http.server parse_request() bug and error reporting #72734

warsaw · 2016-10-28T15:57:10Z

BPO	28548
Nosy	@warsaw, @vadmium
PRs	[Do Not Merge] Convert `Misc/NEWS` so that it is managed by towncrier #552
Files	parse-version.patch parse-version.v2.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-11-19.03:06:33.902>
created_at = <Date 2016-10-28.15:57:09.762>
labels = ['3.7', 'library']
title = 'http.server parse_request() bug and error reporting'
updated_at = <Date 2017-03-31.16:36:25.199>
user = 'https://github.com/warsaw'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:25.199>
actor = 'dstufft'
assignee = 'none'
closed = True
closed_date = <Date 2016-11-19.03:06:33.902>
closer = 'martin.panter'
components = ['Library (Lib)']
creation = <Date 2016-10-28.15:57:09.762>
creator = 'barry'
dependencies = []
files = ['45259', '45300']
hgrepos = []
issue_num = 28548
keywords = ['patch']
message_count = 9.0
messages = ['279611', '279634', '279637', '279639', '279640', '279646', '279673', '281190', '281194']
nosy_count = 3.0
nosy_names = ['barry', 'python-dev', 'martin.panter']
pr_nums = ['552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue28548'
versions = ['Python 3.7']

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warsaw · 2016-10-28T15:57:08Z

This might also affect other Python version; I haven't checked, but I know it affects Python 3.5.

In Mailman 3, we have a subclass of WSGIRequestHandler for our REST API, and we got a bug report about an error condition returning a 400 in the body of the response, but still returning an implicit 200 header.

https://gitlab.com/mailman/mailman/issues/288

This is pretty easily reproducible with the following recipe.

$ git clone https://gitlab.com/mailman/mailman.git
$ cd mailman
$ tox -e py35 --notest -r
$ .tox/py35/bin/python3.5 /home/barry/projects/mailman/trunk/.tox/py35/bin/runner --runner=rest:0:1 -C /home/barry/projects/mailman/trunk/var/etc/mailman.cfg

(Note that you might need to run .tox/py35/bin/mailman info first, and of course you'll have to adjust the directories for your own local fork.)

Now, in another shell, do the following:

$ curl -v -i -u restadmin:restpass "http://localhost:8001/3.0/lists/list example.com"

Note specifically that there is a space right before the "example.com" bit.

Take note also that we're generating an HTTP/1.1 request as per curl default.

The response you get is:

Trying 127.0.0.1...
Connected to localhost (127.0.0.1) port 8001 (#0)
Server auth using Basic with user 'restadmin'

GET /3.0/lists/list example.com HTTP/1.1
Host: localhost:8001
Authorization: Basic cmVzdGFkbWluOnJlc3RwYXNz
User-Agent: curl/7.50.1
Accept: */*

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd"\>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
<title>Error response</title>
</head>
<body>
<h1>Error response</h1>
<p>Error code: 400</p>
<p>Message: Bad request syntax ('GET /3.0/lists/list example.com HTTP/1.1').</p>
<p>Error code explanation: HTTPStatus.BAD_REQUEST - Bad request syntax or unsupported method.</p>
</body>
</html>

Connection #0 to host localhost left intact

Notice the lack of response headers, and thus the implicit 200 return even though the proper error code is in the body of the response. Why does this happen?

Now look at http.server.BaseHTTPRequestHandler. The default_request_version is "HTTP/0.9". Given the request, you'd expect to see the version set to "HTTP/1.1", but that doesn't happen because the extra space messes up the request parsing. parse_request() splits the request line by spaces and when this happens, the wrong number of words shows up. We get 4 words, thus the 'else:' clause in parse_request() gets triggered. So far so good.

This eventually leads us to send_error() and from there into send_response() with the error code (properly 400) and message. From there we get to .send_response_only() and tracing into this function shows you where things go wrong.

send_response_only() has an explicit test on self.request_version != 'HTTP/0.9', in which case it adds nothing to the _header_buffer. Well, because the request parsing got the unexpected number of words, in fact request_version *is* the default HTTP/0.9. Thus the error headers are never added.

There are several problems here. Why are the headers never added when the request is HTTP/0.9? (I haven't read the spec.) Also, since parse_request() isn't setting the request version to 'HTTP/1.1'. It should probably dig out the words[-1] and see if that looks like a version string.

Clearly the request isn't properly escaped, but http.server should not be sending an implicit 200 when the request is bogus.

vadmium · 2016-10-28T22:42:15Z

I think you should be able to reproduce this without Mailman or tox, by just running “python -m http.server”.

The problem is the “HTTP/0.9” protocol that Python is assuming does not include a header section, so there is no place to put a 400 status code or header fields. The HTTP 0.9 response is supposed to only be a HTML body; see <https://www.w3.org/Protocols/HTTP/AsImplemented.html\> and <https://tools.ietf.org/html/rfc1945#section-6\>.

I think we should drop HTTP 0.9 response support from Python’s HTTP server, as well as the attempted but buggy request support. But there was a bit of resistance; see bpo-10721.

Another possibility would be to change default_request_version so that error responses are sent as HTTP 1.0. But there may be more fixes needed for this to continue the buggy HTTP 0.9 support; see bpo-26578.

vadmium · 2016-10-28T22:57:56Z

Parsing the version from words[-1] rather than words[2] may be a minor improvement however.

warsaw · 2016-10-28T23:04:38Z

On Oct 28, 2016, at 10:42 PM, Martin Panter wrote:

I think you should be able to reproduce this without Mailman or tox, by just
running “python -m http.server”.

Yep, indeed.

The problem is the “HTTP/0.9” protocol that Python is assuming does not
include a header section, so there is no place to put a 400 status code or
header fields. The HTTP 0.9 response is supposed to only be a HTML body; see
<https://www.w3.org/Protocols/HTTP/AsImplemented.html\> and
<https://tools.ietf.org/html/rfc1945#section-6\>.

I think we should drop HTTP 0.9 response support from Python’s HTTP server,
as well as the attempted but buggy request support. But there was a bit of
resistance; see bpo-10721.

Another possibility would be to change default_request_version so that error
responses are sent as HTTP 1.0. But there may be more fixes needed for this
to continue the buggy HTTP 0.9 support; see bpo-26578.

I think it may indeed make sense to set default_request_version to 1.0. It's
probably too late to do that for 3.6, but I do think it makes sense for 3.7.
I'm nosying on the issues you mentioned.

I've proposed essentially that workaround for Mailman.

https://gitlab.com/mailman/mailman/issues/288

We can get away with requiring at least HTTP/1.1 for our REST clients.

warsaw · 2016-10-28T23:05:08Z

On Oct 28, 2016, at 10:57 PM, Martin Panter wrote:

Parsing the version from words[-1] rather than words[2] may be a minor
improvement however.

Indeed; I thought about that too.

vadmium · 2016-10-29T02:20:28Z

Here is a patch to parse the version from the request in more cases. Since it is more of a cosmetic improvement for handling erroneous requests, I would probably only apply it to the next Python version (3.7 atm). Or do you think it should go into bug fix versions as well?

warsaw · 2016-10-29T13:53:41Z

On Oct 29, 2016, at 02:20 AM, Martin Panter wrote:

Here is a patch to parse the version from the request in more cases. Since it
is more of a cosmetic improvement for handling erroneous requests, I would
probably only apply it to the next Python version (3.7 atm). Or do you think
it should go into bug fix versions as well?

That would be a RM decision.

python-dev · 2016-11-19T01:19:29Z

New changeset 7c98768368cb by Martin Panter in branch 'default':
Issue bpo-28548: Parse HTTP request version even if too many words received
https://hg.python.org/cpython/rev/7c98768368cb

vadmium · 2016-11-19T03:06:34Z

Okay committed to 3.7 for the moment. I think that is all we can reasonably do unless we drop the pseudo-HTTP-0.9 support.

warsaw added the stdlib Python modules in the Lib dir label Oct 28, 2016

vadmium added the 3.7 (EOL) end of life label Nov 1, 2016

vadmium closed this as completed Nov 19, 2016

ezio-melotti transferred this issue from another repository Apr 10, 2022

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http.server parse_request() bug and error reporting #72734

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http.server parse_request() bug and error reporting #72734

http.server parse_request() bug and error reporting #72734

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warsaw commented Oct 28, 2016

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