diff --git a/Lib/http/cookiejar.py b/Lib/http/cookiejar.py
index e0f1032b2816d7..00cb1250a07e86 100644
--- a/Lib/http/cookiejar.py
+++ b/Lib/http/cookiejar.py
@@ -1145,6 +1145,11 @@ def return_ok_domain(self, cookie, request):
         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
@@ -1157,7 +1162,7 @@ def return_ok_domain(self, cookie, request):
             _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
@@ -1171,7 +1176,11 @@ def domain_return_ok(self, domain, request):
             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 --git a/Lib/test/test_http_cookiejar.py b/Lib/test/test_http_cookiejar.py
index abc625d672a784..6e1b30881310b2 100644
--- a/Lib/test/test_http_cookiejar.py
+++ b/Lib/test/test_http_cookiejar.py
@@ -415,6 +415,7 @@ def test_domain_return_ok(self):
             ("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),
@@ -425,6 +426,8 @@ def test_domain_return_ok(self):
             ("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 = urllib.request.Request(url)
             r = pol.domain_return_ok(domain, request)
@@ -959,6 +962,33 @@ def test_domain_block(self):
         c.add_cookie_header(req)
         self.assertFalse(req.has_header("Cookie"))
 
+        c.clear()
+
+        pol.set_blocked_domains([])
+        req = urllib.request.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 = urllib.request.Request("http://acme.com/")
+        c.add_cookie_header(req)
+        self.assertTrue(req.has_header("Cookie"))
+
+        req = urllib.request.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 = urllib.request.Request("http://acme.com/")
+        c.add_cookie_header(req)
+        self.assertFalse(req.has_header("Cookie"))
+
+        req = urllib.request.Request("http://badacme.com/")
+        c.add_cookie_header(req)
+        self.assertFalse(req.has_header("Cookie"))
+
     def test_secure(self):
         for ns in True, False:
             for whitespace in " ", "":
diff --git a/Misc/NEWS.d/next/Security/2018-10-31-15-39-17.bpo-35121.EgHv9k.rst b/Misc/NEWS.d/next/Security/2018-10-31-15-39-17.bpo-35121.EgHv9k.rst
new file mode 100644
index 00000000000000..d2eb8f1f352c21
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2018-10-31-15-39-17.bpo-35121.EgHv9k.rst
@@ -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:`http.cookiejar.DefaultCookiePolicy` policy. Patch by
+Karthikeyan Singaravelan.

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