diff --git a/dashboard/authcallback.html b/dashboard/authcallback.html
index f023edaea..23b4a5556 100644
--- a/dashboard/authcallback.html
+++ b/dashboard/authcallback.html
@@ -2,19 +2,59 @@
 
 <script>
 
-const tmp = window.location.hash.slice(1).split('&');
+(async function () {
+    const params = new URLSearchParams(window.location.search);
+    const code = params.get('code');
 
-// FIXME: implicit flow (response_type=code token) results in access_token query param. this is not secure
-tmp.forEach(function (pair) {
-    if (pair.indexOf('access_token=') === 0) localStorage.token = pair.split('=')[1];
-});
+    if (!code) {
+        console.error('No authorization code in callback URL');
+        window.location.replace('/');
+        return;
+    }
 
-let redirectTo = '/';
-if (localStorage.getItem('redirectToHash')) {
-    redirectTo += localStorage.getItem('redirectToHash');
-    localStorage.removeItem('redirectToHash');
-}
+    const codeVerifier = sessionStorage.getItem('pkce_code_verifier');
+    const clientId = sessionStorage.getItem('pkce_client_id') || 'cid-webadmin';
+    const apiOrigin = sessionStorage.getItem('pkce_api_origin') || '';
 
-window.location.replace(redirectTo); // this removes us from history
+    sessionStorage.removeItem('pkce_code_verifier');
+    sessionStorage.removeItem('pkce_client_id');
+    sessionStorage.removeItem('pkce_api_origin');
+
+    try {
+        const response = await fetch(apiOrigin + '/openid/token', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+            body: new URLSearchParams({
+                grant_type: 'authorization_code',
+                code: code,
+                client_id: clientId,
+                redirect_uri: window.location.origin + '/authcallback.html',
+                code_verifier: codeVerifier
+            })
+        });
+
+        const data = await response.json();
+
+        if (!response.ok || !data.access_token) {
+            console.error('Token exchange failed', data);
+            window.location.replace('/');
+            return;
+        }
+
+        localStorage.token = data.access_token;
+    } catch (e) {
+        console.error('Token exchange error', e);
+        window.location.replace('/');
+        return;
+    }
+
+    let redirectTo = '/';
+    if (localStorage.getItem('redirectToHash')) {
+        redirectTo += localStorage.getItem('redirectToHash');
+        localStorage.removeItem('redirectToHash');
+    }
+
+    window.location.replace(redirectTo);
+})();
 
 </script>
diff --git a/dashboard/src/Index.vue b/dashboard/src/Index.vue
index 3d7007466..4deabdc63 100644
--- a/dashboard/src/Index.vue
+++ b/dashboard/src/Index.vue
@@ -8,7 +8,7 @@ import { onMounted, onUnmounted, ref, useTemplateRef, provide } from 'vue';
 import { Notification, InputDialog, fetcher } from '@cloudron/pankow';
 import { setLanguage } from './i18n.js';
 import { API_ORIGIN, TOKEN_TYPES } from './constants.js';
-import { redirectIfNeeded } from './utils.js';
+import { redirectIfNeeded, startAuthFlow } from './utils.js';
 import ProfileModel from './models/ProfileModel.js';
 import ProvisionModel from './models/ProvisionModel.js';
 import NotificationsModel from './models/NotificationsModel.js';
@@ -436,8 +436,8 @@ onMounted(async () => {
   if (!localStorage.token) {
     localStorage.setItem('redirectToHash', window.location.hash);
 
-    // start oidc flow
-    window.location.href =  `${API_ORIGIN}/openid/auth?client_id=` + (API_ORIGIN ? TOKEN_TYPES.ID_DEVELOPMENT : TOKEN_TYPES.ID_WEBADMIN) + '&scope=openid email profile&response_type=code token&redirect_uri=' + window.location.origin + '/authcallback.html';
+    const clientId = API_ORIGIN ? TOKEN_TYPES.ID_DEVELOPMENT : TOKEN_TYPES.ID_WEBADMIN;
+    window.location.href = await startAuthFlow(clientId, API_ORIGIN);
 
     return;
   }
diff --git a/dashboard/src/components/SetupAccount.vue b/dashboard/src/components/SetupAccount.vue
index fc90c1fbf..370b792fe 100644
--- a/dashboard/src/components/SetupAccount.vue
+++ b/dashboard/src/components/SetupAccount.vue
@@ -5,6 +5,7 @@ import { marked } from 'marked';
 import { Button, PasswordInput, FormGroup, TextInput } from '@cloudron/pankow';
 import PublicPageLayout from '../components/PublicPageLayout.vue';
 import ProfileModel from '../models/ProfileModel.js';
+import { startAuthFlow } from '../utils.js';
 
 const profileModel = ProfileModel.create();
 
@@ -89,7 +90,7 @@ async function onSubmit() {
   // set token to autologin on first oidc flow
   localStorage.cloudronFirstTimeToken = result.accessToken;
 
-  dashboardUrl.value = '/openid/auth?client_id=cid-webadmin&scope=openid email profile&response_type=code token&redirect_uri=' + window.location.origin + '/authcallback.html';
+  dashboardUrl.value = await startAuthFlow('cid-webadmin', '');
 
   busy.value = false;
   mode.value = MODE.DONE;
diff --git a/dashboard/src/utils.js b/dashboard/src/utils.js
index 605a1e504..7bc69d7c0 100644
--- a/dashboard/src/utils.js
+++ b/dashboard/src/utils.js
@@ -660,6 +660,35 @@ function parseFullBackupPath(fullPath) {
     return { prefix, remotePath };
 }
 
+function base64urlEncode(buffer) {
+  return btoa(String.fromCharCode(...buffer))
+    .replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}
+
+function generateCodeVerifier() {
+  const array = new Uint8Array(32);
+  crypto.getRandomValues(array);
+  return base64urlEncode(array);
+}
+
+async function computeCodeChallenge(verifier) {
+  const hash = await crypto.subtle.digest('SHA-256', new TextEncoder().encode(verifier));
+  return base64urlEncode(new Uint8Array(hash));
+}
+
+async function startAuthFlow(clientId, apiOrigin) {
+  const codeVerifier = generateCodeVerifier();
+  const codeChallenge = await computeCodeChallenge(codeVerifier);
+
+  sessionStorage.setItem('pkce_code_verifier', codeVerifier);
+  sessionStorage.setItem('pkce_client_id', clientId);
+  sessionStorage.setItem('pkce_api_origin', apiOrigin || '');
+
+  const redirectUri = window.location.origin + '/authcallback.html';
+  const base = apiOrigin || '';
+  return `${base}/openid/auth?client_id=${clientId}&scope=openid email profile&response_type=code&redirect_uri=${redirectUri}&code_challenge=${codeChallenge}&code_challenge_method=S256`;
+}
+
 // named exports
 export {
   renderSafeMarkdown,
@@ -679,7 +708,8 @@ export {
   getColor,
   prettySchedule,
   parseSchedule,
-  parseFullBackupPath
+  parseFullBackupPath,
+  startAuthFlow
 };
 
 // default export
@@ -701,5 +731,6 @@ export default {
   getColor,
   prettySchedule,
   parseSchedule,
-  parseFullBackupPath
+  parseFullBackupPath,
+  startAuthFlow
 };
diff --git a/dashboard/src/views/ActivationView.vue b/dashboard/src/views/ActivationView.vue
index f0bb5093e..fd4d1b685 100644
--- a/dashboard/src/views/ActivationView.vue
+++ b/dashboard/src/views/ActivationView.vue
@@ -3,7 +3,7 @@
 import { ref, useTemplateRef, onMounted } from 'vue';
 import { Button, Checkbox, FormGroup, TextInput, PasswordInput, EmailInput } from '@cloudron/pankow';
 import ProvisionModel from '../models/ProvisionModel.js';
-import { redirectIfNeeded } from '../utils.js';
+import { redirectIfNeeded, startAuthFlow } from '../utils.js';
 
 const provisionModel = ProvisionModel.create();
 
@@ -59,7 +59,7 @@ async function onOwnerSubmit() {
   // set token to autologin on first oidc flow
   localStorage.cloudronFirstTimeToken = result;
 
-  window.location.href = '/openid/auth?client_id=cid-webadmin&scope=openid email profile&response_type=code token&redirect_uri=' + window.location.origin + '/authcallback.html';
+  window.location.href = await startAuthFlow('cid-webadmin', '');
 }
 
 onMounted(async () => {
diff --git a/src/oidcclients.js b/src/oidcclients.js
index 28d0f149c..c2bb951f1 100644
--- a/src/oidcclients.js
+++ b/src/oidcclients.js
@@ -61,8 +61,9 @@ async function get(id) {
             id: ID_WEBADMIN,
             secret: 'notused',
             application_type: 'web',
-            response_types: ['code', 'code token'],
-            grant_types: ['authorization_code', 'implicit'],
+            token_endpoint_auth_method: 'none',
+            response_types: ['code'],
+            grant_types: ['authorization_code'],
             loginRedirectUri: `https://${dashboardFqdn}/authcallback.html`
         };
     } else if (id === ID_DEVELOPMENT) {
@@ -70,8 +71,9 @@ async function get(id) {
             id: ID_DEVELOPMENT,
             secret: 'notused',
             application_type: 'native', // have to use native here to support plaintext http on localhost
-            response_types: ['code', 'code token'],
-            grant_types: ['authorization_code', 'implicit'],
+            token_endpoint_auth_method: 'none',
+            response_types: ['code'],
+            grant_types: ['authorization_code'],
             loginRedirectUri: 'http://localhost:4000/authcallback.html'
         };
     } else if (id === ID_CLI) {
diff --git a/src/oidcserver.js b/src/oidcserver.js
index 2a428fab7..b6f249a7a 100644
--- a/src/oidcserver.js
+++ b/src/oidcserver.js
@@ -36,15 +36,14 @@ import mailpasswords from './mailpasswords.js';
 const { log, trace } = logger('oidcserver');
 
 
-// 1. Index.vue starts the OIDC flow by navigating to /openid/auth. Webadmin sets callback url to authcallback.html + implicit flow
+// 1. Index.vue starts the OIDC flow by navigating to /openid/auth. Webadmin uses authorization code flow with PKCE
 // 2. oidcserver starts an interaction and redirects to oidc_login.html
 // 3. oidc_login.html is rendered by renderInteractionPage() with the form submit url /interaction/:uid/login
 // 4. When form is submitted, it invokes interactionLogin(). This validates user creds
 // 5. We enter the scopes confirmation flow which is oidc_interaction_confirm.html rendered by renderInteractionPage()
 // 6. We have no concept of confirmation. The page auto-submits the form immediately without user interaction
 // 7. oidcserver calls interactionConfirm() which finishes it via interactionFinished().
-
-// FIXME: webadmin's implicit flow (response_type=code token) results in authcallback.html being called with access_token query param. We should remove this
+// 8. authcallback.html exchanges the authorization code for an access token via POST to /openid/token with code_verifier
 
 const ROUTE_PREFIX = '/openid';
 
@@ -719,8 +718,8 @@ async function start() {
             keys: [ cookieSecret ]
         },
         pkce: {
-            required: function pkceRequired(/*ctx, client*/) {
-                return false;
+            required: function pkceRequired(ctx, client) {
+                return client.clientId === 'cid-webadmin' || client.clientId === 'cid-development';
             }
         },
         clientBasedCORS(ctx, origin, client) {