcha("s", "t", allowed_hosts=allowed, allow_unattributed=True)
+    assert not await verify_captcha("s", "t", allowed_hosts=allowed | {""})
+
+
+@pytest.mark.asyncio
+async def test_the_flag_does_not_open_the_allowlist(google):
+    """It admits *unattributed* solves, not solves from somewhere else.
+
+    A bot on its own web page still reports that page's hostname, which is
+    still not ours, flag or no flag.
+    """
+    google(success=True, hostname="a-bot-farm.example")
+    assert not await verify_captcha(
+        "s", "t", allowed_hosts=frozenset({"meshbay.org"}), allow_unattributed=True)
 
 
 @pytest.mark.asyncio
@@ -97,10 +136,12 @@ async def test_a_solve_from_somewhere_else_is_refused(google):
 
 
 @pytest.mark.asyncio
-async def test_a_missing_hostname_is_refused_when_checking(google):
-    """An answer with no hostname at all must not read as "any hostname"."""
+async def test_an_absent_hostname_field_reads_like_an_empty_one(google):
+    """`hostname` missing and `hostname: ""` are the same answer."""
     google(success=True)
-    assert not await verify_captcha("s", "t", allowed_hosts=frozenset({"meshbay.org"}))
+    allowed = frozenset({"meshbay.org"})
+    assert not await verify_captcha("s", "t", allowed_hosts=allowed)
+    assert await verify_captcha("s", "t", allowed_hosts=allowed, allow_unattributed=True)
 
 
 @pytest.mark.asyncio
@@ -143,6 +184,15 @@ async def test_a_transport_failure_is_a_refusal_not_an_exception(monkeypatch):
     assert not await verify_captcha("s", "t", allowed_hosts=frozenset({"meshbay.org"}))
 
 
+@pytest.mark.asyncio
+async def test_the_flag_alone_checks_nothing_without_an_allowlist(google):
+    """`allowed_hosts` empty still means "do not check", flag or no flag —
+    the whole block is skipped and reCAPTCHA is doing the origin check."""
+    google(success=True, hostname="somewhere.example")
+    assert await verify_captcha("s", "t", allow_unattributed=False)
+    assert await verify_captcha("s", "t", allow_unattributed=True)
+
+
 def test_the_config_hands_over_a_set_or_nothing():
     """`host_check` is what the caller passes straight through, so "unset" has
     to arrive as None rather than an empty set that refuses everything."""
@@ -179,6 +229,21 @@ def test_a_hub_toml_without_the_key_checks_nothing(tmp_path):
     assert load_config(cfg_file).captcha.host_check is None
 
 
+def test_the_unattributed_flag_comes_from_the_config(tmp_path, monkeypatch):
+    cfg_file = tmp_path / "hub.toml"
+    cfg_file.write_text(
+        '[captcha]\nsite_key = "k"\nsecret_key = "s"\n'
+        'allowed_hosts = ["meshbay.org"]\nallow_unattributed_host = true\n')
+    assert load_config(cfg_file).captcha.allow_unattributed_host is True
+
+    cfg_file.write_text('[captcha]\nsite_key = "k"\nsecret_key = "s"\n')
+    assert load_config(cfg_file).captcha.allow_unattributed_host is False, (
+        "the default has to stay off — it is the looser of the two")
+
+    monkeypatch.setenv("MESHBAY_CAPTCHA_ALLOW_UNATTRIBUTED_HOST", "true")
+    assert load_config(cfg_file).captcha.allow_unattributed_host is True
+
+
 def test_the_environment_can_override_the_list(monkeypatch, tmp_path):
     """Same shape as MESHBAY_ADMIN_USERS, for a container that has no file."""
     monkeypatch.setenv("MESHBAY_CAPTCHA_ALLOWED_HOSTS", "meshbay.org, meshbay ,")
diff --git a/packages/meshbay-hub/tests/test_register_captcha.py b/packages/meshbay-hub/tests/test_register_captcha.py
index d319137..32663fe 100644
--- a/packages/meshbay-hub/tests/test_register_captcha.py
+++ b/packages/meshbay-hub/tests/test_register_captcha.py
@@ -17,7 +17,8 @@ def captcha_on(client, monkeypatch):
     monkeypatch.setattr(_cfg.captcha, "site_key", "test-site")
     monkeypatch.setattr(_cfg.captcha, "secret_key", "test-secret")
 
-    async def fake_verify(secret, token, remote_ip=None, allowed_hosts=None):
+    async def fake_verify(secret, token, remote_ip=None, allowed_hosts=None,
+                          allow_unattributed=False):
         return token == "good-token"
 
     monkeypatch.setattr("meshbay_hub.captcha.verify_captcha", fake_verify)