Microsoft Graph Webhook Listener

The msgraph_webhook gateway platform is an inbound event listener. It's how Hermes receives change notifications from Microsoft Graph — "a Teams meeting ended," "a new message landed in this chat," "this calendar event was updated." Different from the teams platform (which is a chat bot users type to) — this one is M365 telling Hermes something happened, not a person.

Right now the primary consumer is the Teams meeting summary pipeline: Graph notifies when a meeting produces a transcript, the pipeline fetches it, and Hermes posts a summary back into Teams. Other Graph resources (/chats/.../messages, /users/.../events) use the same listener — the pipeline consumers land with their own PRs.

• Microsoft Graph application credentials — Register a Microsoft Graph Application
• A public HTTPS URL that Microsoft Graph can reach (Graph does not call private endpoints). A dev tunnel works for testing; production needs a real domain with a valid certificate.
• A strong shared secret to use as the clientState value. Generate with openssl rand -hex 32 and put it in ~/.hermes/.env as MSGRAPH_WEBHOOK_CLIENT_STATE.

Minimum ~/.hermes/config.yaml:

platforms:
  msgraph_webhook:
    enabled: true
    extra:
      host: 127.0.0.1
      port: 8646
      client_state: "replace-with-a-strong-secret"
      accepted_resources:
        - "communications/onlineMeetings"

Or via env vars in ~/.hermes/.env (auto-merged on startup):

MSGRAPH_WEBHOOK_ENABLED=true
MSGRAPH_WEBHOOK_PORT=8646
MSGRAPH_WEBHOOK_CLIENT_STATE=<generate-with-openssl-rand-hex-32>
MSGRAPH_WEBHOOK_ACCEPTED_RESOURCES=communications/onlineMeetings

Note: the bind host is read from extra.host in config.yaml (see the example above); there is no MSGRAPH_WEBHOOK_HOST env-var override.

Start the gateway: hermes gateway run. The listener exposes:

• POST /msgraph/webhook — change notifications from Graph
• GET /msgraph/webhook?validationToken=... — Graph subscription validation handshake
• GET /health — readiness probe with accepted/duplicate counters

Expose the listener publicly (reverse proxy, dev tunnel, ingress). Your notification URL for Graph subscriptions is your public HTTPS origin followed by /msgraph/webhook:

https://ops.example.com/msgraph/webhook

All settings go under platforms.msgraph_webhook.extra:

Most settings also have an equivalent env var (MSGRAPH_WEBHOOK_*) that merges into the config at gateway startup (the exception is host, which is config-only — see the note above) — see the environment variables reference.

clientState is the primary auth check

Every Graph notification includes the clientState string your subscription registered with. The listener rejects any notification whose clientState doesn't match, using timing-safe comparison. This is Microsoft's documented mechanism — treat the value as a strong shared secret.

If client_state is unset, the listener refuses to start.

Source-IP allowlisting (production deployments)

For production, restrict the listener to Microsoft's published Graph webhook source IP ranges. Microsoft documents the egress ranges under the Office 365 IP Address and URL Web service ↗. Configure them as:

platforms:
  msgraph_webhook:
    enabled: true
    extra:
      host: 0.0.0.0
      client_state: "..."
      allowed_source_cidrs:
        - "52.96.0.0/14"
        - "52.104.0.0/14"
        # ...add the current Microsoft 365 "Common" + "Teams" category egress ranges

Or as an env var:

MSGRAPH_WEBHOOK_ALLOWED_SOURCE_CIDRS="52.96.0.0/14,52.104.0.0/14"

Binding a non-loopback host such as 0.0.0.0, ::, or a LAN IP without allowed_source_cidrs is refused at startup. If you're using a dev tunnel or reverse proxy on the same machine, bind Hermes to 127.0.0.1 or ::1 and leave the allowlist empty there. Invalid CIDR strings log a warning and are ignored. Review the Microsoft IP list quarterly — it changes.

HTTPS termination

The listener speaks plain HTTP. Terminate TLS at your reverse proxy (Caddy, Nginx, Cloudflare Tunnel, AWS ALB) and proxy to the listener over the local network. Graph refuses to deliver to non-HTTPS endpoints, so there's no path for unencrypted traffic to reach you from Graph itself.

Response hygiene

On success the listener returns 202 Accepted with an empty body — internal counters stay out of the wire response. Operators can observe counts via /health, which is guarded by the same source-IP rules as the webhook path.

Status code table:

• Register a Microsoft Graph Application — Azure app registration prereq
• Environment Variables → Microsoft Graph — full env var list
• Microsoft Teams bot setup — the different platform that lets users chat with Hermes in Teams