Catastrophic backtracking in regex allows Denial of Service in Waitress
Impact
When waitress receives a header that contains invalid characters it will cause the regular expression engine to catastrophically backtrack causing the process to use 100% CPU time and blocking any other interactions.
This would allow an attacker to send a single request with an invalid header and take the service offline.
Invalid header example:
Bad-header: xxxxxxxxxxxxxxx\x10Increasing the number of x's in the header will increase the amount of time Waitress spends in the regular expression engine.
This issue was introduced in version 1.4.2 when the regular expression was updated to attempt to match the behaviour required by errata associated with RFC7230.
Patches
The regular expression that is used to validate incoming headers has been updated in version 1.4.3, it is recommended that people upgrade to the new version of Waitress as soon as possible.
Workarounds
If you have deployed a reverse proxy in front of Waitress it may already be rejecting requests that include invalid headers.
Thanks
The Pylons Project would like to thank Fil Zembowicz for reaching out and disclosing this vulnerability!
References
Catastrophic backtracking explained: https://www.regular-expressions.info/catastrophic.html
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: pylons-project-security@googlegroups.com (if security related)
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | waitress | >= 1.4.2, < 1.4.3 | 1.4.3 |
References
- https://github.com/Pylons/waitress/security/advisories/GHSA-73m2-3pwg-5fgc
- https://github.com/Pylons/waitress/commit/6e46f9e3f014d64dd7d1e258eaf626e39870ee1f
- https://nvd.nist.gov/vuln/detail/CVE-2020-5236
- https://github.com/Pylons/waitress
- https://github.com/pypa/advisory-database/tree/main/vulns/waitress/PYSEC-2020-155.yaml
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-400- Platform
- Python (PyPI)
- Package
waitress- Disclosed
- 04.02.2020
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2020-155 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-73m2-3pwg-5fgc - CC BY 4.0
Vulnerability history: waitress
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 10
- First → latest
- 12.2019 10.2024
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 2
- High 4
- Medium 4
Disclosures per year
- 2019: 5
- 2020: 1
- 2021: 0
- 2022: 2
- 2023: 0
- 2024: 2
- 2025: 0
- 2026: 0
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