`SlackAdapter._is_interactive_user_authorized` (approval / slash-confirm /
clarify Block Kit clicks) and the early pre-fetch gate in the message
handler recovered the runner via `_message_handler.__self__`, which is
None on a multiplexed adapter (closure handler) — so both fell to env-only
auth. The fallback read `SLACK_ALLOW_ALL_USERS` raw from `os.environ` and
its `_env` helper fell through to `os.environ` on a scoped miss: the
DEFAULT profile's allow-all flag / allowlist authorized callers on every
other profile's bot.
- Prefer the wired `set_authorization_check` callback (profile-bound
`_make_adapter_auth_check`) at both sites; keep `__self__` introspection
only for adapters wired without one.
- Env-only fallback reads go through `authz_mixin._platform_gate_env`
(scoped miss under multiplex → "", never os.environ); drop the raw
`os.getenv("SLACK_ALLOW_ALL_USERS")` pre-read.
Reapplies #72657 onto current main (original commit carried a bot
co-author trailer). Same class as Telegram #86296 / #65589.
Co-authored-by: MilaArtyNew <261982280+MilaArtyNew@users.noreply.github.com>
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