{"id":"CVE-2019-16786","aliases":["GHSA-g2xc-35jw-c63p","PYSEC-2019-137"],"title":"HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress","summary":"HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress","severity":"high","cvss":7.1,"cvssVector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:N","vendor":"waitress","product":"waitress","ecosystem":"pip","affected":["waitress < 1.4.0"],"patched":["waitress 1.4.0"],"published":"2019-12-20","updated":"2026-07-08","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-g2xc-35jw-c63p","references":[{"url":"https://github.com/Pylons/waitress/security/advisories/GHSA-g2xc-35jw-c63p"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2019-16786"},{"url":"https://github.com/Pylons/waitress/commit/f11093a6b3240fc26830b6111e826128af7771c3"},{"url":"https://access.redhat.com/errata/RHSA-2020:0720"},{"url":"https://docs.pylonsproject.org/projects/waitress/en/latest/#security-fixes"},{"url":"https://github.com/Pylons/waitress"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/waitress/PYSEC-2019-137.yaml"},{"url":"https://lists.debian.org/debian-lts-announce/2022/05/msg00011.html"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/GVDHR2DNKCNQ7YQXISJ45NT4IQDX3LJ7"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/LYEOTGWJZVKPRXX2HBNVIYWCX73QYPM5"},{"url":"https://www.oracle.com/security-alerts/cpuapr2022.html"}],"tags":["osv","pip"],"epss":0.02379,"epssPercentile":0.83,"ingestedAt":"2026-07-08T18:25:49.189Z","slug":"CVE-2019-16786","body":"## Overview\n\n### Impact\n\nWaitress 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.\n\nAccording 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`.\n\nRequests sent with:\n\n```\nTransfer-Encoding: gzip, chunked\n```\n\nWould incorrectly get ignored, and the request would use a `Content-Length` header instead to determine the body size of the HTTP message.\n\nThis could allow for Waitress to treat a single request as multiple requests in the case of HTTP pipelining.\n\n### Patches\n\nThis 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. \n\nWaitress 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`.\n\nThe Pylons Project recommends upgrading as soon as possible, while validating that the changes in Waitress don&#39;t cause any changes in behavior.\n\n### Workarounds\n\nVarious 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&#39;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).\n\n### Issues/more security issues:\n\n* open an issue at https://github.com/Pylons/waitress/issues (if not sensitive or security related)\n* email the Pylons Security mailing list: pylons-project-security@googlegroups.com (if security related)\n\n## Affected packages\n\n- `waitress < 1.4.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `waitress 1.4.0`","depth":"twilight","depthScore":40,"depthScoreParts":{"impact":39.1,"likelihood":0.5,"exploitation":0,"ransomware":0},"changes":[]}