Fixed #26902 -- Allowed is_safe_url() to require an https URL.

Thanks Andrew Nester, Berker Peksag, and Tim Graham for reviews.

django/utils/http.py

@@ -277,12 +277,15 @@ def is_same_domain(host, pattern):
     )
 
 
-def is_safe_url(url, host=None):
+def is_safe_url(url, host=None, require_https=False):
     """
     Return ``True`` if the url is a safe redirection (i.e. it doesn't point to
     a different host and uses a safe scheme).
 
     Always returns ``False`` on an empty url.
+
+    If ``require_https`` is ``True``, only 'https' will be considered a valid
+    scheme, as opposed to 'http' and 'https' with the default, ``False``.
     """
     if url is not None:
         url = url.strip()
@@ -295,10 +298,11 @@ def is_safe_url(url, host=None):
             return False
     # Chrome treats \ completely as / in paths but it could be part of some
     # basic auth credentials so we need to check both URLs.
-    return _is_safe_url(url, host) and _is_safe_url(url.replace('\\', '/'), host)
+    return (_is_safe_url(url, host, require_https=require_https) and
+            _is_safe_url(url.replace('\\', '/'), host, require_https=require_https))
 
 
-def _is_safe_url(url, host):
+def _is_safe_url(url, host, require_https=False):
     # Chrome considers any URL with more than two slashes to be absolute, but
     # urlparse is not so flexible. Treat any url with three slashes as unsafe.
     if url.startswith('///'):
@@ -315,8 +319,13 @@ def _is_safe_url(url, host):
     # URL and might consider the URL as scheme relative.
     if unicodedata.category(url[0])[0] == 'C':
         return False
+    scheme = url_info.scheme
+    # Consider URLs without a scheme (e.g. //example.com/p) to be http.
+    if not url_info.scheme and url_info.netloc:
+        scheme = 'http'
+    valid_schemes = ['https'] if require_https else ['http', 'https']
     return ((not url_info.netloc or url_info.netloc == host) and
-            (not url_info.scheme or url_info.scheme in ['http', 'https']))
+            (not scheme or scheme in valid_schemes))
 
 
 def limited_parse_qsl(qs, keep_blank_values=False, encoding='utf-8',

tests/utils_tests/test_http.py

@@ -140,6 +140,24 @@ class TestUtilsHttp(unittest.TestCase):
         # Basic auth without host is not allowed.
         self.assertFalse(http.is_safe_url(r'http://testserver\@example.com'))
 
+    def test_is_safe_url_secure_param_https_urls(self):
+        secure_urls = (
+            'https://example.com/p',
+            'HTTPS://example.com/p',
+            '/view/?param=http://example.com',
+        )
+        for url in secure_urls:
+            self.assertTrue(http.is_safe_url(url, 'example.com', require_https=True))
+
+    def test_is_safe_url_secure_param_non_https_urls(self):
+        not_secure_urls = (
+            'http://example.com/p',
+            'ftp://example.com/p',
+            '//example.com/p',
+        )
+        for url in not_secure_urls:
+            self.assertFalse(http.is_safe_url(url, 'example.com', require_https=True))
+
     def test_urlsafe_base64_roundtrip(self):
         bytestring = b'foo'
         encoded = http.urlsafe_base64_encode(bytestring)