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CVE-2020-5236

MediumPublished

Catastrophic backtracking in regex allows Denial of Service in Waitress

Published: Feb 4, 2020
Updated: Aug 4, 2024
Assigning CNA: GitHub_M
Evidence observed: Aug 8, 2026

Primary CVSS

6.5/ 10Medium

nvd · CVSS 3.1

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

EPSS probability estimate

2.4%

Low · next 30 days

Percentile: 82.9%
Model date: Sep 21, 2026

EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.

Summary

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.

Sources

CVE-2020-5236PoC
Waitress 1.4.2 ReDoS - CVE-2020-5236 (Blog Sample Code)

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