Tim Burke <tim.bu...@gmail.com> added the comment:

Note that because http.server uses http.client to parse headers [0], this can 
pose a request-smuggling vector depending on how you've designed your system. 
For example, you might have a storage system with a user-facing HTTP server 
that is in charge of

* authenticating and authorizing users,
* determining where data should be stored, and
* proxying the user request to the backend

and a separate (unauthenticated) HTTP server for actually storing that data. If 
the proxy and backend are running different versions of CPython (say, because 
you're trying to upgrade an existing py2 cluster to run on py3), they may 
disagree about where the request begins and ends -- potentially causing the 
backend to process multiple requests, only the first of which was authorized.

See, for example, https://bugs.launchpad.net/swift/+bug/1840507

For what it's worth, most http server libraries (that I tested; take it with a 
grain of salt) seem to implement their own header parsing. Eventlet was a 
notable exception [1].

[0] https://github.com/python/cpython/blob/v3.8.0/Lib/http/server.py#L336-L337
[1] https://github.com/eventlet/eventlet/pull/574

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