cookies: Use `SameSite=None` if in an iframe from another site

settings.json.docker

@@ -336,6 +336,24 @@
    */
   "trustProxy": "${TRUST_PROXY:false}",
 
+  /*
+   * Settings controlling the session cookie issued by Etherpad.
+   */
+  "cookie": {
+    /*
+     * Value of the SameSite cookie property. "Lax" is recommended unless
+     * Etherpad will be embedded in an iframe from another site, in which case
+     * this must be set to "None". Note: "None" will not work (the browser will
+     * not send the cookie to Etherpad) unless https is used to access Etherpad
+     * (either directly or via a reverse proxy with "trustProxy" set to true).
+     *
+     * "Strict" is not recommended because it has few security benefits but
+     * significant usability drawbacks vs. "Lax". See
+     * https://stackoverflow.com/q/41841880 for discussion.
+     */
+    "sameSite": "${COOKIE_SAME_SITE:Lax}"
+  },
+
   /*
    * Privacy: disable IP logging
    */

settings.json.template

@@ -339,6 +339,24 @@
    */
   "trustProxy": false,
 
+  /*
+   * Settings controlling the session cookie issued by Etherpad.
+   */
+  "cookie": {
+    /*
+     * Value of the SameSite cookie property. "Lax" is recommended unless
+     * Etherpad will be embedded in an iframe from another site, in which case
+     * this must be set to "None". Note: "None" will not work (the browser will
+     * not send the cookie to Etherpad) unless https is used to access Etherpad
+     * (either directly or via a reverse proxy with "trustProxy" set to true).
+     *
+     * "Strict" is not recommended because it has few security benefits but
+     * significant usability drawbacks vs. "Lax". See
+     * https://stackoverflow.com/q/41841880 for discussion.
+     */
+    "sameSite": "Lax"
+  },
+
   /*
    * Privacy: disable IP logging
    */

src/node/hooks/express/webaccess.js

@@ -237,9 +237,7 @@ exports.expressConfigure = (hook_name, args, cb) => {
     name: 'express_sid',
     proxy: true,
     cookie: {
-      // `Strict` is not used because it has few security benefits but significant usability
-      // drawbacks vs. `Lax`. See https://stackoverflow.com/q/41841880 for discussion.
-      sameSite: 'Lax',
+      sameSite: settings.cookie.sameSite,
       /*
        * The automatic express-session mechanism for determining if the
        * application is being served over ssl is similar to the one used for

src/node/utils/Settings.js

@@ -268,6 +268,24 @@ exports.sessionKey = false;
  */
 exports.trustProxy = false;
 
+/*
+ * Settings controlling the session cookie issued by Etherpad.
+ */
+exports.cookie = {
+  /*
+   * Value of the SameSite cookie property. "Lax" is recommended unless
+   * Etherpad will be embedded in an iframe from another site, in which case
+   * this must be set to "None". Note: "None" will not work (the browser will
+   * not send the cookie to Etherpad) unless https is used to access Etherpad
+   * (either directly or via a reverse proxy with "trustProxy" set to true).
+   *
+   * "Strict" is not recommended because it has few security benefits but
+   * significant usability drawbacks vs. "Lax". See
+   * https://stackoverflow.com/q/41841880 for discussion.
+   */
+  sameSite: 'Lax',
+};
+
 /*
  * This setting is used if you need authentication and/or
  * authorization. Note: /admin always requires authentication, and

src/static/js/pad_utils.js

@@ -528,13 +528,28 @@ padutils.setupGlobalExceptionHandler = setupGlobalExceptionHandler;
 
 padutils.binarySearch = require('./ace2_common').binarySearch;
 
+// https://stackoverflow.com/a/42660748
+function inThirdPartyIframe() {
+  try {
+    return (!window.top.location.hostname);
+  } catch (e) {
+    return true;
+  }
+}
+
 // This file is included from Node so that it can reuse randomString, but Node doesn't have a global
 // window object.
 if (typeof window !== 'undefined') {
   exports.Cookies = require('js-cookie/src/js.cookie');
+  // Use `SameSite=Lax`, unless Etherpad is embedded in an iframe from another site in which case
+  // use `SameSite=None`. For iframes from another site, only `None` has a chance of working
+  // because the cookies are third-party (not same-site). Many browsers/users block third-party
+  // cookies, but maybe blocked is better than definitely blocked (which would happen with `Lax`
+  // or `Strict`). Note: `None` will not work unless secure is true.
+  //
   // `Strict` is not used because it has few security benefits but significant usability drawbacks
   // vs. `Lax`. See https://stackoverflow.com/q/41841880 for discussion.
-  exports.Cookies.defaults.sameSite = 'Lax';
+  exports.Cookies.defaults.sameSite = inThirdPartyIframe() ? 'None' : 'Lax';
   exports.Cookies.defaults.secure = window.location.protocol === 'https:';
 }
 exports.randomString = randomString;