diff --git a/docs/example.nginx.conf b/docs/example.nginx.conf
index 13fd7789f..be56e10cd 100644
--- a/docs/example.nginx.conf
+++ b/docs/example.nginx.conf
@@ -19,6 +19,16 @@ server {
     set $main_domain "your-main-domain.com";
     set $sandbox_domain "your-sandbox-domain.com";
 
+    # By default CryptPad allows remote domains to embed CryptPad documents in iframes.
+    # This behaviour can be blocked by changing $allowed_origins from "*" to the
+    # sandbox domain, which must be permitted to load content from the main domain
+    # in order for CryptPad to work as expected.
+    #
+    # An example is given below which can be uncommented if you want to block
+    # remote sites from including content from your server
+    set $allowed_origins "*";
+    # set $allowed_origins "https://${sandbox_domain}";
+
     # CryptPad's dynamic content (websocket traffic and encrypted blobs)
     # can be served over separate domains. Using dedicated domains (or subdomains)
     # for these purposes allows you to move them to a separate machine at a later date
@@ -58,7 +68,7 @@ server {
     add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
     add_header X-XSS-Protection "1; mode=block";
     add_header X-Content-Type-Options nosniff;
-    add_header Access-Control-Allow-Origin "*";
+    add_header Access-Control-Allow-Origin "${allowed_origins}";
     # add_header X-Frame-Options "SAMEORIGIN";
 
     # Opt out of Google's FLoC Network
@@ -115,6 +125,9 @@ server {
     # script-src specifies valid sources for javascript, including inline handlers
     set $scriptSrc  "'self' resource: https://${main_domain}";
 
+    # XXX frame-ancestors defines where your cryptpad instance can be embedded...
+    set $frameAncestors "https://${main_domain} $https://${sandbox_domain}";
+
     set $unsafe 0;
     # the following assets are loaded via the sandbox domain
     # they unfortunately still require exceptions to the sandboxing to work correctly.
@@ -135,7 +148,7 @@ server {
     }
 
     # Finally, set all the rules you composed above.
-    add_header Content-Security-Policy "default-src 'none'; child-src $childSrc; worker-src $workerSrc; media-src $mediaSrc; style-src $styleSrc; script-src $scriptSrc; connect-src $connectSrc; font-src $fontSrc; img-src $imgSrc; frame-src $frameSrc;";
+    add_header Content-Security-Policy "default-src 'none'; child-src $childSrc; worker-src $workerSrc; media-src $mediaSrc; style-src $styleSrc; script-src $scriptSrc; connect-src $connectSrc; font-src $fontSrc; img-src $imgSrc; frame-src $frameSrc; frame-ancestors $frameAncestors";
 
     # The nodejs process can handle all traffic whether accessed over websocket or as static assets
     # We prefer to serve static content from nginx directly and to leave the API server to handle
@@ -183,7 +196,7 @@ server {
     # encrypted blobs are immutable and are thus cached for a year
     location ^~ /blob/ {
         if ($request_method = 'OPTIONS') {
-            add_header 'Access-Control-Allow-Origin' '*';
+            add_header 'Access-Control-Allow-Origin' "${allowed_origins}";
             add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
             add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
             add_header 'Access-Control-Max-Age' 1728000;
@@ -192,7 +205,7 @@ server {
             return 204;
         }
         add_header Cache-Control max-age=31536000;
-        add_header 'Access-Control-Allow-Origin' '*';
+        add_header 'Access-Control-Allow-Origin' "${allowed_origins}";
         add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
         add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,Content-Length';
         add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,Content-Length';