diff --git a/django/contrib/admindocs/views.py b/django/contrib/admindocs/views.py
index 9f66ce8545..ab6e75c811 100644
--- a/django/contrib/admindocs/views.py
+++ b/django/contrib/admindocs/views.py
@@ -15,6 +15,7 @@ from django.db import models
 from django.http import Http404
 from django.template.engine import Engine
 from django.urls import get_mod_func, get_resolver, get_urlconf
+from django.utils._os import safe_join
 from django.utils.decorators import method_decorator
 from django.utils.functional import cached_property
 from django.utils.inspect import (
@@ -333,7 +334,7 @@ class TemplateDetailView(BaseAdminDocsView):
         else:
             # This doesn't account for template loaders (#24128).
             for index, directory in enumerate(default_engine.dirs):
-                template_file = Path(directory) / template
+                template_file = Path(safe_join(directory, template))
                 if template_file.exists():
                     template_contents = template_file.read_text()
                 else:
diff --git a/docs/releases/2.2.24.txt b/docs/releases/2.2.24.txt
index 5b71d9939f..9bcf7037c4 100644
--- a/docs/releases/2.2.24.txt
+++ b/docs/releases/2.2.24.txt
@@ -6,4 +6,14 @@ Django 2.2.24 release notes
 
 Django 2.2.24 fixes two security issues in 2.2.23.
 
-...
+CVE-2021-33203: Potential directory traversal via ``admindocs``
+===============================================================
+
+Staff members could use the :mod:`~django.contrib.admindocs`
+``TemplateDetailView`` view to check the existence of arbitrary files.
+Additionally, if (and only if) the default admindocs templates have been
+customized by the developers to also expose the file contents, then not only
+the existence but also the file contents would have been exposed.
+
+As a mitigation, path sanitation is now applied and only files within the
+template root directories can be loaded.
diff --git a/docs/releases/3.1.12.txt b/docs/releases/3.1.12.txt
index 32fd96feb5..7d8ee8447e 100644
--- a/docs/releases/3.1.12.txt
+++ b/docs/releases/3.1.12.txt
@@ -6,4 +6,14 @@ Django 3.1.12 release notes
 
 Django 3.1.12 fixes two security issues in 3.1.11.
 
-...
+CVE-2021-33203: Potential directory traversal via ``admindocs``
+===============================================================
+
+Staff members could use the :mod:`~django.contrib.admindocs`
+``TemplateDetailView`` view to check the existence of arbitrary files.
+Additionally, if (and only if) the default admindocs templates have been
+customized by the developers to also expose the file contents, then not only
+the existence but also the file contents would have been exposed.
+
+As a mitigation, path sanitation is now applied and only files within the
+template root directories can be loaded.
diff --git a/docs/releases/3.2.4.txt b/docs/releases/3.2.4.txt
index 7aab5f8134..7c1b195d00 100644
--- a/docs/releases/3.2.4.txt
+++ b/docs/releases/3.2.4.txt
@@ -6,6 +6,18 @@ Django 3.2.4 release notes
 
 Django 3.2.4 fixes two security issues and several bugs in 3.2.3.
 
+CVE-2021-33203: Potential directory traversal via ``admindocs``
+===============================================================
+
+Staff members could use the :mod:`~django.contrib.admindocs`
+``TemplateDetailView`` view to check the existence of arbitrary files.
+Additionally, if (and only if) the default admindocs templates have been
+customized by the developers to also expose the file contents, then not only
+the existence but also the file contents would have been exposed.
+
+As a mitigation, path sanitation is now applied and only files within the
+template root directories can be loaded.
+
 Bugfixes
 ========
 
diff --git a/tests/admin_docs/test_views.py b/tests/admin_docs/test_views.py
index 8e09c4cfec..085b821a37 100644
--- a/tests/admin_docs/test_views.py
+++ b/tests/admin_docs/test_views.py
@@ -154,6 +154,22 @@ class AdminDocViewTests(TestDataMixin, AdminDocsTestCase):
             self.assertEqual(response.status_code, 200)
 
 
+@unittest.skipUnless(utils.docutils_is_available, 'no docutils installed.')
+class AdminDocViewDefaultEngineOnly(TestDataMixin, AdminDocsTestCase):
+
+    def setUp(self):
+        self.client.force_login(self.superuser)
+
+    def test_template_detail_path_traversal(self):
+        cases = ['/etc/passwd', '../passwd']
+        for fpath in cases:
+            with self.subTest(path=fpath):
+                response = self.client.get(
+                    reverse('django-admindocs-templates', args=[fpath]),
+                )
+                self.assertEqual(response.status_code, 400)
+
+
 @override_settings(TEMPLATES=[{
     'NAME': 'ONE',
     'BACKEND': 'django.template.backends.django.DjangoTemplates',