diff --git a/docs/post-mortems/2026-06-24-kubeadm-oidc-drift-apiserver-upgrade-stall.md b/docs/post-mortems/2026-06-24-kubeadm-oidc-drift-apiserver-upgrade-stall.md
new file mode 100644
index 00000000..9a42d3ed
--- /dev/null
+++ b/docs/post-mortems/2026-06-24-kubeadm-oidc-drift-apiserver-upgrade-stall.md
@@ -0,0 +1,90 @@
+# Post-mortem: kubeadm-config OIDC drift crash-looped the v1.35 apiserver upgrade (2026-06-24)
+
+**Impact:** The autonomous k8s-version-upgrade chain (23:00 UTC nightly) reached
+the master control-plane phase for the first time — preflight passed, etcd
+snapshot taken, master cordoned + drained, etcd upgraded 3.6.5→3.6.6 — then the
+kube-apiserver upgrade to v1.35.6 **crash-looped**. kubeadm waited its 5-minute
+static-pod-hash window across all internal retries, then auto-rolled-back to
+v1.34.9. The cluster stayed healthy on 1.34.9 (apiserver, all 7 nodes Ready), but
+the run left **k8s-master cordoned** and the chain **wedged on `in_flight=1`**
+(which correctly blocks subsequent runs). No data loss; no user-facing outage
+(the master carries control-plane taints, so no workloads were displaced).
+
+**Trigger:** the first *minor* upgrade the chain ever attempted (1.34→1.35).
+Patch upgrades never hit this because the apiserver manifest content is identical
+across patches; a minor upgrade is the first time kubeadm regenerates the
+manifest with a new image.
+
+## Root cause
+
+apiserver authentication was configured in **two** places that were allowed to
+drift from a **third**:
+
+1. `/etc/kubernetes/pki/auth-config.yaml` — a structured `AuthenticationConfiguration`
+   (apiserver.config.k8s.io/v1) carrying **two** JWT issuers (`kubernetes` for
+   kubectl/kubelogin + `k8s-dashboard` for the dashboard's oauth2-proxy), added
+   2026-06-19 (`docs/plans/2026-06-04-k8s-dashboard-sso-design.md`).
+2. the **live** kube-apiserver static-pod manifest — referenced it via
+   `--authentication-config=/etc/kubernetes/pki/auth-config.yaml`.
+3. the **kubeadm-config `ClusterConfiguration` ConfigMap** — still carried the
+   **legacy single-issuer `--oidc-*` extraArgs** (`oidc-issuer-url`,
+   `oidc-client-id`, `oidc-username-claim`, `oidc-groups-claim`). Never updated
+   when (1)+(2) switched to structured auth.
+
+`kubeadm upgrade apply` **regenerates the static-pod manifests from
+kubeadm-config**. So it dropped `--authentication-config` and re-added the four
+`--oidc-*` flags. Proven by `kubeadm upgrade diff v1.35.6`:
+
+```diff
+-    - --authentication-config=/etc/kubernetes/pki/auth-config.yaml
++    - --oidc-issuer-url=https://authentik.viktorbarzin.me/application/o/kubernetes/
++    - --oidc-client-id=kubernetes
++    - --oidc-username-claim=email
++    - --oidc-groups-claim=groups
+```
+
+The regenerated apiserver crash-looped (`CrashLoopBackOff`, `back-off 10s`, 8
+probe failures in the kubelet journal) — it exited within seconds, repeatedly, so
+kubeadm's hash-watch never saw a stable new pod and timed out → rollback. (The
+`--oidc-*` flags are NOT removed in 1.35; the crash is the auth-config swap in the
+live control-plane environment, the only functional delta in the diff. Image
+pull, etcd, OOM, and disk were all ruled out: all v1.35.6 images were pre-pulled,
+etcd upgraded cleanly, no OOM, master root disk at 73%.)
+
+**Why the existing safety net missed it:** `stacks/rbac/modules/rbac/apiserver-oidc.tf`
+already *knew* kubeadm drops `--authentication-config` and published a
+`apiserver-oidc-restore` ConfigMap for the chain to re-run **after** the upgrade.
+But the apiserver crashes *during* `kubeadm upgrade apply`, which never returns
+success, so the post-upgrade restore step is never reached.
+
+## Resolution
+
+1. **Reconciled kubeadm-config live** (2026-06-24, zero cluster impact — the CM is
+   only read during an upgrade): rewrote `apiServer.extraArgs` to drop the
+   `--oidc-*` args and add `--authentication-config`, via `kubeadm init phase
+   upload-config kubeadm`. `kubeadm upgrade diff v1.35.6` then showed **only** the
+   control-plane image bumps — no auth-flag changes.
+2. **Recovered:** uncordoned k8s-master, cleared the stuck `in_flight` gauge +
+   namespace annotation.
+
+## Prevention (all landed in this change)
+
+| Gap | Fix |
+|-----|-----|
+| kubeadm-config not managed alongside the live manifest | `apiserver-oidc.tf`'s remote script now **also** reconciles kubeadm-config (`kubeadm init phase upload-config`). It reaches the cluster two ways: the published `apiserver-oidc-restore` ConfigMap (a plain k8s resource — CI applies it with no ssh) which the chain's `phase_master` re-runs, and a local `-replace` apply with `TF_VAR_ssh_private_key`. (The null_resource trigger deliberately does NOT hash the script: CI has no ssh key, so it must stay a no-op on a plain CI apply.) |
+| The chain drained the master into a crash with no pre-check | new **preflight gate 4b** in `upgrade-step.sh`: runs `kubeadm upgrade diff v$TARGET` and `block`s (k8s_upgrade_blocked=1 → K8sUpgradeBlocked alert) BEFORE snapshot/in-flight/drain if a `-` line would drop `--authentication-config`. Fails safe — blocks only on a positive drift signal. |
+| The live fix had to be applied out-of-band (only `default` Vault policy on the workstation; CI can't ssh) | kubeadm-config reconciled live via `kubeadm init phase upload-config` on the master (2026-06-24); the committed code makes it durable for future upgrades. |
+
+## Lessons
+
+- **Out-of-band control-plane edits must be written back to kubeadm-config.**
+  Anything that edits a static-pod manifest directly (auth, admission, audit, API
+  flags) is silently reverted on the next `kubeadm upgrade` unless kubeadm-config
+  itself carries it. `kubeadm upgrade diff <target>` is the authoritative
+  pre-flight check for "what will the upgrade change?" and is non-mutating.
+- **A post-upgrade fixup can't repair something that breaks the upgrade itself.**
+  The restore-after-upgrade design assumed the apiserver would come up (degraded)
+  and be fixed afterward; it actually crash-looped, so the fix has to be in
+  kubeadm-config *before* `apply`, plus a preflight gate.
+- **Minor upgrades exercise manifest regeneration; patch upgrades don't.** First
+  minor bump is where this whole class of drift surfaces.
diff --git a/docs/runbooks/k8s-version-upgrade.md b/docs/runbooks/k8s-version-upgrade.md
index 08d43926..86d1d31e 100644
--- a/docs/runbooks/k8s-version-upgrade.md
+++ b/docs/runbooks/k8s-version-upgrade.md
@@ -41,6 +41,7 @@ Job 0 — preflight       (pinned: k8s-node1)
   ├── halt-on-alert (kured-style ignore-list)
   ├── 24h-quiet baseline (no Ready transitions <24h ago)
   ├── kubeadm upgrade plan matches target (skipped when master already at target — partial-resume)
+  ├── apiserver-OIDC drift gate: kubeadm upgrade diff must NOT drop --authentication-config (else BLOCK+alert)
   ├── Push k8s_upgrade_in_flight=1, k8s_upgrade_started_timestamp=$(date +%s)
   ├── Trigger backup-etcd Job, wait, verify snapshot byte count
   ├── SSH master: containerd skew fix (if master < workers)
@@ -222,22 +223,34 @@ Exposed in K8s via ExternalSecret `k8s-upgrade-creds` in the `k8s-upgrade` names
 
 ## Common Operations
 
-### Post-upgrade: apiserver OIDC restore (AUTOMATED by the chain since 2026-06-19)
+### apiserver OIDC + kubeadm upgrades (kubeadm-config reconciliation since 2026-06-24)
 
 `kubeadm upgrade apply` **regenerates `/etc/kubernetes/manifests/kube-apiserver.yaml`
-and drops the `--authentication-config` flag**, silently disabling apiserver
-OIDC (kubectl/kubelogin CLI **and** the web dashboard SSO break — tokens get
-401). This used to require a manual re-apply after **every** control-plane bump.
+from kubeadm-config**. apiserver auth uses a structured multi-issuer
+`--authentication-config` (kubectl + dashboard SSO), but kubeadm-config used to
+still carry the legacy single-issuer `--oidc-*` extraArgs — so every upgrade
+reverted the flag. On the **1.34→1.35** bump that regenerated apiserver
+**crash-looped and stalled the whole upgrade mid-flight** (master cordoned, etcd
+already bumped); the post-upgrade restore below never ran because `kubeadm
+upgrade apply` itself never returned success. Post-mortem:
+`docs/post-mortems/2026-06-24-kubeadm-oidc-drift-apiserver-upgrade-stall.md`.
 
-**Now automated:** the `rbac` stack publishes its OIDC restore script to the
-`kube-system/apiserver-oidc-restore` ConfigMap, and the version-upgrade chain's
-`phase_master` re-runs it on master immediately after `kubeadm upgrade apply`
-(while tigera-operator is still quiesced, so the flag-add apiserver restart can't
-crashloop the operator). It's idempotent, health-gates `/livez` with
-auto-rollback, and is **non-fatal** — a failure only lags SSO until the next rbac
-apply (the version upgrade itself already succeeded). So a chain-driven
-control-plane bump no longer breaks SSO. The master phase self-skips when master
-is already at target, so this only runs when master was actually upgraded.
+**Primary fix (2026-06-24):** `stacks/rbac/modules/rbac/apiserver-oidc.tf` now
+**reconciles kubeadm-config** (`kubeadm init phase upload-config kubeadm`, rewriting
+`apiServer.extraArgs`: drop `--oidc-*`, add `--authentication-config`) as part of
+its remote script. So kubeadm regenerates a **correct** manifest and the apiserver
+upgrades with a pure image bump — `kubeadm upgrade diff <target>` shows only the
+image change. Zero live impact (the CM is read only during an upgrade).
+
+**Backstops:**
+- **Preflight gate 4b** runs `kubeadm upgrade diff` and BLOCKs (k8s_upgrade_blocked=1
+  → alert) BEFORE draining the master if `--authentication-config` would still be
+  dropped — so this can never again drain into a crash.
+- The `rbac` stack still publishes its restore script to the
+  `kube-system/apiserver-oidc-restore` ConfigMap, and `phase_master` re-runs it on
+  master right after `kubeadm upgrade apply` (idempotent, `/livez`-gated with
+  auto-rollback, non-fatal) — now redundant belt-and-suspenders that *also*
+  re-reconciles kubeadm-config. Self-skips when master is already at target.
 
 **Manual fallback** — only for an out-of-band/manual `kubeadm` upgrade, or if the
 chain logged `WARN: --authentication-config absent after re-apply`:
diff --git a/stacks/k8s-version-upgrade/scripts/upgrade-step.sh b/stacks/k8s-version-upgrade/scripts/upgrade-step.sh
index 76fdf157..5bafd241 100644
--- a/stacks/k8s-version-upgrade/scripts/upgrade-step.sh
+++ b/stacks/k8s-version-upgrade/scripts/upgrade-step.sh
@@ -416,6 +416,25 @@ phase_preflight() {
     fi
   fi
 
+  # 4b. apiserver-OIDC drift gate (backstop for the rbac stack's kubeadm-config
+  # reconciliation). A `kubeadm upgrade` REGENERATES the apiserver manifest from
+  # kubeadm-config; if kubeadm-config still carries the legacy single-issuer
+  # --oidc-* args instead of --authentication-config, the regenerated apiserver
+  # reverts structured multi-issuer auth and CRASH-LOOPS — stalling the chain
+  # mid-flight with the master cordoned and etcd already bumped (the 2026-06-24
+  # v1.35 stall; docs/post-mortems/2026-06-24-kubeadm-oidc-drift-apiserver-upgrade-stall.md).
+  # `kubeadm upgrade diff` shows exactly what the manifest regen will change; a
+  # '-' line dropping --authentication-config means the drift is still present.
+  # Skip on an at-target master (resume — no apiserver regen). Best-effort: blocks
+  # only on a POSITIVE drift signal, never merely because diff is unavailable.
+  if [ "$master_kubelet_v" != "$TARGET_VERSION" ]; then
+    local apiserver_diff
+    apiserver_diff=$(ssh "${SSH_OPTS[@]}" "$(ssh_target k8s-master)" "sudo kubeadm upgrade diff v$TARGET_VERSION 2>/dev/null" || true)
+    if echo "$apiserver_diff" | grep -qE '^-[[:space:]].*--authentication-config'; then
+      block "kubeadm upgrade would DROP --authentication-config from kube-apiserver (kubeadm-config OIDC drift → apiserver crash-loop). Re-apply the rbac stack (apiserver-oidc.tf reconciles kubeadm-config), then retry. Master NOT drained."
+    fi
+  fi
+
   # 5. Push in-flight + started_timestamp metrics + ns annotations
   $KUBECTL annotate ns "$NS" \
     "viktorbarzin.me/k8s-upgrade-in-flight=$(date -u +%FT%TZ)" \
diff --git a/stacks/rbac/modules/rbac/apiserver-oidc.tf b/stacks/rbac/modules/rbac/apiserver-oidc.tf
index 5165a7d1..a5ad08c8 100644
--- a/stacks/rbac/modules/rbac/apiserver-oidc.tf
+++ b/stacks/rbac/modules/rbac/apiserver-oidc.tf
@@ -10,16 +10,26 @@
 # match the existing RBAC subjects (kind: User, name: <raw email>; group names
 # verbatim). Do NOT add a prefix or existing bindings break.
 #
-# DRIFT WARNING: this edits the kube-apiserver static-pod manifest on the single
-# master. A `kubeadm upgrade` regenerates that manifest and DROPS this flag (this
-# is exactly how OIDC silently broke before — the flag was wiped and the
-# content-hash trigger never re-fired). After any k8s control-plane upgrade,
-# re-apply the rbac stack to restore apiserver OIDC. See
-# docs/plans/2026-06-04-k8s-dashboard-sso-design.md.
+# DRIFT WARNING (and how it's now handled): apiserver auth lives in THREE places
+# that must stay in sync, because a `kubeadm upgrade` REGENERATES the static-pod
+# manifest from kubeadm-config:
+#   1. /etc/kubernetes/pki/auth-config.yaml         — the structured authn file
+#   2. the live kube-apiserver static-pod manifest  — references it via the flag
+#   3. the kubeadm-config ClusterConfiguration CM   — what kubeadm regenerates from
+# Originally only (1)+(2) were managed, so every kubeadm upgrade rewrote the
+# manifest from the STALE CM, reverting --authentication-config to single-issuer
+# --oidc-* flags. On k8s 1.35 that regenerated apiserver CRASH-LOOPED and stalled
+# the whole upgrade mid-flight (master cordoned, etcd already bumped) — see
+# docs/post-mortems/2026-06-24-kubeadm-oidc-drift-apiserver-upgrade-stall.md. The
+# remote script below now ALSO reconciles (3) via `kubeadm init phase
+# upload-config`, so a future kubeadm upgrade regenerates a CORRECT manifest. The
+# k8s-version-upgrade chain additionally GATES on `kubeadm upgrade diff` in
+# preflight and blocks+alerts if --authentication-config would still be dropped.
 #
 # SAFETY: the remote script health-gates on /livez and AUTO-ROLLS-BACK the
 # manifest from a timestamped backup if the apiserver does not recover, so a
-# malformed config cannot leave the single master down.
+# malformed config cannot leave the single master down. Reconciling kubeadm-config
+# is zero-impact on the running cluster (the CM is only read during an upgrade).
 
 variable "k8s_master_host" {
   type    = string
@@ -97,6 +107,40 @@ locals {
     print('flag-inserted' if done else 'ANCHOR-NOT-FOUND')
   PY
 
+  # Reconciles the kubeadm-config ClusterConfiguration's apiServer.extraArgs:
+  # drops the stale single-issuer --oidc-* args and ensures --authentication-config
+  # is present (anchored after --authorization-mode). Stdlib-only (the master is
+  # only guaranteed python3, not pyyaml/yq). Idempotent; preserves all other
+  # fields (etcd args, audit args, extraVolumes) verbatim. Exits 3 if the
+  # authorization-mode anchor is missing (fail loud, leave the CM untouched).
+  kubeadm_oidc_reconcile_py = <<-PY
+    import sys
+    lines = sys.stdin.read().split('\n')
+    out, i, n = [], 0, len(lines)
+    have_authn = any('name: authentication-config' in l for l in lines)
+    inserted = have_authn
+    while i < n:
+        ln = lines[i]; s = ln.strip()
+        if s.startswith('- name: oidc-'):
+            i += 2 if (i + 1 < n and lines[i + 1].strip().startswith('value:')) else 1
+            continue
+        out.append(ln)
+        if (not inserted) and s == '- name: authorization-mode':
+            indent = ln[:len(ln) - len(ln.lstrip())]
+            if i + 1 < n and lines[i + 1].strip().startswith('value:'):
+                out.append(lines[i + 1]); i += 2
+            else:
+                i += 1
+            out.append(indent + '- name: authentication-config')
+            out.append(indent + '  value: /etc/kubernetes/pki/auth-config.yaml')
+            inserted = True
+            continue
+        i += 1
+    if not inserted:
+        sys.stderr.write('ANCHOR-NOT-FOUND: authorization-mode\n'); sys.exit(3)
+    sys.stdout.write('\n'.join(out))
+  PY
+
   # Whole remote operation, base64-embedded for byte-exact transfer (no
   # heredoc/escaping hazards across SSH).
   apiserver_auth_remote_script = <<-SH
@@ -137,6 +181,30 @@ locals {
       echo "rolled back to previous manifest"; exit 1
     fi
     echo "kube-apiserver healthy with multi-issuer --authentication-config"
+
+    # 5. Reconcile kubeadm-config so a FUTURE `kubeadm upgrade` regenerates the
+    #    apiserver manifest WITH --authentication-config instead of reverting to
+    #    the stale single-issuer --oidc-* flags. Without this, kubeadm rewrote the
+    #    manifest from kubeadm-config on every control-plane upgrade and the
+    #    regenerated apiserver crash-looped (the 2026-06-24 v1.35 upgrade stall).
+    #    Zero live impact (the CM is only read at upgrade time); idempotent;
+    #    best-effort (the chain's `kubeadm upgrade diff` preflight gate is the
+    #    backstop if this cannot run).
+    KC="sudo kubectl --kubeconfig /etc/kubernetes/admin.conf"
+    CC=$($KC -n kube-system get cm kubeadm-config -o jsonpath='{.data.ClusterConfiguration}' 2>/dev/null || true)
+    if [ -n "$CC" ] && { echo "$CC" | grep -q 'oidc-issuer-url' || ! echo "$CC" | grep -q 'authentication-config'; }; then
+      echo "Reconciling kubeadm-config (oidc-* -> authentication-config) so kubeadm upgrade keeps structured auth"
+      echo '${base64encode(local.kubeadm_oidc_reconcile_py)}' | base64 -d > /tmp/reconcile_kubeadm_oidc.py
+      if printf '%s' "$CC" | python3 /tmp/reconcile_kubeadm_oidc.py > /tmp/kubeadm-cc-new.yaml \
+         && sudo kubeadm init phase upload-config kubeadm --config /tmp/kubeadm-cc-new.yaml; then
+        echo "kubeadm-config reconciled: future control-plane upgrades keep --authentication-config"
+      else
+        echo "WARN: kubeadm-config reconcile failed; the upgrade-chain preflight gate will block the next upgrade"
+      fi
+      rm -f /tmp/reconcile_kubeadm_oidc.py /tmp/kubeadm-cc-new.yaml
+    else
+      echo "kubeadm-config already uses --authentication-config (no oidc drift)"
+    fi
   SH
 }
 
@@ -155,6 +223,14 @@ resource "null_resource" "apiserver_oidc_config" {
   }
 
   triggers = {
+    # Intentionally hash ONLY the issuer config, NOT the remote script. CI applies
+    # the rbac stack with no ssh_private_key (var defaults to ""), so a re-run of
+    # this SSH provisioner in CI would fail — hence the null_resource must stay a
+    # no-op on a plain CI apply. Script changes (e.g. the 2026-06-24 kubeadm-config
+    # reconciliation) reach the cluster via the apiserver-oidc-restore ConfigMap
+    # below (a plain k8s resource, no ssh) which the upgrade chain re-runs. To force
+    # this provisioner to re-run after a script change, apply locally with
+    # `-replace` + TF_VAR_ssh_private_key (see docs/runbooks/k8s-version-upgrade.md).
     auth_config = sha256(local.apiserver_auth_config_yaml)
   }
 }