Slow IDNA decoding with large strings


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The canonical database for vulnerabilities affecting Python is available on GitHub in the Open Source Vulnerability (OSV) format. This vulnerability can be viewed online at the Open Source Vulnerability Database.

IDNA decoding execution time is not linear in relation to the input string size, which can cause slowness with large inputs:

  • 10 chars = 0.016 seconds
  • 100 chars = 0.047 seconds
  • 1000 chars = 2.883 seconds
  • 2500 chars = 17.724 seconds
  • 5000 chars = 1 min 10 seconds

According to Unicode Technical Standard #46, an IDNA label must not be longer than 63 characters. The Python idna module enforces the restriction, but too late.


  • Disclosure date: 2022-10-19 (Python issue gh-98433 reported)
  • Reported at: 2022-09-09 (email to PSRT)
  • Reported by: Guido Vranken

Fixed In

Python issue

Slow IDNA decoding with large strings [CVE-2022-45061].

  • Python issue: gh-98433
  • Creation date: 2022-10-19
  • Reporter: guidovranken


An issue was discovered in Python before 3.11.1. An unnecessary quadratic algorithm exists in one path when processing some inputs to the IDNA (RFC 3490) decoder, such that a crafted, unreasonably long name being presented to the decoder could lead to a CPU denial of service. Hostnames are often supplied by remote servers that could be controlled by a malicious actor; in such a scenario, they could trigger excessive CPU consumption on the client attempting to make use of an attacker-supplied supposed hostname. For example, the attack payload could be placed in the Location header of an HTTP response with status code 302. A fix is planned in 3.11.1, 3.10.9, 3.9.16, 3.8.16, and 3.7.16.


Timeline using the disclosure date 2022-10-19 as reference: