---
id: CVE-2019-16785
aliases:
  - GHSA-pg36-wpm5-g57p
  - PYSEC-2019-136
title: 'HTTP Request Smuggling: LF vs CRLF handling in Waitress'
summary: 'HTTP Request Smuggling: LF vs CRLF handling in Waitress'
severity: high
cvss: 7.1
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:N'
vendor: waitress
product: waitress
ecosystem: pip
affected:
  - waitress < 1.4.0
patched:
  - waitress 1.4.0
published: '2019-12-20'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:49:24.779958466Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-pg36-wpm5-g57p'
references:
  - url: 'https://github.com/Pylons/waitress/security/advisories/GHSA-pg36-wpm5-g57p'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2019-16785'
  - url: >-
      https://github.com/Pylons/waitress/commit/8eba394ad75deaf9e5cd15b78a3d16b12e6b0eba
  - url: 'https://access.redhat.com/errata/RHSA-2020:0720'
  - url: 'https://docs.pylonsproject.org/projects/waitress/en/latest/#security-fixes'
  - url: 'https://github.com/Pylons/waitress'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/waitress/PYSEC-2019-136.yaml
  - url: 'https://lists.debian.org/debian-lts-announce/2022/05/msg00011.html'
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/GVDHR2DNKCNQ7YQXISJ45NT4IQDX3LJ7
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/LYEOTGWJZVKPRXX2HBNVIYWCX73QYPM5
  - url: 'https://www.oracle.com/security-alerts/cpuapr2022.html'
tags:
  - osv
  - pip
epss: 0.02537
epssPercentile: 0.84269
ingestedAt: '2026-09-12T03:13:01.715Z'
---

## Overview

### Impact

Waitress implemented a &amp;quot;MAY&amp;quot; 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.

The Pylons Project recommends upgrading as soon as possible, while validating that the changes in Waitress don&amp;#39;t cause any changes in behavior.

### Workarounds

Various reverse proxies may have protections against sending potentially bad HTTP requests to the backend, and or hardening against potential issues like this. If the reverse proxy doesn&amp;#39;t use HTTP/1.1 for connecting to the backend issues are also somewhat mitigated, as HTTP pipelining does not exist in HTTP/1.0 and Waitress will close the connection after every single request (unless the Keep Alive header is explicitly sent... so this is not a fool proof security method)

### Issues/more security issues:

* open an issue at https://github.com/Pylons/waitress/issues (if not sensitive or security related)
* email the Pylons Security mailing list: pylons-project-security@googlegroups.com (if security related)

## Affected packages

- `waitress < 1.4.0`

## Remediation

Upgrade to a patched release:

- `waitress 1.4.0`