urllib.parse should sanitize urls containing ASCII newline and tabs.

A security issue was reported by Mike Lissner wherein an attacker was able to use \r\n in the url path, the urlparse method didn’t sanitize and allowed those characters be present in the request:

>>> from urllib.parse import urlsplit
>>> urlsplit("java\nscript:alert('bad')")
SplitResult(scheme='', netloc='', path="java\nscript:alert('bad')", query='', fragment='')

Firefox and other browsers ignore newlines in the scheme. From the browser console:

>> new URL("java\nscript:alert(bad)")
<< URL { href: "javascript:alert(bad)", origin: "null", protocol:
"javascript:", username: "", password: "", host: "", hostname: "", port: "", pathname: "alert(bad)", search: ""

Mozilla Developers informed about the controlling specification for URLs is in fact defined by the “URL Spec” from WHATWG which updates RFC 3986 and specifies that tabs and newlines should be stripped from the scheme.

See: https://url.spec.whatwg.org/#concept-basic-url-parser

That link defines an automaton for URL parsing. From that link, steps 2 and 3 of scheme parsing read:

If input contains any ASCII tab or newline, validation error. 3. Remove all ASCII tab or newline from input.

urlparse module behavior should be updated, and an ASCII tab or newline should be removed from the url (sanitized) before it is sent to the request, as WHATWG spec.

More commits:

Doc changes:


  • Disclosure date: 2021-04-18 (Python issue bpo-43882 reported)
  • Reported at: 2021-03-16 (email sent to the PSRT list)
  • Reported by: Mike Lissner

Fixed In

Python issue

[security] urllib.parse should sanitize urls containing ASCII newline and tabs.

  • Python issue: bpo-43882
  • Creation date: 2021-04-18
  • Reporter: Senthil Kumaran


Timeline using the disclosure date 2021-04-18 as reference: