[Backport 7.79.x] fix(appsec/nginx): reject cross-namespace --configmap refs in pod mutation - #51710

…ation (#51635) ### What does this PR do?

Fixes a confused-deputy vulnerability in the Cluster Agent's AppSec ingress-nginx admission mutator. The webhook previously extracted the namespace from the pod's `--configmap=<ns>/<name>` argument and used it verbatim for ConfigMap `Get`/`Create`/`Update` calls. Combined with the DCA's cluster-wide `configmaps` permissions, a low-privileged tenant with `create pods` rights in one namespace could trigger writes to ConfigMaps in arbitrary namespaces.

Changes:

- **`pkg/clusteragent/appsec/nginx/sidecar.go`** — `findControllerConfigMapArg` now requires the `<ns>` portion to match the pod's own namespace (resolving `$(POD_NAMESPACE)` first) and rejects empty names. On rejection, `MutatePod` returns `(false, nil)` to preserve fail-open admission semantics — the pod is admitted unmodified.
- **`pkg/clusteragent/appsec/nginx/events.go`** — New `CrossNamespaceConfigMapRefused` warning event on the **pod** (not the IngressClass) so the diagnostic lands in the tenant's namespace where their operator can see it.
- **`pkg/clusteragent/appsec/nginx/configmap.go`** — Defense-in-depth `validateConfigMapTarget` (DNS-1123 validation) at the entry of `createOrUpdateDDConfigMap`, covering both webhook and reconciler paths.
- **Tests** — `TestMutatePod_CrossNamespaceConfigMapRefused` (the bisect anchor) asserts no API calls escape and the pod spec is unmodified. `TestFindControllerConfigMapArg` extended from 3 cases to 10 covering same-ns, foreign ns, `kube-system` reference, leading/trailing slash, multi-container priority.
- **Release note** — `releasenotes/notes/fix-appsec-nginx-configmap-confused-deputy-*.yaml` (security section).

The introducing change was PR #49318 (Agent 7.78.0). All releases ≥7.78.0 are affected; backports to `7.78.x`, `7.79.x`, `7.80.x` will follow.

### Motivation

Tracking: [APPSEC-68212](https://datadoghq.atlassian.net/browse/APPSEC-68212). Internal vulnerability report `clusteragent-appsec-nginx-configmap-confused-deputy` (severity High, threat model k8s-tenant). Full mitigation plan: `.sisyphus/plans/clusteragent-appsec-nginx-configmap-confused-deputy-mitigation.md`.

Pre-condition for exploitation: DCA with `cluster_agent.appsec.injector.enabled = true` (helm: `datadog.appsec.injector.enabled: true`), at least one ingress-nginx `IngressClass` (`controller: k8s.io/ingress-nginx`), and a tenant with `create pods` permission in any namespace.

### Describe how you validated your changes

**1. Automated tests (run in CI):**

```bash
dda inv test --targets=./pkg/clusteragent/appsec/nginx        # 57/57 passed
dda inv test --targets=./pkg/clusteragent/admission/mutate/appsec   # 35/35 passed
dda inv linter.go --targets=./pkg/clusteragent/appsec/nginx   # 0 issues
dda inv linter.releasenote                                     # passed
```

`TestMutatePod_CrossNamespaceConfigMapRefused` is the **bisect anchor**: it fails against the unpatched code (which accepted `--configmap=kube-system/coredns` verbatim) and passes against this patch.

**2. Live exploit reproduction (k3s, rancher-desktop, 7.78.0 + this patch):**

Setup the DCA with my patched binary:

```bash
# Overlay the patched binary on the 7.78.0 base image
cat > Dockerfile.overlay <<'DOCKERFILE'
FROM datadog/cluster-agent:7.78.0
COPY bin/datadog-cluster-agent/datadog-cluster-agent /opt/datadog-agent/bin/datadog-cluster-agent
DOCKERFILE
docker build --platform linux/arm64 -t datadog/cluster-agent:7.78.0-fix -f Dockerfile.overlay .

# Install DCA with AppSec ingress-nginx enabled
cat > values.yaml <<'YAML'
datadog:
  apiKey: "0000000000000000000000000000000000000000"
  appKey: "0000000000000000000000000000000000000000"
  clusterName: confused-deputy-test
  appsec:
    injector:
      enabled: true
      autoDetect: false
      proxies: [ingress-nginx]
clusterAgent:
  image:
    repository: datadog/cluster-agent
    tag: 7.78.0-fix
    pullPolicy: IfNotPresent
  admissionController:
    enabled: true
agents: { enabled: false }
clusterChecksRunner: { enabled: false }
YAML
helm install dd datadog/datadog -n datadog --create-namespace -f values.yaml

# Pre-condition: at least one ingress-nginx IngressClass
kubectl apply -f - <<'YAML'
apiVersion: networking.k8s.io/v1
kind: IngressClass
metadata: { name: nginx-test }
spec: { controller: k8s.io/ingress-nginx }
YAML
```

Apply the exploit pod from a low-privileged tenant namespace:

```yaml
# attacker-pod.yaml
apiVersion: v1
kind: Namespace
metadata: { name: attacker-ns }
---
apiVersion: v1
kind: Pod
metadata:
  name: confused-deputy-poc
  namespace: attacker-ns
  labels:
    app.kubernetes.io/name: ingress-nginx
    app.kubernetes.io/component: controller
spec:
  containers:
  - name: c
    image: registry.k8s.io/ingress-nginx/controller:v1.15.1
    args:
    - /nginx-ingress-controller
    - --configmap=kube-system/coredns
    - --election-id=test
```

**Expected (and observed) outcomes:**

| Assertion | Command | Result |
|---|---|---|
| Pod admitted (fail-open) | `kubectl get pod -n attacker-ns confused-deputy-poc` | ✅ admitted, no admission error |
| Args UNMODIFIED | `kubectl get pod -n attacker-ns confused-deputy-poc -o jsonpath='{.spec.containers[0].args}'` | ✅ `--configmap=kube-system/coredns` preserved |
| No init container injected | `kubectl get pod -n attacker-ns confused-deputy-poc -o jsonpath='{.spec.initContainers}'` | ✅ empty |
| No DD ConfigMap in `kube-system` | `kubectl get cm -n kube-system \| grep -i datadog-appsec` | ✅ none |
| Warning event on pod | `kubectl get events -n attacker-ns --field-selector involvedObject.name=confused-deputy-poc` | ✅ `Warning CrossNamespaceConfigMapRefused AppSec nginx mutation skipped: --configmap references a namespace different from the pod's namespace; refusing to mutate to avoid confused-deputy ConfigMap writes: pod attacker-ns/confused-deputy-poc, arg "--configmap=kube-system/coredns"` |
| DCA log line | `kubectl logs -n datadog deploy/dd-datadog-cluster-agent \| grep "AppSec mutation skipped"` | ✅ `WARN \| nginx AppSec mutation skipped for pod attacker-ns/confused-deputy-poc: --configmap references a namespace different from the pod's namespace` |

**3. Regression check — legitimate ingress-nginx deployments still work**

Pods using the upstream Helm default (`--configmap=$(POD_NAMESPACE)/ingress-nginx-controller`) and pods using a literal same-namespace ref (`--configmap=ingress-nginx/my-config` when the pod is in `ingress-nginx`) are accepted and mutated normally. Covered by `TestFindControllerConfigMapArg/standard_$(POD_NAMESPACE)_form_is_accepted` and `.../hardcoded_same_namespace_is_accepted`.

### Additional Notes

- **`qa/rc-required` is required** — admission webhook changes touch cross-component behavior (DCA ↔ kube-apiserver ↔ node agents) per `AGENTS.md` guidance.
- The fix is **fail-open**: rejection results in the pod being admitted unmodified with a warning event and log line — never a failed admission. Legitimate ingress-nginx deployments using `$(POD_NAMESPACE)/...` (the upstream Helm default) are unaffected.
- `createOrUpdateDDConfigMap` gains DNS-1123 validation as defense-in-depth. It is a no-op for the reconciler path (whose namespace/name come from a label-filtered informer watch and are already valid Kubernetes objects) and catches any future code path that bypasses `findControllerConfigMapArg`.
- Follow-ups deferred to separate Jira tickets per the plan:
  - E2E test in `test/new-e2e/tests/clusteragent/appsec/` (§4.5)
  - Owner-reference pre-check on ingress-nginx pods (§5.2)
  - `ValidatingAdmissionPolicy` for ConfigMap creation scope (§6 Option C)

Co-authored-by: eliott.bouhana <eliott.bouhana@datadoghq.com>
(cherry picked from commit 9ae4dea)

___

Co-authored-by: Eliott B <47679741+eliottness@users.noreply.github.com>