diff --git a/tools/serve_header/README.md b/tools/serve_header/README.md
index 0d0ed69f6..bdf2c60ec 100644
--- a/tools/serve_header/README.md
+++ b/tools/serve_header/README.md
@@ -60,6 +60,10 @@ int main() {
 `serve_header.py` will try to read a configuration file `serve_header.yml` in the top level or project root directory, and will fall back on built-in defaults if the file cannot be read.
 An annotated example configuration can be found in `tools/serve_header/serve_header.yml.example`.
 
+By default, the server listens on `localhost` only, and only web pages from Compiler Explorer (`https://godbolt.org` and `https://compiler-explorer.com`) may read the header.
+Set `bind` to serve other machines as well; anyone who can reach the server can then trigger `make` runs in your working trees.
+Set `cors_origins` to allow other web pages.
+
 ## Serving `json.hpp` from multiple project directory instances or working trees
 
 `serve_header.py` was designed with the goal of supporting multiple project roots or working trees at the same time.
diff --git a/tools/serve_header/serve_header.py b/tools/serve_header/serve_header.py
index e2da2dad0..1f29cb589 100755
--- a/tools/serve_header/serve_header.py
+++ b/tools/serve_header/serve_header.py
@@ -26,6 +26,10 @@ HEADER = 'json.hpp'
 
 DATETIME_FORMAT = '%Y-%m-%d %H:%M:%S'
 
+# origins whose pages may read the served header from a browser; Compiler
+# Explorer downloads #include <https://...> headers client-side
+DEFAULT_CORS_ORIGINS = ['https://godbolt.org', 'https://compiler-explorer.com']
+
 JSON_VERSION_RE = re.compile(r'\s*#\s*define\s+NLOHMANN_JSON_VERSION_MAJOR\s+')
 
 class ExitHandler(logging.StreamHandler):
@@ -247,6 +251,8 @@ class WorkTrees(FileSystemEventHandler):
         self.observer.join()
 
 class HeaderRequestHandler(SimpleHTTPRequestHandler): # lgtm[py/missing-call-to-init]
+    cors_origins = DEFAULT_CORS_ORIGINS
+
     def __init__(self, request, client_address, server):
         """."""
         self.worktrees = server.worktrees
@@ -310,8 +316,11 @@ class HeaderRequestHandler(SimpleHTTPRequestHandler): # lgtm[py/missing-call-to-
 
         # set content length
         super().send_header('Content-Length', length)
-        # CORS header
-        self.send_header('Access-Control-Allow-Origin', '*')
+        # CORS header; only for the configured origins
+        origin = self.headers.get('Origin')
+        if origin in self.cors_origins:
+            self.send_header('Access-Control-Allow-Origin', origin)
+        self.send_header('Vary', 'Origin')
         # prevent caching
         self.send_header('Cache-Control', 'no-cache, no-store, must-revalidate')
         self.send_header('Pragma', 'no-cache')
@@ -383,8 +392,15 @@ if __name__ == '__main__':
         # find and monitor working trees
         worktrees = WorkTrees(config.get('root', '.'))
 
-        # start web server
-        infos = socket.getaddrinfo(config.get('bind', None), config.get('port', 8443),
+        # origins allowed to read the header from a browser
+        cors_origins = config.get('cors_origins', DEFAULT_CORS_ORIGINS)
+        if isinstance(cors_origins, str):
+            cors_origins = [cors_origins]
+        HeaderRequestHandler.cors_origins = cors_origins
+
+        # start web server; only reachable from this machine unless configured
+        # otherwise (bind: null listens on all interfaces)
+        infos = socket.getaddrinfo(config.get('bind', 'localhost'), config.get('port', 8443),
                                    type=socket.SOCK_STREAM, flags=socket.AI_PASSIVE)
         DualStackServer.address_family = infos[0][0]
         HeaderRequestHandler.protocol_version = 'HTTP/1.0'
diff --git a/tools/serve_header/serve_header.yml.example b/tools/serve_header/serve_header.yml.example
index 42310910e..ec75e2b49 100644
--- a/tools/serve_header/serve_header.yml.example
+++ b/tools/serve_header/serve_header.yml.example
@@ -10,6 +10,13 @@
 #  cert_file: localhost.pem
 #  key_file: localhost-key.pem
 
-# address and port for the server to listen on
-# bind: null
+# address and port for the server to listen on; by default, only this machine
+# can connect. Binding to a network address, or to null for all interfaces,
+# lets other machines connect, and every request runs make in a working tree.
+# bind: localhost
 # port: 8443
+
+# origins whose web pages may read the header (CORS)
+# cors_origins:
+#   - https://godbolt.org
+#   - https://compiler-explorer.com