I recently came across a bug during a pentest that's allowed me to perform some really interesting attacks on a target. While originally discovered in requests, I had been forwarded to one of the urllib3 developers after agreeing that fixing it at it's lowest level would be preferable. I was informed that the vulnerability is also present in http.client and that I should report it here as well.

The 'method' parameter is not filtered to prevent the injection from altering the entire request.

For example:
>>> conn = http.client.HTTPConnection("localhost", 80)
>>> conn.request(method="GET / HTTP/1.1\r\nHost: abc\r\nRemainder:", url="/index.html")

This will result in the following request being generated:
GET / HTTP/1.1
Host: abc
Remainder: /index.html HTTP/1.1
Host: localhost
Accept-Encoding: identity

This was originally found in an HTTP proxy that was utilising Requests. It allowed me to manipulate the original path to access different files from an internal server since the developers had assumed that the method would filter out non-standard HTTP methods.

The recommended solution is to only allow the standard HTTP methods of GET, HEAD, POST, PUT, DELETE, CONNECT, OPTIONS, TRACE, and PATCH.

An alternate solution that would allow programmers to use non-standard methods would be to only support characters [a-z] and stop reading at any special characters (especially newlines and spaces).

> The recommended solution is to only allow the standard HTTP methods of GET, HEAD, POST, PUT, DELETE, CONNECT, OPTIONS, TRACE, and PATCH.

I don't think that we have to be so strict. We can maybe restrict the HTTP method to ASCII letters, or just reject control characters (U+0000-U+001f).

Similar issues (fixed):

* https://python-security.readthedocs.io/vuln/http-header-injection2.html
* https://python-security.readthedocs.io/vuln/http-header-injection.html

I agree that the solution is quite restrictive.
Restricting to ASCII characters alone would certainly work.

can I work on it?!

@vstinner sorry to bother you, I have a quick question.

the request(...) method is like this:

def request(self, method, url, body=None, headers={}, *,          
            encode_chunked=False):                                
    """Send a complete request to the server."""                  
    self._send_request(method, url, body, headers, encode_chunked)

'request' calls '_send_request' method and '_send_request' calls 'putrequest' inside itself.

So is it good if I encode 'method' parameter to ASCII inside 'putrequest'??!

Welcome to work on the patch, Amir.

* We shouldn't be encoding anything.
* Create reject for Unicode control characters and reject the request if the request contains any control character. Write tests for this.

It will similar to one of the examples Victor has shared.

Hey, it's been a week since the last activity here...
Amir, if you are not working on it I'd be glad to work on it as well :)

New changeset 8ca8a2e8fb068863c1138f07e3098478ef8be12e by AMIR in branch 'master':
bpo-39603: Prevent header injection in http methods (GH-18485)
https://github.com/python/cpython/commit/8ca8a2e8fb068863c1138f07e3098478ef8be12e

The 3.9 and 3.8 backports are waiting for tests to complete. The 3.7 and 3.6 backports need to be merged by the RM (Ned). Then someone can close this issue.

New changeset 668d321476d974c4f51476b33aaca870272523bf by Miss Islington (bot) in branch '3.8':
bpo-39603: Prevent header injection in http methods (GH-18485)
https://github.com/python/cpython/commit/668d321476d974c4f51476b33aaca870272523bf

New changeset 27b811057ff5e93b68798e278c88358123efdc71 by Miss Islington (bot) in branch '3.9':
bpo-39603: Prevent header injection in http methods (GH-18485)
https://github.com/python/cpython/commit/27b811057ff5e93b68798e278c88358123efdc71

New changeset ca75fec1ed358f7324272608ca952b2d8226d11a by Miss Islington (bot) in branch '3.7':
bpo-39603: Prevent header injection in http methods (GH-18485) (GH-21538)
https://github.com/python/cpython/commit/ca75fec1ed358f7324272608ca952b2d8226d11a

New changeset f02de961b9f19a5db0ead56305fe0057a78787ae by Miss Islington (bot) in branch '3.6':
bpo-39603: Prevent header injection in http methods (GH-18485) (GH-21539)
https://github.com/python/cpython/commit/f02de961b9f19a5db0ead56305fe0057a78787ae

Merged for release in 3.9.0b5, 3.8.5, 3.7.9, and 3.6.12. Thanks, everyone!

New changeset 580fbb018fd0844806119614d752b41fc69660f9 by Łukasz Langa in branch '3.8':
Python 3.8.5
https://github.com/python/cpython/commit/580fbb018fd0844806119614d752b41fc69660f9

I've just noticed an issue with the current version of the patch. It should also include 0x20 (space) since that can also be used to manipulate the request.

> It should also include 0x20 (space) since that can also be used to manipulate the request.

Can you indicate how to use a space in the HTTP verb as part of an attack?

New changeset 524b8de630036a29ca340bc2ae6fd6dc7dda8f40 by Victor Stinner in branch '3.5':
bpo-39603: Prevent header injection in http methods (GH-18485) (#21946)
https://github.com/python/cpython/commit/524b8de630036a29ca340bc2ae6fd6dc7dda8f40

Hello,

CVE-2020-26116 has been requested/assigned for this flaw via MITRE form: https://cveform.mitre.org/

I suggest mentioning it in the related vulnerability page: https://python-security.readthedocs.io/vuln/http-header-injection-method.html

Also note that httplib (python-2.7.18) seems to be affected too. Any particular reason for it not to be listed in the same vulnerability page?

Thank you,

> Also note that httplib (python-2.7.18) seems to be affected too. Any particular reason for it not to be listed in the same vulnerability page?

Yes: 2.7 has been end-of-lifed and is no longer supported.

Mauro Matteo Cascella: "CVE-2020-26116 has been requested/assigned for this flaw via MITRE form: https://cveform.mitre.org/ I suggest mentioning it in the related vulnerability page: https://python-security.readthedocs.io/vuln/http-header-injection-method.html"

Thanks, done.