---
id: CVE-2019-16786
aliases:
  - GHSA-g2xc-35jw-c63p
  - PYSEC-2019-137
title: 'HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress'
summary: 'HTTP Request Smuggling: Invalid Transfer-Encoding 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: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-g2xc-35jw-c63p'
references:
  - url: 'https://github.com/Pylons/waitress/security/advisories/GHSA-g2xc-35jw-c63p'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2019-16786'
  - url: >-
      https://github.com/Pylons/waitress/commit/f11093a6b3240fc26830b6111e826128af7771c3
  - 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-2019-137.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: 0.02379
epssPercentile: 0.8314
ingestedAt: '2026-07-08T18:25:49.189Z'
---

## Overview

### Impact

Waitress would parse the `Transfer-Encoding` header and only look for a single string value, if that value was not `chunked` it would fall through and use the `Content-Length` header instead.

According to the HTTP standard `Transfer-Encoding` should be a comma separated list, with the inner-most encoding first, followed by any further transfer codings, ending with `chunked`.

Requests sent with:

```
Transfer-Encoding: gzip, chunked
```

Would incorrectly get ignored, and the request would use a `Content-Length` header instead to determine the body size of the HTTP message.

This could allow for Waitress to treat a single request as multiple requests in the case of HTTP pipelining.

### 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 will now return a 501 Not Implemented error if the `Transfer-Encoding` is not `chunked` or contains multiple elements. Waitress does not support any transfer codings such as `gzip` or `deflate`.

The Pylons Project recommends upgrading as soon as possible, while validating that the changes in Waitress don&#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&#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`