How severe is CVE-2019-16786?

CVE-2019-16786 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2019-16786?

CVE-2019-16786 affects the following packages: waitress (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2019-16786?

Update waitress to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-16786 is resolved across your whole dependency graph.

How do I detect CVE-2019-16786 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for waitress, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2019-16786 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2019-16786 actively exploited in the wild?

No public exploit code has been indexed for CVE-2019-16786 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2019-16786?

CVE-2019-16786 has an EPSS (Exploit Prediction Scoring System) score of 2.4%, placing it in the 84th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2019-16786?

CVE-2019-16786 is classified as CWE-444 (CWE-444). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2019-16786 published, and has it been updated?

CVE-2019-16786 was published on December 20, 2019 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

HIGH severity

CVE-2019-16786 — waitress

HIGHFix: Pylons/waitress@f11093a

CVE-2019-16786 is a high-severity (CVSS 7.5) CWE-444 vulnerability in waitress. A fix is available for waitress — see the affected versions and patch details below.

HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress

Also known asGHSA-g2xc-35jw-c63pPYSEC-2019-137

Published

Dec 20, 2019

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs84th percentile — riskier than 84% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐍waitress

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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'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'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: [email protected] (if security related)

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`waitress`	all versions	1.4.0`pip install --upgrade 'waitress==1.4.0'`

Affected Products

5 products · 6 configurations

Application

waitressagendaless

< 1.3.1

range

debian linuxdebian

1 version

9.0

fedorafedoraproject

2 versions

3031

Application

communications cloud native core network function cloud native environmentoracle

1 version

1.10.0

Application

openstackredhat

1 version

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for waitress, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update waitress to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-16786 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

All affected Red Hat products ship but do not use the flawed version of python-waitress. The impact for these products is therefore rated as having a security impact of Low. In Red Hat OpenStack Platform 13, because the flawed code is not used and the fix would require a substantial amount of development, no update…

Product	Fixed in	Advisory
Red Hat OpenStack Platform 15.0 (Stein)	`python-waitress-0:1.4.2-1.el8ost`	RHSA-2020:0720
Red Hat Quay 3	`quay/clair-rhel8:v3.4.0-25`	RHSA-2021:0420

Frequently Asked Questions

### 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