HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.1 · CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:N/VA:N/SC:H/SI:L/SA:N · score source

EPSS

• CVE-2019-16786: 2.38% estimated exploitation activity in the next 30 days; percentile 83.91 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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: 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…

Published by the source

20 Dec 2019, 23:04 UTC

Last modified by the source

8 Jul 2026, 05:59 UTC

Source last checked

26 Oct 2026, 13:27 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-g2xc-35jw-c63p

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-g2xc-35jw-c63p

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# Patch Next advisory brief: CVE-2019-16786

Advisory: HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress
IDs: CVE-2019-16786, GHSA-g2xc-35jw-c63p
Record: https://patchnext.com/advisories/cve-2019-16786 (revision 3, source checked 26 Oct 2026, 13:27 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: waitress 1.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI waitress: affected < 1.4.0; fixed in 1.4.0

## Suggested inspection (read-only)
- `python -m pip show waitress`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on waitress.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-g2xc-35jw-c63p (OSV record GHSA-g2xc-35jw-c63p lists affected ranges: PyPI waitress >=0 <1.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-g2xc-35jw-c63p (GitHub-reviewed advisory GHSA-g2xc-35jw-c63p lists affected ranges: PyPI waitress >=0 <1.4.0)

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