Summary
HTTP Request Smuggling: Invalid whitespace characters in headers in Waitress
Workarounds
You may enable additional protections on front-end servers, those that follow RFC7230 correctly would drop the request with a 400 Bad Request.
Waitress will now correctly responds to the request with a 400 Bad Request, and will drop the connection to avoid any potential HTTP pipelining issues.
References
This was mentioned in https://portswigger.net/research/http-desync-attacks-what-happened-next and was specifically mentioned as being an issue in HAProxy which did not properly filter it in this article: https://nathandavison.com/blog/haproxy-http-request-smuggling
Thanks
The Pylons Project would like to thank ZeddYu Lu for doing extended testing against Waitress 1.4.0 and bringing this to our attention!
For more information
If you have any questions or comments about this advisory:
- 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)
Impact
If a proxy server is used in front of waitress, an invalid request may be sent by an attacker that bypasses the front-end and is parsed differently by waitress leading to a potential for HTTP request smuggling.
Content-Length: 10
Transfer-Encoding: [\x0b]chunked
For clarity:
0x0b == vertical tab
Would get parsed by Waitress as being a chunked request, but a front-end server would use the Content-Length instead as the Transfer-Encoding header is considered invalid due to containing invalid characters.
If a front-end server does HTTP pipelining to a backend Waitress server this could lead to HTTP request splitting which may lead to potential cache poisoning or unexpected information disclosure.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Please upgrade to Waitress 1.4.1 which fixes this issue with stricter HTTP field validation.
Waitress 1.4.1 due to this change has become much more strict in what is allowed in header values, while the maintainers don't believe that these changes will cause any issues, it may cause failures with non-conformist reverse proxies or clients, and it is highly recommend that users validate the changes in their environment and make sure it won't cause any unacceptable failures.
Frequently Asked Questions
- What is GHSA-M5FF-3WJ3-8PH4? GHSA-M5FF-3WJ3-8PH4 is a high-severity security vulnerability in waitress (pip), affecting versions < 1.4.1. It is fixed in 1.4.1.
- Which versions of waitress are affected by GHSA-M5FF-3WJ3-8PH4? waitress (pip) versions < 1.4.1 is affected.
- Is there a fix for GHSA-M5FF-3WJ3-8PH4? Yes. GHSA-M5FF-3WJ3-8PH4 is fixed in 1.4.1. Upgrade to this version or later.
- Is GHSA-M5FF-3WJ3-8PH4 exploitable, and should I be worried? Whether GHSA-M5FF-3WJ3-8PH4 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-M5FF-3WJ3-8PH4 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-M5FF-3WJ3-8PH4? Upgrade
waitressto 1.4.1 or later.