diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index 9db23e6ce04bd5..680b7d619f5fef 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -145,6 +145,8 @@ def test_password_manager(self):
         mgr = urllib.request.HTTPPasswordMgr()
         add = mgr.add_password
         find_user_pass = mgr.find_user_password
+        is_suburi = mgr.is_suburi
+        reduce_uri = mgr.reduce_uri
 
         add("Some Realm", "http://example.com/", "joe", "password")
         add("Some Realm", "http://example.com/ni", "ni", "ni")
@@ -211,6 +213,25 @@ def test_password_manager(self):
         self.assertEqual(find_user_pass("Some Realm", "e.example.com:3128"),
                          ('5', 'e'))
 
+        # is_suburi
+
+        true = self.assertTrue
+        false = self.assertFalse
+        true(is_suburi(reduce_uri('http://example.com'),
+                       reduce_uri('http://example.com/sub_dir')))
+        true(is_suburi(reduce_uri('http://example.com/sub_dir'),
+                       reduce_uri('http://example.com/sub_dir/sub_dir')))
+        false(is_suburi(reduce_uri('http://example.com/sub_dir/sub_dir'),
+                        reduce_uri('http://example.com/sub_dir')))
+        false(is_suburi(reduce_uri('http://example.com/second_dir'),
+                        reduce_uri('http://example.com/first_dir/second_dir')))
+        false(is_suburi(reduce_uri('http://example.com/sub_dir'),
+                        reduce_uri('http://example.com/sub')))
+        false(is_suburi(reduce_uri('http://example.com/sub_dir'),
+                        reduce_uri('http://exmaple.com/sub_dir_diff')))
+        false(is_suburi(reduce_uri('http://example.com/sub_dir_diff'),
+                        reduce_uri('http://exmaple.com/sub_dir_second_diff')))
+
     def test_password_manager_default_port(self):
         """
         The point to note here is that we can't guess the default port if
diff --git a/Lib/urllib/request.py b/Lib/urllib/request.py
index e5febe61f556d3..227485d336c3f6 100644
--- a/Lib/urllib/request.py
+++ b/Lib/urllib/request.py
@@ -881,15 +881,15 @@ def reduce_uri(self, uri, default_port=True):
         return authority, path
 
     def is_suburi(self, base, test):
-        """Check if test is below base in a URI tree
+        """Check if test is equal or below base in a URI tree
 
         Both args must be URIs in reduced form.
         """
         if base == test:
             return True
-        if base[0] != test[0]:
+        if base[0] != test[0] or len(test[1]) < len(base[1]):
             return False
-        common = posixpath.commonprefix((base[1], test[1]))
+        common = posixpath.commonpath((base[1], test[1]))
         if len(common) == len(base[1]):
             return True
         return False
diff --git a/Misc/ACKS b/Misc/ACKS
index 211455b4dfc3c2..b8c901407acf75 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -552,6 +552,7 @@ Stefan Franke
 Martin Franklin
 Kent Frazier
 Bruce Frederiksen
+Yair Frid
 Jason Fried
 Robin Friedrich
 Bradley Froehle
diff --git a/Misc/NEWS.d/next/Library/2021-01-09-22-36-01.bpo-42766.6ybdUr.rst b/Misc/NEWS.d/next/Library/2021-01-09-22-36-01.bpo-42766.6ybdUr.rst
new file mode 100644
index 00000000000000..fe22bbb1035085
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-01-09-22-36-01.bpo-42766.6ybdUr.rst
@@ -0,0 +1,3 @@
+Fixed 2 bugs in urllib.request.HTTPPasswordMgr.is_suburi, added some tests
+for it. :func:`urllib.request.HTTPPasswordMgr.is_suburi`. Patch by Yair
+Frid.`