diff --git a/django/middleware/common.py b/django/middleware/common.py
index bea3f7448a..a18fbe7b47 100644
--- a/django/middleware/common.py
+++ b/django/middleware/common.py
@@ -7,6 +7,7 @@ from django.core.mail import mail_managers
 from django.http import HttpResponsePermanentRedirect
 from django.urls import is_valid_path
 from django.utils.deprecation import MiddlewareMixin
+from django.utils.http import escape_leading_slashes
 
 
 class CommonMiddleware(MiddlewareMixin):
@@ -79,6 +80,8 @@ class CommonMiddleware(MiddlewareMixin):
         POST, PUT, or PATCH.
         """
         new_path = request.get_full_path(force_append_slash=True)
+        # Prevent construction of scheme relative urls.
+        new_path = escape_leading_slashes(new_path)
         if settings.DEBUG and request.method in ('POST', 'PUT', 'PATCH'):
             raise RuntimeError(
                 "You called this URL via %(method)s, but the URL doesn't end "
diff --git a/django/urls/resolvers.py b/django/urls/resolvers.py
index ce8c7ffa32..5bfab0c067 100644
--- a/django/urls/resolvers.py
+++ b/django/urls/resolvers.py
@@ -17,7 +17,7 @@ from django.core.checks.urls import check_resolver
 from django.core.exceptions import ImproperlyConfigured
 from django.utils.datastructures import MultiValueDict
 from django.utils.functional import cached_property
-from django.utils.http import RFC3986_SUBDELIMS
+from django.utils.http import RFC3986_SUBDELIMS, escape_leading_slashes
 from django.utils.regex_helper import normalize
 from django.utils.translation import get_language
 
@@ -592,9 +592,7 @@ class URLResolver:
                     # safe characters from `pchar` definition of RFC 3986
                     url = quote(candidate_pat % text_candidate_subs, safe=RFC3986_SUBDELIMS + '/~:@')
                     # Don't allow construction of scheme relative urls.
-                    if url.startswith('//'):
-                        url = '/%%2F%s' % url[2:]
-                    return url
+                    return escape_leading_slashes(url)
         # lookup_view can be URL name or callable, but callables are not
         # friendly in error messages.
         m = getattr(lookup_view, '__module__', None)
diff --git a/django/utils/http.py b/django/utils/http.py
index caaab4f9e5..5a063a9956 100644
--- a/django/utils/http.py
+++ b/django/utils/http.py
@@ -435,3 +435,14 @@ def limited_parse_qsl(qs, keep_blank_values=False, encoding='utf-8',
             value = unquote(value, encoding=encoding, errors=errors)
             r.append((name, value))
     return r
+
+
+def escape_leading_slashes(url):
+    """
+    If redirecting to an absolute path (two leading slashes), a slash must be
+    escaped to prevent browsers from handling the path as schemaless and
+    redirecting to another host.
+    """
+    if url.startswith('//'):
+        url = '/%2F{}'.format(url[2:])
+    return url
diff --git a/docs/releases/1.11.15.txt b/docs/releases/1.11.15.txt
index 397681d52e..fca551e772 100644
--- a/docs/releases/1.11.15.txt
+++ b/docs/releases/1.11.15.txt
@@ -5,3 +5,16 @@ Django 1.11.15 release notes
 *August 1, 2018*
 
 Django 1.11.15 fixes a security issue in 1.11.14.
+
+CVE-2018-14574: Open redirect possibility in ``CommonMiddleware``
+=================================================================
+
+If the :class:`~django.middleware.common.CommonMiddleware` and the
+:setting:`APPEND_SLASH` setting are both enabled, and if the project has a
+URL pattern that accepts any path ending in a slash (many content management
+systems have such a pattern), then a request to a maliciously crafted URL of
+that site could lead to a redirect to another site, enabling phishing and other
+attacks.
+
+``CommonMiddleware`` now escapes leading slashes to prevent redirects to other
+domains.
diff --git a/docs/releases/2.0.8.txt b/docs/releases/2.0.8.txt
index 8a03071075..849f80d3f8 100644
--- a/docs/releases/2.0.8.txt
+++ b/docs/releases/2.0.8.txt
@@ -6,6 +6,19 @@ Django 2.0.8 release notes
 
 Django 2.0.8 fixes a security issue and several bugs in 2.0.7.
 
+CVE-2018-14574: Open redirect possibility in ``CommonMiddleware``
+=================================================================
+
+If the :class:`~django.middleware.common.CommonMiddleware` and the
+:setting:`APPEND_SLASH` setting are both enabled, and if the project has a
+URL pattern that accepts any path ending in a slash (many content management
+systems have such a pattern), then a request to a maliciously crafted URL of
+that site could lead to a redirect to another site, enabling phishing and other
+attacks.
+
+``CommonMiddleware`` now escapes leading slashes to prevent redirects to other
+domains.
+
 Bugfixes
 ========
 
diff --git a/tests/middleware/tests.py b/tests/middleware/tests.py
index f3c8b9ca06..88e33348e6 100644
--- a/tests/middleware/tests.py
+++ b/tests/middleware/tests.py
@@ -130,6 +130,25 @@ class CommonMiddlewareTest(SimpleTestCase):
         self.assertEqual(r.status_code, 301)
         self.assertEqual(r.url, '/needsquoting%23/')
 
+    @override_settings(APPEND_SLASH=True)
+    def test_append_slash_leading_slashes(self):
+        """
+        Paths starting with two slashes are escaped to prevent open redirects.
+        If there's a URL pattern that allows paths to start with two slashes, a
+        request with path //evil.com must not redirect to //evil.com/ (appended
+        slash) which is a schemaless absolute URL. The browser would navigate
+        to evil.com/.
+        """
+        # Use 4 slashes because of RequestFactory behavior.
+        request = self.rf.get('////evil.com/security')
+        response = HttpResponseNotFound()
+        r = CommonMiddleware().process_request(request)
+        self.assertEqual(r.status_code, 301)
+        self.assertEqual(r.url, '/%2Fevil.com/security/')
+        r = CommonMiddleware().process_response(request, response)
+        self.assertEqual(r.status_code, 301)
+        self.assertEqual(r.url, '/%2Fevil.com/security/')
+
     @override_settings(APPEND_SLASH=False, PREPEND_WWW=True)
     def test_prepend_www(self):
         request = self.rf.get('/path/')
diff --git a/tests/middleware/urls.py b/tests/middleware/urls.py
index 8c6621d059..d623e7d6af 100644
--- a/tests/middleware/urls.py
+++ b/tests/middleware/urls.py
@@ -6,4 +6,6 @@ urlpatterns = [
     url(r'^noslash$', views.empty_view),
     url(r'^slash/$', views.empty_view),
     url(r'^needsquoting#/$', views.empty_view),
+    # Accepts paths with two leading slashes.
+    url(r'^(.+)/security/$', views.empty_view),
 ]
diff --git a/tests/utils_tests/test_http.py b/tests/utils_tests/test_http.py
index 05b43c814f..1cbb0d96bf 100644
--- a/tests/utils_tests/test_http.py
+++ b/tests/utils_tests/test_http.py
@@ -5,10 +5,10 @@ from django.test import SimpleTestCase, ignore_warnings
 from django.utils.datastructures import MultiValueDict
 from django.utils.deprecation import RemovedInDjango30Warning
 from django.utils.http import (
-    base36_to_int, cookie_date, http_date, int_to_base36, is_safe_url,
-    is_same_domain, parse_etags, parse_http_date, quote_etag, urlencode,
-    urlquote, urlquote_plus, urlsafe_base64_decode, urlsafe_base64_encode,
-    urlunquote, urlunquote_plus,
+    base36_to_int, cookie_date, escape_leading_slashes, http_date,
+    int_to_base36, is_safe_url, is_same_domain, parse_etags, parse_http_date,
+    quote_etag, urlencode, urlquote, urlquote_plus, urlsafe_base64_decode,
+    urlsafe_base64_encode, urlunquote, urlunquote_plus,
 )
 
 
@@ -275,3 +275,14 @@ class HttpDateProcessingTests(unittest.TestCase):
     def test_parsing_asctime(self):
         parsed = parse_http_date('Sun Nov  6 08:49:37 1994')
         self.assertEqual(datetime.utcfromtimestamp(parsed), datetime(1994, 11, 6, 8, 49, 37))
+
+
+class EscapeLeadingSlashesTests(unittest.TestCase):
+    def test(self):
+        tests = (
+            ('//example.com', '/%2Fexample.com'),
+            ('//', '/%2F'),
+        )
+        for url, expected in tests:
+            with self.subTest(url=url):
+                self.assertEqual(escape_leading_slashes(url), expected)