CVE-2021-3733: ReDoS in urllib.request

The regular expression used by the AbstractBasicAuthHandler class of the urllib module is inefficient and can be abused by an attacker with a maliciuous HTTP server to cause a denial of service.


  • Disclosure date: 2021-01-30 (Python issue bpo-43075 reported)

Fixed In

Python issue

CVE-2021-3733: ReDoS in urllib.request.

  • Python issue: bpo-43075
  • Creation date: 2021-01-30
  • Reporter: yeting li


There’s a flaw in urllib’s AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is sent by the server to the client. The greatest threat that this flaw poses is to application availability.


Timeline using the disclosure date 2021-01-30 as reference: