HTTP Request Smuggling Vulnerability in Waitress
Summary
GHSA-m5ff-3wj3-8ph4: HTTP Request Smuggling: Invalid whitespace characters in headers in Waitress. Affects waitress versions < 1.4.1. Fixed in 1.4.1. How to fix ↓
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Background
Waitress is a Python web server that serves web applications written in Python. It is designed to be simple and efficient, providing a reliable foundation for hosting web applications. However, a vulnerability has been discovered in Waitress that could potentially allow for HTTP request smuggling attacks.
Vulnerability Detail
The vulnerability in Waitress arises from the parsing of invalid whitespace characters in headers. If a proxy server is used in front of Waitress, an attacker could send an invalid request that bypasses the front-end and is parsed differently by Waitress. This could lead to HTTP request smuggling, where the request is split or manipulated in a way that could result in cache poisoning or unexpected information disclosure. The issue has been addressed in Waitress 1.4.1, which includes stricter HTTP field validation.
How To Fix
Latest published version: 3.0.2 as of 22 Oct 2026
pip install --upgrade "waitress==3.0.2"Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
1.4.1
This is the first release that contains the fix.
pip install --upgrade "waitress==1.4.1"Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
If upgrading to Waitress 1.4.1 is not immediately feasible, additional protections can be enabled on front-end servers. Servers that follow RFC7230 correctly would drop the request with a 400 Bad Request response. Waitress 1.4.1 now responds to the invalid request with a 400 Bad Request and drops the connection to avoid potential HTTP pipelining issues.
Conclusion
The HTTP Request Smuggling vulnerability in Waitress can be mitigated by upgrading to Waitress 1.4.1 or later. It is important to validate the changes in your environment to ensure compatibility. If immediate upgrade is not possible, additional protections can be enabled on front-end servers. For further support on vulnerability remediation, please contact DevNack.com.
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FAQ
The vulnerability could potentially allow for HTTP request smuggling, which may lead to cache poisoning or unexpected information disclosure.
You can fix this vulnerability by upgrading to Waitress 1.4.1 or later, which includes the necessary fixes.
If immediate upgrade is not possible, you can enable additional protections on front-end servers to drop the invalid requests.
Waitress 1.4.1 has stricter header validation, which may cause failures with non-conformist reverse proxies or clients. It is recommended to validate the changes in your environment before upgrading.
You can find more information about this vulnerability in the Pylons Project's advisory and the references provided in the vulnerability details.
References
Related vulnerabilities
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