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CVE-2019-16785 — HTTP Request Smuggling: LF vs CRLF handling in Waitress

2026-09-29T06:55:26.849274+00:00

Affected: Pylons Waitress

Waitress through version 1.3.1 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. This issue is fixed in Waitress 1.4.0.

GHSA-pg36-wpm5-g57p — HTTP Request Smuggling: LF vs CRLF handling in Waitress

2026-09-29T06:55:26.849393+00:00

Affected: PyPI: waitress

### Impact

Waitress implemented a "MAY" part of the RFC7230 (https://tools.ietf.org/html/rfc7230#section-3.5) 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.

Example:

``` Content-Length: 100[CRLF] X-Header: x[LF]Content-Length: 0[CRLF] ```

Would get treated by Waitress as if it were:

``` Content-Length: 100 X-Header: x Content-Length: 0 ```

This could potentially get used by attackers to split the HTTP request and smuggle a second request in the body of the first.

### Patches

This issue is fixed in Waitress 1.4.0. This brings a range of changes to harden Waitress against potential HTTP request confusions, and may change the behaviour of Waitress behind non-conformist proxies.

Waitress no longer implements the MAY part of the specification and instead requires that all lines are terminated correctly with CRLF. If any lines are found with a bare CR or LF a 400 Bad Request is sent back to the requesting entity…

PYSEC-2019-136

2026-09-29T06:55:26.849502+00:00

Affected: PyPI: waitress