CVE-2020-5236Public exploitNetwork attack vector

Service hang via catastrophic header-regex backtracking

Published Feb 4, 2020 · Updated Aug 4, 2024

Regular expression denial of service in Pylons Project Waitress 1.4.2 allows attackers to hang the service with a malformed HTTP header. The HEADER_FIELD pattern in waitress/rfc7230.py nests repeating field-value groups, causing exponential backtracking when an invalid byte follows a long valid prefix. Exploitation requires a request to reach Waitress without an intermediary rejecting the invalid header; one request can monopolize CPU and block other interactions.

CVSS severity6.9
Medium
EPSS probability2.36%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in Pylons Project Waitress 1.4.2 allows attackers to hang the service with a malformed HTTP header. The HEADER_FIELD pattern in waitress/rfc7230.py nests repeating field-value groups, causing exponential backtracking when an invalid byte follows a long valid prefix. Exploitation requires a request to reach Waitress without an intermediary rejecting the invalid header; one request can monopolize CPU and block other interactions.

The record

CVE
CVE-2020-5236
Published
Feb 4, 2020
Updated
Aug 4, 2024
Vendor
Pylons Project
Product
Waitress
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
Unavailable

Timeline

How it unfolded

  1. Feb 4, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion== 1.4.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesUnavailable in this record.

What is affected?

Pylons Project · Waitressversion== 1.4.2

Published CVSS scores

6.5NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
6.8NVDAV:N/AC:L/Au:S/C:N/I:N/A:C
6.9GitHubCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
5.7GitHub, Inc. CNACVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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