diff --git a/internal/web/context/store.go b/internal/web/context/store.go
index 5998f22..496ee62 100644
--- a/internal/web/context/store.go
+++ b/internal/web/context/store.go
@@ -1,11 +1,14 @@
 package context
 
 import (
+	"net/http"
+	"path/filepath"
+	"strings"
+
 	"github.com/gorilla/sessions"
 	"github.com/markbates/goth/gothic"
 	"github.com/thomiceli/opengist/internal/config"
 	"github.com/thomiceli/opengist/internal/session"
-	"path/filepath"
 )
 
 type Store struct {
@@ -19,10 +22,28 @@ func NewStore(sessionsPath string) *Store {
 	s := &Store{sessionsPath: sessionsPath}
 
 	s.flashStore = sessions.NewCookieStore([]byte("opengist"))
+	hardenCookie(s.flashStore.Options)
 	encryptKey, _ := session.GenerateSecretKey(filepath.Join(s.sessionsPath, "session-encrypt.key"))
 	s.UserStore = sessions.NewFilesystemStore(s.sessionsPath, config.SecretKey, encryptKey)
 	s.UserStore.MaxLength(10 * 1024)
+	hardenCookie(s.UserStore.Options)
 	gothic.Store = s.UserStore
 
 	return s
 }
+
+// hardenCookie applies secure defaults to a session cookie. HttpOnly keeps the
+// cookie out of reach of JavaScript (so an XSS cannot read the auth session),
+// and SameSite=Lax mitigates CSRF. Secure is only set when the configured
+// external URL is HTTPS, otherwise the cookie would never be sent over a
+// plain-HTTP deployment and logins would silently break.
+func hardenCookie(opts *sessions.Options) {
+	if opts == nil {
+		return
+	}
+	opts.HttpOnly = true
+	opts.SameSite = http.SameSiteLaxMode
+	if strings.HasPrefix(strings.ToLower(config.C.ExternalUrl), "https://") {
+		opts.Secure = true
+	}
+}
diff --git a/internal/web/handlers/gist/gist.go b/internal/web/handlers/gist/gist.go
index b4730b2..7d6b276 100644
--- a/internal/web/handlers/gist/gist.go
+++ b/internal/web/handlers/gist/gist.go
@@ -6,6 +6,7 @@ import (
 	gojson "encoding/json"
 	"fmt"
 	"net/url"
+	"os"
 	"strings"
 	"time"
 
@@ -44,6 +45,7 @@ func GistIndex(ctx *context.Context) error {
 	ctx.SetData("hasMoreFiles", hasMoreFiles)
 	ctx.SetData("revision", revision)
 	ctx.SetData("htmlTitle", gist.Title)
+	setGistCSP(ctx)
 	return ctx.Html("gist.html")
 }
 
@@ -301,3 +303,22 @@ func escapeJavaScriptContent(htmlContent, cssUrl, themeUrl string, autoMode bool
 
 	return js, nil
 }
+
+func setGistCSP(ctx *context.Context) {
+	if os.Getenv("OG_DEV") == "1" {
+		return
+	}
+	nonce, _ := ctx.GetData("cspNonce").(string)
+	ctx.Response().Header().Set("Content-Security-Policy",
+		"default-src 'self'; "+
+			"script-src 'self' 'nonce-"+nonce+"'; "+
+			"style-src 'self' 'unsafe-inline'; "+
+			"img-src 'self' data:; "+
+			"font-src 'self' data:; "+
+			"connect-src 'self'; "+
+			// 'self' (not 'none') so same-origin PDF previews keep working via
+			// the <embed> element PDFObject inserts.
+			"object-src 'self'; "+
+			"base-uri 'self'; "+
+			"frame-ancestors 'self'")
+}
diff --git a/internal/web/server/middlewares.go b/internal/web/server/middlewares.go
index c7ac564..7ebbc00 100644
--- a/internal/web/server/middlewares.go
+++ b/internal/web/server/middlewares.go
@@ -1,7 +1,9 @@
 package server
 
 import (
+	"crypto/rand"
 	"crypto/subtle"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"html/template"
@@ -134,6 +136,7 @@ func isClientGone(err error) bool {
 func dataInit(next Handler) Handler {
 	return func(ctx *context.Context) error {
 		ctx.SetData("loadStartTime", time.Now())
+		ctx.SetData("cspNonce", newCSPNonce())
 
 		if err := loadSettings(ctx); err != nil {
 			return ctx.ErrorRes(500, "Cannot load settings", err)
@@ -352,6 +355,20 @@ func sessionInit(next Handler) Handler {
 	}
 }
 
+// newCSPNonce returns a fresh random nonce for the Content-Security-Policy of
+// the current request. The same value is exposed to templates (so inline
+// scripts can carry a matching nonce attribute) and used in the CSP header set
+// by handlers that opt into a strict policy (e.g. the gist view page).
+func newCSPNonce() string {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		// rand.Read never returns an error on supported platforms; fall back to
+		// an empty nonce rather than serving a non-functional inline script.
+		return ""
+	}
+	return base64.RawStdEncoding.EncodeToString(b)
+}
+
 func csrfInit(next Handler) Handler {
 	return func(ctx *context.Context) error {
 		var csrf string
diff --git a/package-lock.json b/package-lock.json
index 4ed1dea..a562bea 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -17,6 +17,7 @@
         "@tailwindcss/typography": "^0.5.20",
         "@tailwindcss/vite": "^4.3.0",
         "codemirror": "^6.0.2",
+        "dompurify": "^3.4.11",
         "github-markdown-css": "^5.9.0",
         "highlight.js": "^11.11.1",
         "jdenticon": "^3.3.0",
@@ -904,6 +905,14 @@
         "undici-types": "~7.13.0"
       }
     },
+    "node_modules/@types/trusted-types": {
+      "version": "2.0.7",
+      "resolved": "https://registry.npmjs.org/@types/trusted-types/-/trusted-types-2.0.7.tgz",
+      "integrity": "sha512-ScaPdn1dQczgbl0QFTeTOmVHFULt394XJgOQNoyVhZ6r2vLnMLJfBPd53SB52T/3G36VI1/g2MZaX0cwDuXsfw==",
+      "dev": true,
+      "license": "MIT",
+      "optional": true
+    },
     "node_modules/anymatch": {
       "version": "3.1.3",
       "resolved": "https://registry.npmjs.org/anymatch/-/anymatch-3.1.3.tgz",
@@ -1066,6 +1075,16 @@
         "node": ">=8"
       }
     },
+    "node_modules/dompurify": {
+      "version": "3.4.11",
+      "resolved": "https://registry.npmjs.org/dompurify/-/dompurify-3.4.11.tgz",
+      "integrity": "sha512-zhlUV12GsaRzMsf9q5M254YhA4+VuF0fG+QFqu6aYpoGlKtz+w8//jBcGVYBgQkR5GHjUomejY84AV+/uPbWdw==",
+      "dev": true,
+      "license": "(MPL-2.0 OR Apache-2.0)",
+      "optionalDependencies": {
+        "@types/trusted-types": "^2.0.7"
+      }
+    },
     "node_modules/enhanced-resolve": {
       "version": "5.23.0",
       "resolved": "https://registry.npmjs.org/enhanced-resolve/-/enhanced-resolve-5.23.0.tgz",
diff --git a/package.json b/package.json
index 4290cc4..1aabf81 100644
--- a/package.json
+++ b/package.json
@@ -19,6 +19,7 @@
     "@tailwindcss/typography": "^0.5.20",
     "@tailwindcss/vite": "^4.3.0",
     "codemirror": "^6.0.2",
+    "dompurify": "^3.4.11",
     "github-markdown-css": "^5.9.0",
     "highlight.js": "^11.11.1",
     "jdenticon": "^3.3.0",
diff --git a/public/ts/ipynb.ts b/public/ts/ipynb.ts
index 0b5a10c..8f3c054 100644
--- a/public/ts/ipynb.ts
+++ b/public/ts/ipynb.ts
@@ -1,6 +1,13 @@
 import hljs from 'highlight.js';
 import latex from './latex';
 import { marked } from 'marked';
+import DOMPurify from 'dompurify';
+
+// Notebook content is attacker-controlled: any user can store a `.ipynb` gist
+// whose JSON ends up injected into the page. Every fragment that reaches an
+// `innerHTML` sink must therefore be sanitized to prevent stored XSS (see the
+// security advisory describing the original markdown-cell / output sinks).
+const sanitize = (html: string): string => DOMPurify.sanitize(html);
 
 class IPynb {
   private element: HTMLElement;
@@ -10,7 +17,9 @@ class IPynb {
 
   constructor(element: HTMLElement) {
     this.element = element;
-    let notebookContent = element.innerText;
+    // textContent yields the raw notebook JSON the server escaped into the
+    // <pre>; it is parsed as data, never injected as markup.
+    let notebookContent = element.textContent || '';
 
     try {
       this.notebook = JSON.parse(notebookContent);
@@ -49,10 +58,10 @@ class IPynb {
         outputElement.appendChild(textElement);
       } else if (output.output_type === 'display_data' || output.output_type === 'execute_result') {
         if (output.data['text/plain']) {
-          outputElement.innerHTML += `\n<pre>${output.data['text/plain']}</pre>`;
+          outputElement.innerHTML += sanitize(`\n<pre>${output.data['text/plain']}</pre>`);
         }
         if (output.data['text/html']) {
-          outputElement.innerHTML += '\n' + output.data['text/html'];
+          outputElement.innerHTML += '\n' + sanitize(output.data['text/html']);
         }
 
         const images = Object.keys(output.data).filter(key => key.startsWith('image/'));
@@ -60,19 +69,7 @@ class IPynb {
           const imgEl = document.createElement('img');
           const imgType = images[0]; // Use the first image type found
           imgEl.src = `data:${imgType};base64,${output.data[imgType]}`;
-          outputElement.innerHTML += imgEl.outerHTML;
-        }
-      } else if (output.output_type === 'execute_result') {
-        if (output.data['text/plain']) {
-          outputElement.innerHTML += `<pre>${output.data['text/plain']}</pre>`;
-        }
-        if (output.data['text/html']) {
-          outputElement.innerHTML += output.data['text/html'];
-        }
-        if (output.data['image/png']) {
-          const imgEl = document.createElement('img');
-          imgEl.src = `data:image/png;base64,${output.data['image/png']}`;
-          outputElement.appendChild(imgEl);
+          outputElement.innerHTML += sanitize(imgEl.outerHTML);
         }
       } else if (output.output_type === 'error') {
         outputElement.classList.add('error');
@@ -91,13 +88,24 @@ class IPynb {
     switch (cell.cell_type) {
       case 'markdown':
         cellElement.classList.add('markdown-cell');
-        cellElement.innerHTML = `<div class="markdown-body">${marked.parse(latex.render(source))}</div>`;
+        cellElement.innerHTML = sanitize(
+          `<div class="markdown-body">${marked.parse(latex.render(source)) as string}</div>`
+        );
         break;
-      case 'code':
+      case 'code': {
         cellElement.classList.add('code-cell');
-        cellElement.innerHTML = `<pre class="hljs"><code class="language-${this.language}">${source}</code></pre>`;
-        hljs.highlightElement(cellElement.querySelector('code') as HTMLElement);
+        // Build the code block via DOM APIs so the source is treated as text,
+        // not markup, before highlight.js processes it.
+        const pre = document.createElement('pre');
+        pre.classList.add('hljs');
+        const code = document.createElement('code');
+        code.classList.add(`language-${this.language}`);
+        code.textContent = source;
+        pre.appendChild(code);
+        cellElement.appendChild(pre);
+        hljs.highlightElement(code);
         break;
+      }
       default:
         break;
     }
diff --git a/templates/base/base_header.html b/templates/base/base_header.html
index 3c64a01..fdc49ce 100644
--- a/templates/base/base_header.html
+++ b/templates/base/base_header.html
@@ -8,12 +8,12 @@
     {{ end }}
 
     <base href="{{ $.c.ExternalUrl }}" />
-
+    
     {{ if .canonicalUrl }}
         <link rel="canonical" href="{{ .canonicalUrl }}" />
     {{ end }}
-
-    <script>
+  
+    <script nonce="{{ .cspNonce }}">
         window.opengist_base_url = "{{ $.c.ExternalUrl }}";
         window.opengist_locale = "{{ .locale.Code }}".substring(0, 2);
         const checkTheme = () => {