HTTP Request Smuggling: Invalid whitespace characters in headers in Waitress

Exploitation

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

CVSS

not scored

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Impact If a proxy server is used in front of waitress, an invalid request may be sent by an attacker that bypasses the front-end and is parsed differently by waitress leading to a potential for HTTP request smuggling. For clarity: Would get parsed by Waitress as being a chunked request, but a front-end server would use the Content-Length instead as the Transfer-Encoding header is considered invalid due to containing invalid characters. If a front-end server does HTTP pipelining to a backend Waitress server this could lead to HTTP request splitting which may lead to potential cache poisoning or unexpected information disclosure. Patches Please upgrade to Waitress 1.4.1 which fixes this issue with stricter HTTP field validation. Waitress 1.4.1 due to this change has become much more strict in what is allowed in header values, while the maintainers don't believe that these changes will cause any issues, it may cause failures with non-conformist reverse proxies or clients, and it is…

Published by the source

26 Dec 2019, 16:34 UTC

Last modified by the source

17 Feb 2025, 05:27 UTC

Source last checked

26 Oct 2026, 13:12 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-m5ff-3wj3-8ph4

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-m5ff-3wj3-8ph4

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# Patch Next advisory brief: GHSA-m5ff-3wj3-8ph4

Advisory: HTTP Request Smuggling: Invalid whitespace characters in headers in Waitress
IDs: GHSA-m5ff-3wj3-8ph4
Record: https://patchnext.com/advisories/ghsa-m5ff-3wj3-8ph4 (revision 2, source checked 26 Oct 2026, 13:12 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: waitress 1.4.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI waitress: affected < 1.4.1; fixed in 1.4.1

## 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-m5ff-3wj3-8ph4 (OSV record GHSA-m5ff-3wj3-8ph4 lists affected ranges: PyPI waitress >=0 <1.4.1 (fixed))
- Verified: https://github.com/advisories/GHSA-m5ff-3wj3-8ph4 (GitHub-reviewed advisory GHSA-m5ff-3wj3-8ph4 lists affected ranges: PyPI waitress >=0 <1.4.1)

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