CVE-2020-5236

Name: CVE-2020-5236
Creator: NVD / NIST
Published: 2020-02-04T03:15:10.75+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 2.62%

Last modified Jun 17, 2026

CVE-2020-5236 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. Waitress version 1.4.2 allows a DOS attack When waitress receives a header that contains invalid characters. When a header like "Bad-header: xxxxxxxxxxxxxxx\x10" is received, it will cause the regular expression engine to catastrophically backtrack causing the process to use 100% CPU time and blocking any other interactions. EPSS estimates a 2.62% chance of exploitation in the next 30 days.

Description

Waitress version 1.4.2 allows a DOS attack When waitress receives a header that contains invalid characters. When a header like "Bad-header: xxxxxxxxxxxxxxx\x10" is received, it will cause the regular expression engine to catastrophically backtrack causing the process to use 100% CPU time and blocking any other interactions. This allows an attacker to send a single request with an invalid header and take the service offline. 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. 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.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

2.62%

83.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Agendaless	Waitress	1.4.2

References

https://github.com/Pylons/waitress/commit/6e46f9e3f014d64dd7d1e258eaf626e39870ee1fPatch, Third Party Advisory
https://github.com/Pylons/waitress/security/advisories/GHSA-73m2-3pwg-5fgcThird Party Advisory
https://github.com/Pylons/waitress/commit/6e46f9e3f014d64dd7d1e258eaf626e39870ee1fPatch, Third Party Advisory
https://github.com/Pylons/waitress/security/advisories/GHSA-73m2-3pwg-5fgcThird Party Advisory

Timeline

Published: Feb 4, 2020
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-5236?

How severe is CVE-2020-5236?

CVE-2020-5236 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 2.62% probability of exploitation in the next 30 days.

How do I fix CVE-2020-5236?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2020-5236?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST