HTTP Request Smuggling: LF vs CRLF handling 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-2019-16785: 2.54% estimated exploitation activity in the next 30 days; percentile 84.97 (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 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: Would get treated by Waitress as if it were: 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…

Published by the source

20 Dec 2019, 23:03 UTC

Last modified by the source

10 Sep 2026, 03:49 UTC

Source last checked

26 Oct 2026, 13:27 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-pg36-wpm5-g57p

2. Revision 2 · amended · 25 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-pg36-wpm5-g57p

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2019-16785

Advisory: HTTP Request Smuggling: LF vs CRLF handling in Waitress
IDs: CVE-2019-16785, GHSA-pg36-wpm5-g57p
Record: https://patchnext.com/advisories/cve-2019-16785 (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-pg36-wpm5-g57p (OSV record GHSA-pg36-wpm5-g57p lists affected ranges: PyPI waitress >=0 <1.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-pg36-wpm5-g57p (GitHub-reviewed advisory GHSA-pg36-wpm5-g57p lists affected ranges: PyPI waitress >=0 <1.4.0)

Commands are suggestions: review each one before running it.