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23 Microsoft Graph Webhook Listener Receive Microsoft Graph change notifications (meetings, calendar, chat, etc.) in Hermes

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.

Prerequisites

  • 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.

Quick Start

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_HOST=127.0.0.1
MSGRAPH_WEBHOOK_PORT=8646
MSGRAPH_WEBHOOK_CLIENT_STATE=<generate-with-openssl-rand-hex-32>
MSGRAPH_WEBHOOK_ACCEPTED_RESOURCES=communications/onlineMeetings

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

Configuration

All settings go under platforms.msgraph_webhook.extra:

Setting Default Description
host 0.0.0.0 Bind address for the HTTP listener. Non-loopback binds require allowed_source_cidrs; loopback (127.0.0.1 / ::1) is the easiest dev-tunnel / reverse-proxy setup.
port 8646 Bind port.
webhook_path /msgraph/webhook URL path Graph POSTs to.
health_path /health Readiness endpoint.
client_state — Shared secret Graph echoes in every notification. Compared with hmac.compare_digest — generate with openssl rand -hex 32.
accepted_resources [] (accept all) Allowlist of Graph resource paths/patterns. Trailing * acts as prefix match. Leading / is tolerated. Example: ["communications/onlineMeetings", "chats/*/messages"].
max_seen_receipts 5000 Dedupe cache size for notification IDs. Oldest entries evicted when the cap is hit.
allowed_source_cidrs [] Required for non-loopback binds. Leave empty only when the listener is bound to loopback and fronted by a local tunnel / reverse proxy.

Each setting also has an equivalent env var (MSGRAPH_WEBHOOK_*) that merges into the config at gateway startup — see the environment variables reference.

Security Hardening

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:

Outcome Status
Notification(s) accepted or deduped 202
Validation handshake (GET with validationToken) 200 (echoes the token)
Every item in batch failed clientState 403
Malformed JSON / missing value array / unknown resource 400
Source IP not in allowlist 403
Bare GET without validationToken 400

Troubleshooting

Problem What to check
Graph subscription validation fails Public URL is reachable, /msgraph/webhook path matches, GET with validationToken echoes the token verbatim as text/plain within 10 seconds.
Notifications POST but nothing ingests client_state matches what you registered the subscription with. Re-run openssl rand -hex 32 and create a new subscription if the value drifted. Check accepted_resources includes the resource path Graph is sending.
Every notification 403s clientState mismatch (forged, or subscription registered with a different value). Re-create the subscription with hermes teams-pipeline subscribe --client-state "$MSGRAPH_WEBHOOK_CLIENT_STATE" ... (ships with the pipeline runtime PR).
Listener refuses to start on 0.0.0.0 Set allowed_source_cidrs to Microsoft's current webhook egress ranges, or bind Hermes to 127.0.0.1 / ::1 behind your tunnel or reverse proxy.
Listener starts but curl http://localhost:8646/health hangs Port binding collision. Check ss -tlnp | grep 8646 and change port: if needed.
Real Graph requests from Microsoft get 403'd Source IP allowlist is too narrow. Widen the list to include the current Microsoft egress ranges. If you're still validating the tunnel path, bind Hermes to loopback and let the tunnel handle public exposure.

Related Docs