diff -uNrp a/Lib/cookielib.py b/Lib/cookielib.py
--- a/Lib/cookielib.py	2019-12-21 16:06:12.476000000 +0800
+++ b/Lib/cookielib.py	2019-12-21 16:09:31.556000000 +0800
@@ -1139,6 +1139,12 @@ class DefaultCookiePolicy(CookiePolicy):
         req_host, erhn = eff_request_host(request)
         domain = cookie.domain
 
+        if domain and not domain.startswith("."):
+            dotdomain = "." + domain
+        else:
+            dotdomain = domain
+
+
         # strict check of non-domain cookies: Mozilla does this, MSIE5 doesn't
         if (cookie.version == 0 and
             (self.strict_ns_domain & self.DomainStrictNonDomain) and
@@ -1151,7 +1157,7 @@ class DefaultCookiePolicy(CookiePolicy):
             _debug("   effective request-host name %s does not domain-match "
                    "RFC 2965 cookie domain %s", erhn, domain)
             return False
-        if cookie.version == 0 and not ("."+erhn).endswith(domain):
+        if cookie.version == 0 and not ("."+erhn).endswith(dotdomain):
             _debug("   request-host %s does not match Netscape cookie domain "
                    "%s", req_host, domain)
             return False
@@ -1165,7 +1171,11 @@ class DefaultCookiePolicy(CookiePolicy):
             req_host = "."+req_host
         if not erhn.startswith("."):
             erhn = "."+erhn
-        if not (req_host.endswith(domain) or erhn.endswith(domain)):
+        if domain and not domain.startswith("."):
+            dotdomain = "." + domain
+        else:
+            dotdomain = domain
+        if not (req_host.endswith(dotdomain) or erhn.endswith(dotdomain)):
             #_debug("   request domain %s does not match cookie domain %s",
             #       req_host, domain)
             return False
diff -uNrp a/Lib/test/test_cookielib.py b/Lib/test/test_cookielib.py
--- a/Lib/test/test_cookielib.py	2019-12-21 16:06:12.640000000 +0800
+++ b/Lib/test/test_cookielib.py	2019-12-21 16:11:53.888000000 +0800
@@ -368,6 +368,7 @@ class CookieTests(TestCase):
             ("http://foo.bar.com/", ".foo.bar.com", True),
             ("http://foo.bar.com/", "foo.bar.com", True),
             ("http://foo.bar.com/", ".bar.com", True),
+            ("http://foo.bar.com/", "bar.com", True),
             ("http://foo.bar.com/", "com", True),
             ("http://foo.com/", "rhubarb.foo.com", False),
             ("http://foo.com/", ".foo.com", True),
@@ -378,6 +379,8 @@ class CookieTests(TestCase):
             ("http://foo/", "foo", True),
             ("http://foo/", "foo.local", True),
             ("http://foo/", ".local", True),
+            ("http://barfoo.com", ".foo.com", False),
+            ("http://barfoo.com", "foo.com", False),
             ]:
             request = urllib2.Request(url)
             r = pol.domain_return_ok(domain, request)
@@ -938,6 +941,34 @@ class CookieTests(TestCase):
         c.add_cookie_header(req)
         self.assertFalse(req.has_header("Cookie"))
 
+        c.clear()
+
+        pol.set_blocked_domains([])
+        req = Request("http://acme.com/")
+        res = FakeResponse(headers, "http://acme.com/")
+        cookies = c.make_cookies(res, req)
+        c.extract_cookies(res, req)
+        self.assertEqual(len(c), 1)
+
+        req = Request("http://acme.com/")
+        c.add_cookie_header(req)
+        self.assertTrue(req.has_header("Cookie"))
+
+        req = Request("http://badacme.com/")
+        c.add_cookie_header(req)
+        self.assertFalse(pol.return_ok(cookies[0], req))
+        self.assertFalse(req.has_header("Cookie"))
+
+        p = pol.set_blocked_domains(["acme.com"])
+        req = Request("http://acme.com/")
+        c.add_cookie_header(req)
+        self.assertFalse(req.has_header("Cookie"))
+
+        req = Request("http://badacme.com/")
+        c.add_cookie_header(req)
+        self.assertFalse(req.has_header("Cookie"))
+
+
     def test_secure(self):
         from cookielib import CookieJar, DefaultCookiePolicy
 
diff -uNrp a/Misc/NEWS.d/next/Security/2019-05-20-00-35-12.bpo-35121.RRi-HU.rst b/Misc/NEWS.d/next/Security/2019-05-20-00-35-12.bpo-35121.RRi-HU.rst
--- a/Misc/NEWS.d/next/Security/2019-05-20-00-35-12.bpo-35121.RRi-HU.rst	1970-01-01 08:00:00.000000000 +0800
+++ b/Misc/NEWS.d/next/Security/2019-05-20-00-35-12.bpo-35121.RRi-HU.rst	2019-12-21 16:12:17.416000000 +0800
@@ -0,0 +1,4 @@
+Don't send cookies of domain A without Domain attribute to domain B when
+domain A is a suffix match of domain B while using a cookiejar with
+:class:`cookielib.DefaultCookiePolicy` policy. Patch by Karthikeyan
+Singaravelan.
