Background

RFC 3986 defines a scheme like this:

• scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

RFC 2234 defines an ALPHA like this:

• ALPHA = %x41-5A / %x61-7A

The WHATWG URL spec defines a scheme like this:

• "A URL-scheme string must be one ASCII alpha, followed by zero or more of ASCII alphanumeric, U+002B (+), U+002D (-), and U+002E (.)."

The bug

This is the scheme string parsing code from Lib/urllib/parse.py:462-468:

    i = url.find(':')
    if i > 0:
        for c in url[:i]:
            if c not in scheme_chars:
                break
        else:
            scheme, url = url[:i].lower(), url[i+1:]

This is the definition of scheme_chars from Lib/urllib/parse.py:77-80:

scheme_chars = ('abcdefghijklmnopqrstuvwxyz'
                'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
                '0123456789'
                '+-.')

This will erroneously validate schemes that begin with any of ('.', '-', '+', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'). This behavior is in violation of both specifications.

This bug is reproducible with the following snippet:

>>> from urllib.parse import urlparse
>>> urlparse(".://") # Should error, but doesn't
ParseResult(scheme='.', netloc='', path='', params='', query='', fragment='')

My environment

• CPython versions tested on:
  • 3.12.0a1+ (fb844e1)
  • 3.10.8
• Operating system and architecture:
  • Arch Linux x86_64

• PR: gh-99418: Make urllib.parse.urlparse enforce that a scheme must begin with an alphabetical ASCII character. #99421

• PR: [3.11] gh-99418: Make urllib.parse.urlparse enforce that a scheme must begin with an alphabetical ASCII character. (GH-99421) #99446