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Waitress WSGI server

Project description

latest version of waitress on PyPI master Documentation Status IRC Freenode

Waitress is meant to be a production-quality pure-Python WSGI server with very acceptable performance. It has no dependencies except ones which live in the Python standard library. It runs on CPython on Unix and Windows under Python 2.7+ and Python 3.5+. It is also known to run on PyPy 1.6.0+ on UNIX. It supports HTTP/1.0 and HTTP/1.1.

For more information, see the “docs” directory of the Waitress package or visit

1.4.4 (2020-06-01)

  • Fix an issue with keep-alive connections in which memory usage was higher than expected because output buffers were being reused across requests on a long-lived connection and each buffer would not be freed until it was full or the connection was closed. Buffers are now rotated per-request to stabilize their behavior.


  • Waitress threads have been updated to contain their thread number. This will allow loggers that use that information to print the thread that the log is coming from.


1.4.3 (2020-02-02)

Security Fixes

  • In Waitress version 1.4.2 a new regular expression was added to validate the headers that Waitress receives to make sure that it matches RFC7230. Unfortunately the regular expression was written in a way that with invalid input it leads to catastrophic backtracking which allows for a Denial of Service and CPU usage going to a 100%.

    This was reported by Fil Zembowicz to the Pylons Project. Please see for more information.

1.4.2 (2020-01-02)

Security Fixes

  • This is a follow-up to the fix introduced in 1.4.1 to tighten up the way Waitress strips whitespace from header values. This makes sure Waitress won’t accidentally treat non-printable characters as whitespace and lead to a potental HTTP request smuggling/splitting security issue.

    Thanks to ZeddYu Lu for the extra test cases.

    Please see the security advisory for more information:

    CVE-ID: CVE-2019-16789


1.4.1 (2019-12-24)

Security Fixes

  • Waitress did not properly validate that the HTTP headers it received were properly formed, thereby potentially allowing a front-end server to treat a request different from Waitress. This could lead to HTTP request smuggling/splitting.

    Please see the security advisory for more information:

    CVE-ID: CVE-2019-16789

1.4.0 (2019-12-20)


  • Waitress used to slam the door shut on HTTP pipelined requests without setting the Connection: close header as appropriate in the response. This is of course not very friendly. Waitress now explicitly sets the header when responding with an internally generated error such as 400 Bad Request or 500 Internal Server Error to notify the remote client that it will be closing the connection after the response is sent.
  • Waitress no longer allows any spaces to exist between the header field-name and the colon. While waitress did not strip the space and thereby was not vulnerable to any potential header field-name confusion, it should have sent back a 400 Bad Request. See

Security Fixes

  • Waitress implemented a “MAY” part of the RFC7230 ( which states:

    Although the line terminator for the start-line and header fields is the sequence CRLF, a recipient MAY recognize a single LF as a line terminator and ignore any preceding CR.

    Unfortunately if a front-end server does not parse header fields with an LF the same way as it does those with a CRLF it can lead to the front-end and the back-end server parsing the same HTTP message in two different ways. This can lead to a potential for HTTP request smuggling/splitting whereby Waitress may see two requests while the front-end server only sees a single HTTP message.

    For more information I can highly recommend the blog post by ZeddYu Lu

    Please see the security advisory for more information:

    CVE-ID: CVE-2019-16785

  • Waitress used to treat LF the same as CRLF in Transfer-Encoding: chunked requests, while the maintainer doesn’t believe this could lead to a security issue, this is no longer supported and all chunks are now validated to be properly framed with CRLF as required by RFC7230.

  • Waitress now validates that the Transfer-Encoding header contains only transfer codes that it is able to decode. At the moment that includes the only valid header value being chunked.

    That means that if the following header is sent:

    Transfer-Encoding: gzip, chunked

    Waitress will send back a 501 Not Implemented with an error message stating as such, as while Waitress supports chunked encoding it does not support gzip and it is unable to pass that to the underlying WSGI environment correctly.

    Waitress DOES NOT implement support for Transfer-Encoding: identity eventhough identity was valid in RFC2616, it was removed in RFC7230. Please update your clients to remove the Transfer-Encoding header if the only transfer coding is identity or update your client to use Transfer-Encoding: chunked instead of Transfer-Encoding: identity, chunked.

    Please see the security advisory for more information:

    CVE-ID: CVE-2019-16786

  • While validating the Transfer-Encoding header, Waitress now properly handles line-folded Transfer-Encoding headers or those that contain multiple comma seperated values. This closes a potential issue where a front-end server may treat the request as being a chunked request (and thus ignoring the Content-Length) and Waitress using the Content-Length as it was looking for the single value chunked and did not support comma seperated values.

  • Waitress used to explicitly set the Content-Length header to 0 if it was unable to parse it as an integer (for example if the Content-Length header was sent twice (and thus folded together), or was invalid) thereby allowing for a potential request to be split and treated as two requests by HTTP pipelining support in Waitress. If Waitress is now unable to parse the Content-Length header, a 400 Bad Request is sent back to the client.

    Please see the security advisory for more information:

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