When SIGNAL_ALLOWED_USERS (or any platform-specific or global allowlist)
is set, the gateway was still sending automated pairing-code messages to
every unauthorized sender. This forced pairing-code spam onto personal
contacts of anyone running Hermes on a primary personal account with a
whitelist, and exposed information about the bot's existence.
Root cause
----------
_get_unauthorized_dm_behavior() fell through to the global default
('pair') even when an explicit allowlist was configured. An allowlist
signals that the operator has deliberately restricted access; offering
pairing codes to unknown senders contradicts that intent.
Fix
---
Extend _get_unauthorized_dm_behavior() to inspect the active per-platform
and global allowlist env vars. When any allowlist is set and the operator
has not written an explicit per-platform unauthorized_dm_behavior override,
the method now returns 'ignore' instead of 'pair'.
Resolution order (highest → lowest priority):
1. Explicit per-platform unauthorized_dm_behavior in config — always wins.
2. Explicit global unauthorized_dm_behavior != 'pair' in config — wins.
3. Any platform or global allowlist env var present → 'ignore'.
4. No allowlist, no override → 'pair' (open-gateway default preserved).
This fixes the spam for Signal, Telegram, WhatsApp, Slack, and all other
platforms with per-platform allowlist env vars.
Testing
-------
6 new tests added to tests/gateway/test_unauthorized_dm_behavior.py:
- test_signal_with_allowlist_ignores_unauthorized_dm (primary #9337 case)
- test_telegram_with_allowlist_ignores_unauthorized_dm (same for Telegram)
- test_global_allowlist_ignores_unauthorized_dm (GATEWAY_ALLOWED_USERS)
- test_no_allowlist_still_pairs_by_default (open-gateway regression guard)
- test_explicit_pair_config_overrides_allowlist_default (operator opt-in)
- test_get_unauthorized_dm_behavior_no_allowlist_returns_pair (unit)
All 15 tests in the file pass.
Fixes#9337
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