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Fencer Vulns
medium 5.7

CVE-2020-5236 json

Catastrophic backtracking in regex allows Denial of Service in Waitress

published 2020-02-04 6 y ago modified 2026-09-10 1 mo ago also GHSA-73m2-3pwg-5fgc , PYSEC-2020-155 , EUVD-2020-0238

cvss
5.7
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
2.4%
EPSS 2.4% chance of exploitation in 30 days, percentile 83%
public exploits
0
none known

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.

Patch / fix

  • pypi waitress 1.4.3

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2020-5236 updated cvss 6.5

waitress: header with invalid characters can lead to DoS

  • Red Hat OpenStack Platform 15 (Stein) python-waitress, python3-waitress
    affected: no fix planned “Out of support scope”
  • Red Hat OpenStack Platform 16 (Train) python-waitress, python3-waitress
    affected: no fix available “Fix deferred”
  • Red Hat Ceph Storage 3 python-waitress Red Hat Ceph Storage 4 python-waitress, python3-waitress Red Hat OpenShift Container Platform 4 python3-waitress Red Hat OpenStack Platform 13 (Queens) python-waitress, python2-waitress Red Hat Quay 3 python-waitress
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

2.36% chance of exploitation in the next 30 days, higher than 83% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 5.7 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 6.8 AV:N/AC:L/Au:S/C:N/I:N/A:C
GitHub 3.1 5.7 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
pypi waitress
>=1.4.2, <1.4.3
<1.4.3
1.4.3

Affected products

  • Pylons Waitress

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.