Here is some input on subject o authentication against Azure Active Directory (AAD) using oauth2_proxy with kubernetes.

First you need to create an application in AAD and add it email, profile and User.Read permissions to Microsoft Graph.

The default behavior of authentication flow, is that after login against Microsoft authentication server, you will be redirected to root of website with authentication code (e.g. https://exampler.com/). You would expect the Access Token to be visible there -this is a faulty assumption. The url that Access Token is injected into is https://exampler.com/oauth2 !!!

Successful configuration of oauth2_proxt that worked is below.

oauth2-proxy.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    k8s-app: oauth2-proxy
  name: oauth2-proxy
  namespace: oa2p
spec:
  replicas: 1
  selector:
    matchLabels:
      k8s-app: oauth2-proxy
  template:
    metadata:
      labels:
        k8s-app: oauth2-proxy
    spec:
      containers:
      - args:
        - --provider=oidc
        - --azure-tenant=88888888-aaaa-bbbb-cccc-121212121212
        - --email-domain=example.com
        - --http-address=0.0.0.0:4180
        - --set-authorization-header=true
        - --set-xauthrequest=true
        - --pass-access-token=true
        - --pass-authorization-header=true
        - --pass-user-headers=true
        - --pass-host-header=true
        - --skip-jwt-bearer-tokens=true
        - --oidc-issuer-url=https://login.microsoftonline.com/88888888-aaaa-bbbb-cccc-121212121212/v2.0
        env:
        - name: OAUTH2_PROXY_CLIENT_ID
          valueFrom:
            secretKeyRef:
              name: oauth2-proxy-secret
              key: OAUTH2_PROXY_CLIENT_ID
        - name: OAUTH2_PROXY_CLIENT_SECRET
          valueFrom:
            secretKeyRef:
              name: oauth2-proxy-secret
              key: OAUTH2_PROXY_CLIENT_SECRET
        - name: OAUTH2_PROXY_COOKIE_SECRET
          valueFrom:
            secretKeyRef:
              name: oauth2-proxy-secret
              key: OAUTH2_PROXY_COOKIE_SECRET
        image: quay.io/oauth2-proxy/oauth2-proxy:v7.1.3
        imagePullPolicy: Always
        name: oauth2-proxy
        ports:
        - containerPort: 4180
          protocol: TCP
---
apiVersion: v1
kind: Service
metadata:
  labels:
    k8s-app: oauth2-proxy
  name: oauth2-proxy
  namespace: oa2p
spec:
  ports:
  - name: http
    port: 4180
    protocol: TCP
    targetPort: 4180
  selector:
    k8s-app: oauth2-proxy

ingress.yaml

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: oa2p
  namespace: oa2p
  annotations:
  annotations:
    kubernetes.io/ingress.class: nginx
    nginx.ingress.kubernetes.io/force-ssl-redirect: "true"
    nginx.ingress.kubernetes.io/ssl-redirect: "true"
    nginx.ingress.kubernetes.io/limit-rps: "1"
    nginx.ingress.kubernetes.io/auth-url: "https://$host/oauth2/auth"
    nginx.ingress.kubernetes.io/auth-signin: "https://$host/oauth2/start?rd=$escaped_request_uri"
    nginx.ingress.kubernetes.io/auth-response-headers: "X-Auth-Request-Email,X-Auth-Request-Preferred-Username"
spec:
  tls:
  - hosts:
    - oa2p.example.com
    secretName: oa2p-tls
  rules:
    - host: oa2p.example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: oa2p
                port:
                  number: 8080
---
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: oa2p-proxy
  namespace: oa2p
  annotations:
    kubernetes.io/ingress.class: nginx
    nginx.ingress.kubernetes.io/limit-rps: "1"
    nginx.ingress.kubernetes.io/proxy-buffer-size: "8k"
spec:
  tls:
  - hosts:
    - oa2p.example.com
    secretName: oa2p-tls
  rules:
    - host: oa2p.example.com
      http:
        paths:
          - path: /oauth2
            pathType: Prefix
            backend:
              service:
                name: oauth2-proxy
                port:
                  number: 4180