fix(worker): bind Slack API auth to a server-side session

[posthog]

Summary

• Who is hurt: anyone who knows a workspace's Slack team ID can read its private channel list and post as its bot — a pre-authentication token-disclosure flaw that every fork of this public template inherits.
• Root cause: the SPA-facing endpoints trusted the bkb_team_id and bkb_user_id cookies as identity. cookies.ts writes them as plaintext, unsigned, with no server-side record, so a client can set them and load the bot token from KV. The same pattern loaded a member's user token in the sendAsUser branch.
• Fix: mint a random session ID at both OAuth callbacks, store { team_id, user_id } server-side in the new SLACK_SESSIONS KV under that ID, and set only the opaque ID as the bkb_session cookie. Every protected endpoint resolves identity through the session, never from client-supplied values.

Reachable through /api/slack/channels, /api/slack/emojis, /api/slack/messages/send, /api/slack/modals/send, and the /api/slack/me/can-send-as-user enumeration oracle.

Path	Before	After
requireBotInstall	team ID read from bkb_team_id cookie	team ID read from the server-side session
sendAsUser / modal send	user ID read from bkb_user_id cookie	user ID read from the session
can-send-as-user	open oracle — any forged cookie pair answered	answers only for a valid session

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs
• Build / CI / deps
• Other

Area

• Builder UI / React SPA (src/)
• Bot OAuth (/slack/install)
• User-token OAuth (/slack/user-install)
• Worker API routes (/api/slack/*)
• Slack events ingress (/slack/events)
• Setup scripts (scripts/)
• Deploy / CI (.github/workflows, wrangler.jsonc)
• Docs

Test plan

• pnpm run build passes
• pnpm run dev:tunnel exercised end-to-end where relevant
• Manual checks listed below

Manual checks:

• pnpm run build bundles the worker and client.
• pnpm run typecheck reports only the pre-existing src/client/App.tsx prop-type error (present on clean main, unrelated to this change); no errors in the worker.

Screenshots / recordings

N/A — server-side auth change, no UI.

Related issues

Closes ENG-5917, ENG-5918 (linked in the footer).

Checklist

• No secrets, tokens, or signing keys committed
• Updated README.md / .dev.vars.example / wrangler.jsonc if config or setup changed
• Considered backwards compatibility for existing forks of the template

Important

Breaking setup change for forks. This adds a fifth KV namespace, SLACK_SESSIONS. Forks must run pnpm run setup:kv again and paste the new ID into wrangler.jsonc (the REPLACE_WITH_SESSIONS_KV_ID placeholder). After deploy, the old bkb_team_id / bkb_user_id cookies stop working; users re-run /slack/install (and /slack/user-install for "send as me") to get a session. Stored installations in KV stay valid.

Agent context

• Instrumentation for rejected sessions (suggested in the report) was left out on purpose: it would add a telemetry SDK to a public template, which is a maintainer decision beyond this P0 fix. The worker already logs to wrangler tail / pnpm run logs.

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Created with PostHog Desktop from this inbox report, addressing ENG-5917, ENG-5918.

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