urlsplit does not handle NFKC normalization

URLs encoded with Punycode/IDNA use NFKC normalization to decompose characters. This can result in some characters introducing new segments into a URL.

See Unicode® Technical Standard #46: Unicode IDNA Compatibility Processing.


  • Disclosure date: 2019-03-06 (Python issue bpo-36216 reported)
  • Reported at: 2019-02-16 (email to PSRT)
  • Reported by: Jonathan Birch of Microsoft Corporation and Panayiotis Panayiotou

Fixed In

Python issue

CVE-2019-9636: urlsplit does not handle NFKC normalization.

  • Python issue: bpo-36216
  • Creation date: 2019-03-06
  • Reporter: Steve Dower


Python 2.7.x through 2.7.16 and 3.x through 3.7.2 is affected by: Improper Handling of Unicode Encoding (with an incorrect netloc) during NFKC normalization. The impact is: Information disclosure (credentials, cookies, etc. that are cached against a given hostname). The components are: urllib.parse.urlsplit, urllib.parse.urlparse. The attack vector is: A specially crafted URL could be incorrectly parsed to locate cookies or authentication data and send that information to a different host than when parsed correctly. This is fixed in: v2.7.17, v2.7.17rc1, v2.7.18, v2.7.18rc1; v3.5.10, v3.5.10rc1, v3.5.7, v3.5.8, v3.5.8rc1, v3.5.8rc2, v3.5.9; v3.6.10, v3.6.10rc1, v3.6.11, v3.6.11rc1, v3.6.12, v3.6.9, v3.6.9rc1; v3.7.3, v3.7.3rc1, v3.7.4, v3.7.4rc1, v3.7.4rc2, v3.7.5, v3.7.5rc1, v3.7.6, v3.7.6rc1, v3.7.7, v3.7.7rc1, v3.7.8, v3.7.8rc1, v3.7.9.


Timeline using the disclosure date 2019-03-06 as reference: