diff --git a/config.js b/config.js
index c5c8e50..a8ffa56 100644
--- a/config.js
+++ b/config.js
@@ -1,15 +1,26 @@
+const MATCH_EVERYTHING_STRING = '.*';
 const env = process.env.APP__ENV_NAME || "local";
 const isLocal = env === "local" || env === "test";
 const strippedPath = process.env.APP__STRIPPED_PATH || '';
+const hostsWhitelistRegex = (() => {
+    try {
+        return new RegExp(process.env.APP__HOSTS_WHITELIST_REGEX || MATCH_EVERYTHING_STRING);
+    } catch (e) {
+        console.error(`APP__HOSTS_WHITELIST_REGEX=${process.env.APP__HOSTS_WHITELIST_REGEX} cannot be converted to RegExp:`, e, '\nUsing the default.');
+        return new RegExp(MATCH_EVERYTHING_STRING);
+    }
+})();
 
 console.log("Environment variables:");
 console.log(`APP__STRIPPED_PATH=${strippedPath} (path added to original host in analytics scripts)`);
 console.log(`APP__ENV_NAME=${env} (should not be local nor test in production)`);
+console.log(`APP__HOSTS_WHITELIST_REGEX=${hostsWhitelistRegex}${hostsWhitelistRegex.toString() === `/${MATCH_EVERYTHING_STRING}/` ? ' (YAY!! Anyone can use your proxy!)' : ''}`);
 
 export default {
     isLocalEnv: isLocal,
     httpPort: process.env.PORT || 80,
     strippedPath,
+    hostsWhitelistRegex: hostsWhitelistRegex,
     proxyDomain: "",          // Domain to proxy calls through. Leave it empty to use the requested domain as a proxy domain
     proxy: {                  // Proxy configuration is here
         domains: [            // These domains are replaced in any proxied response (including scripts, URLs and redirects)
diff --git a/readme.md b/readme.md
index 35a7cac..59a8821 100644
--- a/readme.md
+++ b/readme.md
@@ -95,8 +95,17 @@ APP__STRIPPED_PATH=/gtm-proxy
 # reaching analytics-saviour so that next front end requests land to the same prefixed path
 # on your domain e.g. example.com/gtm-proxy/*(d3d3Lmdvb2dsZS1hbmFseXRpY3MuY29t)*/collect?..
 # Because of this, she path you strip must be explicitly provided.
+
 APP__ENV_NAME=local
 # APP__ENV_NAME=local or APP__ENV_NAME=test (default) will display static content from `static-test`.
+
+APP__HOSTS_WHITELIST_REGEX="^(example\\.com|mysecondwebsite\\.com)$"
+# A JavaScript regular expression that the host must match. By default, it matches ANY HOST, MAKING
+# YOUR PROXY AVAILABLE TO ANYONE. Make sure you screen all special regexp characters here. Examples:
+# APP__HOSTS_WHITELIST_REGEX="^example\\.com$" (only the domain example.com is allowed to access the proxy)
+# APP__HOSTS_WHITELIST_REGEX="^.*\\.example\\.com$" (only subdomains of example.com are allowed)
+# APP__HOSTS_WHITELIST_REGEX="^.*\\.?example\\.com$" (example.com and all its subdomains are allowed)
+# APP__HOSTS_WHITELIST_REGEX="^(example\\.com|mysecondwebsite\\.com)$" (multiple specified domains are allowed)
 ```
 
 ### NodeJS Application
diff --git a/src/proxy/configured-domains.js b/src/proxy/configured-domains.js
index 5300ff4..69c324e 100644
--- a/src/proxy/configured-domains.js
+++ b/src/proxy/configured-domains.js
@@ -1,6 +1,7 @@
 import { createDefaultProxy } from "../modules/proxy";
 import config from "../../config";
 import { unmask } from "../modules/mask";
+import { info } from "../logger";
 
 const domains = new Set(config.proxy.domains);
 const proxies = new Map(Array.from(domains).map((domain) => {
@@ -11,6 +12,12 @@ const proxies = new Map(Array.from(domains).map((domain) => {
 
 export function enableDefaultProxy (expressApp) {
     expressApp.use("/", (req, res, next) => {
+        if (!req.headers || !req.headers.host || !config.hostsWhitelistRegex.test(req.headers.host)) {
+            info(`FORBIDDEN: proxy request from host "${req.headers.host}" was canceled as it doesn't match with ${config.hostsWhitelistRegex}.`);
+            return res.status(403).send({
+                error: `Requests from host ${req.headers.host} are restricted.`
+            });
+        }
         const domain = unmask(req.url.split(/\//g)[1]);
         return domains.has(domain)
             ? proxies.get(domain)(req, res, next) // Use proxy for configured domains