CVE-2020-5236
A public proof of concept exists; no exploitation attested.
- Published2020-02-04
- Fixed in1.4.3
- WeaknessCWE-400
Am I affected?
Debian, Ubuntu or Red Hat sometimes backport a fix without changing the upstream version number: such a version reads « to verify », never « affected ».With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.
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.
Timeline
dates in UTC- 2020-02-0403:15CVE publishedNVD
- 2026-10-2319:23First public proof of conceptPoC-in-GitHub
Observed activity
isMalicious corpus · updated 2026-10-23 21:37 UTCBlock these indicators
- Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2020-5236?format=txt
- CSV export: https://api.ismalicious.com/edl/cve/CVE-2020-5236?format=csv
- TAXII collection of exploited CVEs
- The EDL and CSV take an API key on the Pro plan.
Versions per branch
OSV · NVD| Branch | Affected | Fixed in | Source |
|---|---|---|---|
| 1.4 | < 1.4.3 | 1.4.3 | OSV |
| 1.4 | ≥ 1.4.2, < 1.4.3 | 1.4.3 | OSV |
| Branch | Affected | Fixed in | Source |
|---|---|---|---|
| 1.4 | = 1.4.2 | no fix published | NVD |
as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.
Fix
- Update waitress to 1.4.3
Advisories and patches
- github.comPatch
Detection
read 2026-10-23 19:23 UTC- Nuclei template
- none so far ProjectDiscovery
- Exploit-DB
- no entry Exploit-DB
Sources
- CNACVSS 3.15.7read 2026-10-23
- NVDCVSS 3.16.5read 2026-10-23
- NVDCVSS 2.06.8read 2026-10-23
- CNAweakness (CWE)CWE-400read 2026-10-23
- NVDweakness (CWE)CWE-400read 2026-10-23
- FIRST EPSSEPSS score2.4 %2026-10-23
- PoC-in-GitHubproof of concept1read 2026-10-23
- OSVaffected packages—
Technical details
- CVSS v3 Vector
- CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- Published
- 2020-02-04
- Last Modified
- 2026-06-17
- NVD status
- Modified
Frequently asked questions
What is CVE-2020-5236?
Is CVE-2020-5236 actively exploited?
What is the CVSS score for CVE-2020-5236?
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Related vulnerabilities
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