Alertmanager (alerts.renner.uno, monitoring ns, :9093) had no auth of its
own, and its v2 API is writable: an unauthenticated LAN client could POST
/api/v2/silences and suppress ALL alerting — a tamper capability, not just
disclosure (2026-07-10 audit, highest-teeth of the deferred unauth
findings).
Gate it with the same Authentik proxy-outpost pattern as
paperless/vscode/shelly-manager:
- New proxy provider + application + dedicated outpost
(ak-outpost-alertmanager-gate), mode proxy, forwarding to the ClusterIP
Service (kube-prometheus-stack-alertmanager.monitoring.svc:9093).
- New git-owned "Monitoring Access" group (k.renner) bound to the app.
Named generically so it can also gate the still-deferred Prometheus UI.
- The Helm-chart-generated Alertmanager Ingress is disabled
(alertmanager.ingress.enabled=false); a standalone gate Ingress
(alertmanager-gate-ingress.yaml) backends the outpost ExternalName —
two Ingresses for the same host would conflict. Mirrors loki-ingress.
- Scoped CNPs: monitoring ingress rule allowing only the outpost -> :9093;
matching egress rule in the authentik namespace.
implicit-consent flow, NOT passkey step-up: Alertmanager is a
frequently-checked monitoring UI, and requiring an authenticated Authentik
session already closes the unauthenticated-LAN exposure. Passkey re-auth on
every visit is disproportionate friction for a dashboard (paperless-gate
uses implicit-consent for the same reason). Bump to
passkey-stepup-authorization if the write API warrants it.
No skip_path_regex: the Alertmanager UI is a plain Elm SPA with no service
worker and no manifest (verified 2026-07-10 — index.html has no
serviceWorker/manifest/webmanifest), so none of the credentials:'omit' PWA
fetches that forced skip carve-outs elsewhere.
Prometheus's alerting path is UNAFFECTED: it posts to the ClusterIP Service
(verified: spec.alerting.alertmanagers targets
monitoring/kube-prometheus-stack-alertmanager:http-web), not this ingress.
Prometheus's own ingress is untouched (still enabled) — its unauth exposure
is a separate deferred finding, and it is read-only.
Verified: kubectl kustomize builds for authentik, monitoring and
network-policies; the embedded outpost survived the blueprint insert; the
rendered ingress backends ak-outpost-alertmanager-gate:9000, egress is
scoped to alertmanager:9093, and alertmanager.ingress.enabled=false while
prometheus.ingress.enabled=true.
Post-merge checks to run: only the gate Ingress exists for alerts.renner.uno
(the Helm one is gone); unauthenticated GET -> 302 to auth.renner.uno;
POST /api/v2/silences unauthenticated is refused; the outpost forwards to
:9093 after login; Prometheus is still delivering alerts to Alertmanager.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
k.rennerandClaude Opus 4.8