Enable TLS certificate verification for OIDC IdP upstream connections.

Updates `openid_connect.server_conf` to enforce secure TLS settings on all IdP-bound requests (`/_token`, `/_refresh`, `/_jwks_uri`). This adds:
- `proxy_ssl_verify on` to enforce verification of the OP’s TLS certificate.
- `proxy_ssl_trusted_certificate /etc/ssl/certs/ca-certificates.crt` to use the system (Debian/Ubuntu/Alpine) CA bundle for trust.
- `proxy_ssl_verify_depth 2` to allow certificate chains up to one intermediate CA.

Author: route443

README.md

@@ -190,6 +190,8 @@ Manual configuration involves reviewing the following files so that they match y
     * No changes are usually required here
     * Modify the `resolver` directive to match a DNS server that is capable of resolving the IdP defined in `$oidc_token_endpoint` and `$oidc_end_session_endpoint`
     * If using [`auth_jwt_key_request`](http://nginx.org/en/docs/http/ngx_http_auth_jwt_module.html#auth_jwt_key_request) to automatically fetch the JWK file from the IdP then modify the validity period and other caching options to suit your IdP
+    * TLS certificate verification for all IdP-bound requests (token, refresh, JWKS) is enabled by default. NGINX Plus uses the system CA bundle at `/etc/ssl/certs/ca-certificates.crt` (via [`proxy_ssl_trusted_certificate`](http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_trusted_certificate)) to validate the IdP’s TLS certificate. If the IdP’s certificate is signed by a private or custom CA, append that CA to this bundle or update the `proxy_ssl_trusted_certificate` path accordingly.
+    * The [`proxy_ssl_verify_depth`](http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_verify_depth) directive is set to **2** by default, allowing one intermediate CA in the chain. This is sufficient for most public IdPs.
 
   * **openid_connect.js** - this is the JavaScript code for performing the authorization code exchange and nonce hashing
     * No changes are required unless modifying the code exchange or validation process
@@ -320,3 +322,4 @@ This reference implementation for OpenID Connect is supported for NGINX Plus sub
   * **R28** Access token support. Added support for access token to authorize NGINX to access protected backend.
   * **R32** Added support for `client_secret_basic` client authentication method.
   * **R33** Refactor code to use async/await. Implement Front-Channel Logout endpoint.
+  * **R36** Enable TLS certificate verification for all IdP-bound requests by default.

openid_connect.server_conf

@@ -13,7 +13,12 @@
         proxy_cache jwk;                              # Cache the JWK Set received from IdP
         proxy_cache_valid 200 12h;                    # How long to consider keys "fresh"
         proxy_cache_use_stale error timeout updating; # Use old JWK Set if cannot reach IdP
-        proxy_ssl_server_name on;                     # For SNI to the IdP
+
+        proxy_ssl_verify on;                          # Enforce TLS certificate verification
+        proxy_ssl_verify_depth 2;                     # Allow intermediate CA chains of depth 2
+        proxy_ssl_server_name on;                     # Send SNI to IdP host
+        proxy_ssl_trusted_certificate /etc/ssl/certs/ca-certificates.crt; # Use system CA bundle
+
         proxy_method GET;                             # In case client request was non-GET
         proxy_set_header Content-Length "";           # ''
         proxy_pass $oidc_jwt_keyfile;                 # Expecting to find a URI here
@@ -43,7 +48,11 @@
         # Exclude client headers to avoid CORS errors with certain IdPs (e.g., Microsoft Entra ID)
         proxy_pass_request_headers off;
 
-        proxy_ssl_server_name on; # For SNI to the IdP
+        proxy_ssl_verify on;         # Enforce TLS certificate verification
+        proxy_ssl_verify_depth 2;    # Allow intermediate CA chains of depth 2
+        proxy_ssl_server_name on;    # Send SNI to IdP host
+        proxy_ssl_trusted_certificate /etc/ssl/certs/ca-certificates.crt; # Use system CA bundle
+
         proxy_set_header      Content-Type "application/x-www-form-urlencoded";
         proxy_set_header      Authorization $arg_secret_basic;
         proxy_pass            $oidc_token_endpoint;
@@ -58,7 +67,11 @@
         # Exclude client headers to avoid CORS errors with certain IdPs (e.g., Microsoft Entra ID)
         proxy_pass_request_headers off;
 
-        proxy_ssl_server_name on; # For SNI to the IdP
+        proxy_ssl_verify on;         # Enforce TLS certificate verification
+        proxy_ssl_verify_depth 2;    # Allow intermediate CA chains of depth 2
+        proxy_ssl_server_name on;    # Send SNI to IdP host
+        proxy_ssl_trusted_certificate /etc/ssl/certs/ca-certificates.crt; # Use system CA bundle
+
         proxy_set_header      Content-Type "application/x-www-form-urlencoded";
         proxy_set_header      Authorization $arg_secret_basic;
         proxy_pass            $oidc_token_endpoint;