diff --git a/src/node/db/SecurityManager.js b/src/node/db/SecurityManager.js
index c37da0a69..0f3de33c3 100644
--- a/src/node/db/SecurityManager.js
+++ b/src/node/db/SecurityManager.js
@@ -28,6 +28,7 @@ const settings = require('../utils/Settings');
 const webaccess = require('../hooks/express/webaccess');
 const log4js = require('log4js');
 const authLogger = log4js.getLogger('auth');
+const {padutils} = require('../../static/js/pad_utils');
 
 const DENY = Object.freeze({accessStatus: 'deny'});
 
@@ -106,6 +107,11 @@ exports.checkAccess = async (padID, sessionCookie, token, userSettings) => {
     authLogger.debug('access denied: HTTP API session is required');
     return DENY;
   }
+  if (!sessionAuthorID && token != null && !padutils.isValidAuthorToken(token)) {
+    // The author token should be kept secret, so do not log it.
+    authLogger.debug('access denied: invalid author token');
+    return DENY;
+  }
 
   const grant = {
     accessStatus: 'grant',
diff --git a/src/static/js/pad.js b/src/static/js/pad.js
index 07fffa6d2..c37920ead 100644
--- a/src/static/js/pad.js
+++ b/src/static/js/pad.js
@@ -177,7 +177,7 @@ const sendClientReady = (isReconnect) => {
   }
 
   let token = Cookies.get('token');
-  if (token == null) {
+  if (token == null || !padutils.isValidAuthorToken(token)) {
     token = padutils.generateAuthorToken();
     Cookies.set('token', token, {expires: 60});
   }
diff --git a/src/static/js/pad_utils.js b/src/static/js/pad_utils.js
index ac45d9ca3..e10841f50 100644
--- a/src/static/js/pad_utils.js
+++ b/src/static/js/pad_utils.js
@@ -88,6 +88,9 @@ const urlRegex = (() => {
       `(?:${withAuth}|${withoutAuth}|www\\.)${urlChar}*(?!${postUrlPunct})${urlChar}`, 'g');
 })();
 
+// https://stackoverflow.com/a/68957976
+const base64url = /^(?=(?:.{4})*$)[A-Za-z0-9_-]*(?:[AQgw]==|[AEIMQUYcgkosw048]=)?$/;
+
 const padutils = {
   /**
    * Prints a warning message followed by a stack trace (to make it easier to figure out what code
@@ -328,6 +331,21 @@ const padutils = {
     }
   }),
 
+  /**
+   * Returns whether a string has the expected format to be used as a secret token identifying an
+   * author. The format is defined as: 't.' followed by a non-empty base64url string (RFC 4648
+   * section 5 with padding).
+   *
+   * Being strict about what constitutes a valid token enables unambiguous extensibility (e.g.,
+   * conditional transformation of a token to a database key in a way that does not allow a
+   * malicious user to impersonate another user).
+   */
+  isValidAuthorToken: (t) => {
+    if (typeof t !== 'string' || !t.startsWith('t.')) return false;
+    const v = t.slice(2);
+    return v.length > 0 && base64url.test(v);
+  },
+
   /**
    * Returns a string that can be used in the `token` cookie as a secret that authenticates a
    * particular author.