diff --git a/Lib/cookielib.py b/Lib/cookielib.py
index 1d56d3fe4c0a2e..e76d09d8a500d4 100644
--- a/Lib/cookielib.py
+++ b/Lib/cookielib.py
@@ -205,10 +205,14 @@ def _str2time(day, mon, yr, hr, min, sec, tz):
        (?::(\d\d))?    # optional seconds
     )?                 # optional clock
        \s*
-    ([-+]?\d{2,4}|(?![APap][Mm]\b)[A-Za-z]+)? # timezone
+    (?:
+       ([-+]?\d{2,4}|(?![APap][Mm]\b)[A-Za-z]+) # timezone
+       \s*
+    )?
+    (?:
+       \(\w+\)         # ASCII representation of timezone in parens.
        \s*
-    (?:\(\w+\))?       # ASCII representation of timezone in parens.
-       \s*$""", re.X)
+    )?$""", re.X)
 def http2time(text):
     """Returns time in seconds since epoch of time represented by a string.
 
@@ -266,7 +270,7 @@ def http2time(text):
     return _str2time(day, mon, yr, hr, min, sec, tz)
 
 ISO_DATE_RE = re.compile(
-    """^
+    r"""^
     (\d{4})              # year
        [-\/]?
     (\d\d?)              # numerical month
@@ -278,9 +282,11 @@ def http2time(text):
       (?::?(\d\d(?:\.\d*)?))?  # optional seconds (and fractional)
    )?                    # optional clock
       \s*
-   ([-+]?\d\d?:?(:?\d\d)?
-    |Z|z)?               # timezone  (Z is "zero meridian", i.e. GMT)
-      \s*$""", re.X)
+   (?:
+      ([-+]?\d\d?:?(:?\d\d)?
+       |Z|z)             # timezone  (Z is "zero meridian", i.e. GMT)
+      \s*
+   )?$""", re.X)
 def iso2time(text):
     """
     As for http2time, but parses the ISO 8601 formats:
diff --git a/Lib/test/test_cookielib.py b/Lib/test/test_cookielib.py
index a93bbfb640b6a9..f3711b966e5722 100644
--- a/Lib/test/test_cookielib.py
+++ b/Lib/test/test_cookielib.py
@@ -6,7 +6,7 @@
 import re
 import time
 
-from cookielib import http2time, time2isoz, time2netscape
+from cookielib import http2time, time2isoz, iso2time, time2netscape
 from unittest import TestCase
 
 from test import test_support
@@ -117,6 +117,19 @@ def test_http2time_garbage(self):
                          "http2time(test) %s" % (test, http2time(test))
                          )
 
+    def test_http2time_redos_regression_actually_completes(self):
+        # LOOSE_HTTP_DATE_RE was vulnerable to malicious input which caused catastrophic backtracking (REDoS).
+        # If we regress to cubic complexity, this test will take a very long time to succeed.
+        # If fixed, it should complete within a fraction of a second.
+        http2time("01 Jan 1970{}00:00:00 GMT!".format(" " * 10 ** 5))
+        http2time("01 Jan 1970 00:00:00{}GMT!".format(" " * 10 ** 5))
+
+    def test_iso2time_performance_regression(self):
+        # If ISO_DATE_RE regresses to quadratic complexity, this test will take a very long time to succeed.
+        # If fixed, it should complete within a fraction of a second.
+        iso2time('1994-02-03{}14:15:29 -0100!'.format(' '*10**6))
+        iso2time('1994-02-03 14:15:29{}-0100!'.format(' '*10**6))
+
 
 class HeaderTests(TestCase):
 
diff --git a/Misc/ACKS b/Misc/ACKS
index eba64ae02586ce..fd28ce7fe23ec0 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -210,6 +210,7 @@ Ralph Butler
 Zach Byrne
 Nicolas Cadou
 Jp Calderone
+Ben Caller
 Arnaud Calmettes
 Daniel Calvelo
 Tony Campbell
diff --git a/Misc/NEWS.d/next/Security/2019-11-15-00-54-42.bpo-38804.vjbM8V.rst b/Misc/NEWS.d/next/Security/2019-11-15-00-54-42.bpo-38804.vjbM8V.rst
new file mode 100644
index 00000000000000..1f45142d9f7437
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2019-11-15-00-54-42.bpo-38804.vjbM8V.rst
@@ -0,0 +1 @@
+Fixes a ReDoS vulnerability in :mod:`http.cookiejar`. Patch by Ben Caller.