CVE-2019-16786 | HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress

Waitress through version 1.3.1 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. This issue is fixed in Waitress 1.4.0.

Published: 2019-12-20 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2019-16786 is rated Moderate Risk (59.2/100): CVSS High severity, with medium exploitation likelihood (EPSS 2.54%). EPSS rose +1.75% over the last day, indicating growing attacker interest. Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2019-16786

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.80% 2.54% +1.75%
2 2026-05-30 0.90% 0.80% -0.10%
3 2026-01-24 — 0.90% —

Full EPSS history (21 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2019-16786

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.1 3.1 HIGH
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 1.8 4.7 [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:P/A:N Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:N)
No availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2019-16786

GitHub Security Advisory for CVE-2019-16786

GHSA-g2xc-35jw-c63p · Severity: medium · Ecosystem: pip — HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress

OS Trackers for CVE-2019-16786

vendor priority summary link
alpine high CVE-2019-16786: 1 source package rows (py3-waitress); 7 state rows across 7 repos (3.17-community, 3.18-community, 3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 7, open 0. https://security.alpinelinux.org/vuln/CVE-2019-16786
debian not yet assigned CVE-2019-16786 not yet assigned priority: Debian including 1 source packages (waitress), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2019-16786
redhat high — https://access.redhat.com/security/cve/CVE-2019-16786
suse high CVE-2019-16786 severity important: SUSE including 298 source package names (1.1.1.0.1.5.334:python3-waitress-1.4.3-3.3.1, 1.2.0.0.1.5.338:python3-waitress-1.4.3-3.3.1, …), 542 product×package rows across 38 product lines (Container ses/6/cephcsi/cephcsi, Container ses/6/rook/ceph, … (38 product lines)): Fixed 542. https://www.suse.com/security/cve/CVE-2019-16786/
ubuntu low CVE-2019-16786 low priority: Ubuntu including 1 source packages (waitress), 18 status rows across 18 suites (bionic, disco, eoan, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 12, ignored 2, needed 2, DNE 1, released 1. https://ubuntu.com/security/CVE-2019-16786

Affected software / configurations for CVE-2019-16786

Vendor Product Version Raw CPE
agendaless waitress < 1.3.1 cpe:2.3:a:agendaless:waitress:*:*:*:*:*:*:*:*
oracle communications_cloud_native_core_network_function_cloud_native_environment 1.10.0 cpe:2.3:a:oracle:communications_cloud_native_core_network_function_cloud_native_environment:1.10.0:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
fedoraproject fedora 30 cpe:2.3:o:fedoraproject:fedora:30:*:*:*:*:*:*:*
fedoraproject fedora 31 cpe:2.3:o:fedoraproject:fedora:31:*:*:*:*:*:*:*
redhat openstack 15 cpe:2.3:a:redhat:openstack:15:*:*:*:*:*:*:*

References for CVE-2019-16786

URL Tags
https://access.redhat.com/errata/RHSA-2020:0720 Third Party Advisory
https://docs.pylonsproject.org/projects/waitress/en/latest/#security-fixes Release Notes Vendor Advisory
https://github.com/Pylons/waitress/commit/f11093a6b3240fc26830b6111e826128af7771c3 Patch Third Party Advisory
https://github.com/Pylons/waitress/security/advisories/GHSA-g2xc-35jw-c63p Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/05/msg00011.html Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/GVDHR2DNKCNQ7YQXISJ45NT4IQDX3LJ7/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/LYEOTGWJZVKPRXX2HBNVIYWCX73QYPM5/
https://www.oracle.com/security-alerts/cpuapr2022.html Patch Third Party Advisory
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