diff --git a/backend/server.js b/backend/server.js
index 1302033df..1056969e7 100644
--- a/backend/server.js
+++ b/backend/server.js
@@ -39,6 +39,27 @@ if (
 
 const { CONTRACTS_VERSION, GI_VERSION } = process.env
 
+const securityHeaders = {
+  // This is the most likely to break things now; it may need changing when sandboxing or federation is implemented.
+  // - Not setting media-src since Background.vue dynamically adds sound assets.
+  // - The localhost:3000 entries are for BrowserSync support.
+  'Content-Security-Policy': "child-src 'none'; connect-src 'self' blob: http://localhost:3001 ws://localhost:3001; font-src 'self'; form-action 'self'; frame-ancestors 'none'; frame-src 'none'; img-src 'self' https://unpkg.com blob: data:; manifest-src 'self'; object-src 'none'; script-src 'unsafe-eval'; script-src-attr 'none'; script-src-elem 'self' 'unsafe-inline'; style-src 'self'; style-src-attr 'none'; style-src-elem 'self' 'unsafe-inline'; upgrade-insecure-requests; worker-src 'self'",
+  // Block embedding cross-origin resources that don't explicitly allow it. Disabled because it blocks emoji sheet rendering.
+  // 'Cross-Origin-Embedder-Policy': 'require-corp',
+  // Don't share context with potentially untrusted sites.
+  'Cross-Origin-Opener-Policy': 'same-origin',
+  // Block hotlinking.
+  'Cross-Origin-Resource-Policy': 'same-origin',
+  // List generated using https://www.permissionspolicy.com.
+  'Permissions-Policy': 'accelerometer=(), ambient-light-sensor=(), autoplay=(), battery=(), camera=(), cross-origin-isolated=(self), display-capture=(), document-domain=(), encrypted-media=(), execution-while-not-rendered=(self), execution-while-out-of-viewport=(self), fullscreen=(self), geolocation=(), gyroscope=(), keyboard-map=(), magnetometer=(), microphone=(), midi=(), navigation-override=(self), payment=(), picture-in-picture=(), publickey-credentials-get=(), screen-wake-lock=(), sync-xhr=(), usb=(), web-share=(self), xr-spatial-tracking=(), clipboard-read=(), clipboard-write=(self), gamepad=(), speaker-selection=()',
+  // This tells the user agent under which circumstances to send a referrer header and what information it should contain.
+  // Setting it can be useful to prevent destination servers of links in GI from learning the exact URL (which can contain, e.g., a group) the user came from.
+  'Referrer-Policy': 'strict-origin-when-cross-origin',
+  'X-Content-Type-Options': 'nosniff',
+  // Block loading the page in a frame.
+  'X-Frame-Options': 'DENY'
+}
+
 const hapi = new Hapi.Server({
   // debug: false, // <- Hapi v16 was outputing too many unnecessary debug statements
   //               // v17 doesn't seem to do this anymore so I've re-enabled the logging
@@ -67,9 +88,13 @@ hapi.ext({
       // but custom headers can be manually added using `.output.headers`.
       // See https://hapi.dev/module/boom/api/.
       if (typeof request.response.header === 'function') {
-        request.response.header('X-Frame-Options', 'deny')
+        for (const [name, value] of Object.entries(securityHeaders)) {
+          request.response.header(name, value)
+        }
       } else {
-        request.response.output.headers['X-Frame-Options'] = 'deny'
+        for (const [name, value] of Object.entries(securityHeaders)) {
+          request.response.output.headers[name] = value
+        }
       }
     } catch (err) {
       console.warn(chalk.yellow('[backend] Could not set X-Frame-Options header:', err.message))
diff --git a/frontend/assets/style/_misc.scss b/frontend/assets/style/_misc.scss
index 8cafb5497..f28904e2a 100644
--- a/frontend/assets/style/_misc.scss
+++ b/frontend/assets/style/_misc.scss
@@ -3,10 +3,10 @@
    ========================================================================== */
 
 .browserupgrade {
-  margin: 0.2em 0;
+  display: none;
   background: #ccc;
   color: #000;
-  padding: 0.2em 0;
+  padding: 0.2em;
 }
 
 /* ==========================================================================
diff --git a/frontend/index.html b/frontend/index.html
index f89085275..3ff31d65d 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -16,7 +16,7 @@
       <img src="/assets/images/group-income-icon-transparent.png" alt="logo" />
     </div>
 
-    <div class="modal is-active" id="fallback" style="display: none;">
+    <div class="browserupgrade modal is-active" id="fallback">
       <div class="modal-content">
         <h1>Outdated browser</h1>
         <p>You are using an <strong>outdated</strong> or unsupported browser. Please upgrade your browser to improve your experience.</p>
diff --git a/test/avatar-caching.test.js b/test/avatar-caching.test.js
index c2647f0dc..7db2fac62 100644
--- a/test/avatar-caching.test.js
+++ b/test/avatar-caching.test.js
@@ -117,7 +117,7 @@ describe('avatar file serving', function () {
     assert.equal(headers.get('content-type'), 'application/octet-stream')
     assert.equal(headers.get('etag'), `"${manifestCid}"`)
     // Not checking for a `last-modified` header.
-    assert.equal(headers.get('x-frame-options'), 'deny')
+    assert.equal(headers.get('x-frame-options'), 'DENY')
 
     const { headers: cHeaders } = await fetch(`${apiURL}/file/${chunkCid}`)
     assert.match(cHeaders.get('cache-control'), /immutable/)
@@ -126,6 +126,6 @@ describe('avatar file serving', function () {
     assert.equal(cHeaders.get('content-type'), 'application/octet-stream')
     assert.equal(cHeaders.get('etag'), `"${chunkCid}"`)
     // Not checking for a `last-modified` header.
-    assert.equal(cHeaders.get('x-frame-options'), 'deny')
+    assert.equal(cHeaders.get('x-frame-options'), 'DENY')
   })
 })