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Copy-pasted from https://bugs.python.org/issue30458#msg347282
================
The commit b7378d77289c911ca6a0c0afaf513879002df7d5 is incomplete: it doesn't seem to check for control characters in the "host" part of the URL, only in the "path" part of the URL. Example:
---
try:
 from urllib import request as urllib_request
except ImportError:
 import urllib2 as urllib_request
import socket
def bug(*args):
 raise Exception(args)
# urlopen() must not call create_connection()
socket.create_connection = bug
urllib_request.urlopen('http://127.0.0.1\r\n\x20hihi\r\n :11211')
---
The URL comes from the first message of this issue:
https://bugs.python.org/issue30458#msg294360
Development branches 2.7 and master produce a similar output:
---
Traceback (most recent call last):
 ...
Exception: (('127.0.0.1\r\n hihi\r\n ', 11211), ..., None)
---
So urllib2/urllib.request actually does a real network connection (DNS query), whereas it should reject control characters in the "host" part of the URL.
***
A second problem comes into the game. Some C libraries like glibc strip the end of the hostname (strip at the first newline character) and so HTTP Header injection is still possible is this case:
https://bugzilla.redhat.com/show_bug.cgi?id=1673465
***
According to the RFC 3986, the "host" grammar doesn't allow any control character, it looks like:
 host = IP-literal / IPv4address / reg-name
 ALPHA (letters)
 DIGIT (decimal digits)
 unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
 pct-encoded = "%" HEXDIG HEXDIG
 sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
 / "*" / "+" / "," / ";" / "="
 reg-name = *( unreserved / pct-encoded / sub-delims )
 IP-literal = "[" ( IPv6address / IPvFuture ) "]"
 IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
 IPv6address = 6( h16 ":" ) ls32
 / "::" 5( h16 ":" ) ls32
 / [ h16 ] "::" 4( h16 ":" ) ls32
 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
 / [ *4( h16 ":" ) h16 ] "::" ls32
 / [ *5( h16 ":" ) h16 ] "::" h16
 / [ *6( h16 ":" ) h16 ] "::"
 h16 = 1*4HEXDIG
 ls32 = ( h16 ":" h16 ) / IPv4address
 IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
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CVE-2019-18348 was assigned to this flaw, which is similar to CVE-2019-9947 and CVE-2019-9740 but it is about the *host* part of a url.

Can't see the specifics of that "restricted" redhat bug, but this was interesting bug and I wanted to ask if perhaps the domain in such cases should be IDN / punycoded ://xn--n28h.ws/ for example is ://💩.la

The glibc issue mentioned in the first comment is CVE-2016-10739 .

Just to say this is reproducible only on rather old enterprise Linux distributions, where CVE-2016-10739 bug in glibc has not been fixed. I believe it means RHEL-6, SUSE SLE-10, 11, 12 (not sure whether it applies to some old Debian as well).

New changeset 9165addc22d05e776a54319a8531ebd0b2fe01ef by Ashwin Ramaswami in branch 'master':
bpo-38576: Disallow control characters in hostnames in http.client (GH-18995)
https://github.com/python/cpython/commit/9165addc22d05e776a54319a8531ebd0b2fe01ef

Thanks for the PR Ashwin!

New changeset 34f85af3229f86c004a954c3f261ceea1f5e9f95 by Miss Islington (bot) in branch '3.7':
bpo-38576: Disallow control characters in hostnames in http.client (GH-18995)
https://github.com/python/cpython/commit/34f85af3229f86c004a954c3f261ceea1f5e9f95

New changeset ff69c9d12c1b06af58e5eae5db4630cedd94740e by Miss Islington (bot) in branch '3.8':
bpo-38576: Disallow control characters in hostnames in http.client (GH-18995)
https://github.com/python/cpython/commit/ff69c9d12c1b06af58e5eae5db4630cedd94740e

New changeset 83fc70159b24f5b11a5ef87c9b05c2cf4c7faeba by Miss Islington (bot) in branch '3.6':
bpo-38576: Disallow control characters in hostnames in http.client (GH-18995) (GH-19002)
https://github.com/python/cpython/commit/83fc70159b24f5b11a5ef87c9b05c2cf4c7faeba

If anyone cares about 2.7, the *final* release is coming up in a few weeks. They'll need to figure out what it looks like there and get a 2.7 PR reviewed by the release manager.

marking as a 2.7 release blocker just to get benjamin's RM attention before the final 2.7.

New changeset e176e0c105786e9f476758eb5438c57223b65e7f by Matěj Cepl in branch '2.7':
[2.7] closes bpo-38576: Disallow control characters in hostnames in http.client. (GH-19052)
https://github.com/python/cpython/commit/e176e0c105786e9f476758eb5438c57223b65e7f

New changeset 09d8172837b6985c4ad90ee025f6b5a554a9f0ac by Tapas Kundu in branch '3.5':
[3.5] closes bpo-38576: Disallow control characters in hostnames in http.client. (#19300)
https://github.com/python/cpython/commit/09d8172837b6985c4ad90ee025f6b5a554a9f0ac