security(nginx): replace wildcard CORS with Office origin allowlist

Replace `Access-Control-Allow-Origin: *` in nginx/default.conf with a
map-based allowlist that echoes the request Origin back only for the
trusted Microsoft origins (Office.js CDN + Outlook hosts). Any other
origin gets an empty value, so nginx omits the header entirely and
arbitrary sites can no longer read add-in responses cross-origin.

Also add `Vary: Origin` so caches don't serve one origin's CORS
response to another.

Refs GHSA-m957-9cxh-72q7, closes #115

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

nginx/default.conf

@@ -9,6 +9,21 @@ map $sent_http_content_type $addin_cache_control {
     ~*application/wasm       "public, max-age=86400";
 }
 
+# Origin allowlist for CORS. The add-in's own assets are served same-origin
+# inside the Office iframe, so most requests need no CORS at all; the entries
+# below cover the Microsoft origins that legitimately fetch add-in assets
+# cross-origin (Office.js CDN and the Outlook hosts). The request Origin is
+# echoed back only when it is on the allowlist — any other origin gets an
+# empty value, and nginx then omits the Access-Control-Allow-Origin header
+# entirely, so arbitrary sites cannot read responses cross-origin.
+map $http_origin $addin_cors_origin {
+    default                               "";
+    "https://appsforoffice.microsoft.com" $http_origin;
+    "https://outlook.office.com"          $http_origin;
+    "https://outlook.office365.com"       $http_origin;
+    "https://outlook.live.com"            $http_origin;
+}
+
 server {
     listen      80 default_server;
     server_name _;
@@ -21,9 +36,13 @@ server {
     add_header Referrer-Policy        "strict-origin-when-cross-origin" always;
     add_header Cache-Control          $addin_cache_control always;
 
-    # Permissive CORS so the Office runtime (and the manifest fetcher) can
-    # load assets across origins.
-    add_header Access-Control-Allow-Origin "*" always;
+    # Restrict CORS to the allowlisted Office/Outlook origins (see the
+    # $addin_cors_origin map above). For any other origin the value is empty
+    # and nginx skips the header, so only the trusted Microsoft origins can
+    # read add-in responses cross-origin. Vary: Origin keeps caches from
+    # serving one origin's CORS response to another.
+    add_header Access-Control-Allow-Origin $addin_cors_origin always;
+    add_header Vary                         Origin always;
 
     location = /health {
         access_log off;