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Copy-pasted from https://bugs.python.org/issue30458#msg347282
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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.