5.7

CVSS Score

3.1

-

CVSS Score

Basic Information

CVE ID

CVE-2020-5236

GHSA ID

GHSA-73m2-3pwg-5fgc

EPSS Score

0.94034%

CWE

CWE-400

Published

2/4/2020

Updated

11/19/2024

KEV Status

Technology

Python

Basic Information

CVE ID

GHSA ID

EPSS Score

CWE

Published

Updated

KEV Status

Technology

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CVE-2020-5236

CVE-2020-5236: Catastrophic backtracking in regex allows Denial of Service in Waitress

Impact

When waitress receives a header that contains invalid characters it will cause the regular expression engine to catastrophically backtrack causing the process to use 100% CPU time and blocking any other interactions.

This would allow an attacker to send a single request with an invalid header and take the service offline.

Invalid header example:

Bad-header: xxxxxxxxxxxxxxx\x10

Increasing the number of x's in the header will increase the amount of time Waitress spends in the regular expression engine.

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.

Patches

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.

Workarounds

If you have deployed a reverse proxy in front of Waitress it may already be rejecting requests that include invalid headers.

Thanks

The Pylons Project would like to thank Fil Zembowicz for reaching out and disclosing this vulnerability!

References

Catastrophic backtracking explained: https://www.regular-expressions.info/catastrophic.html

For more information

If you have any questions or comments about this advisory:

open an issue at https://github.com/Pylons/waitress/issues (if not sensitive or security related)
email the Pylons Security mailing list: pylons-project-security@googlegroups.com (if security related)

(GitHub Advisory)

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CVE-2020-5236

CVE-2020-5236:

5.7

CVSS Score

3.1

-

CVSS Score

Basic Information

CVE ID

CVE-2020-5236

GHSA ID

GHSA-73m2-3pwg-5fgc

EPSS Score

0.94034%

CWE

CWE-400

Published

2/4/2020

Updated

11/19/2024

KEV Status

Technology

Python

Basic Information

CVE ID

GHSA ID

EPSS Score

CWE

Published

Updated

KEV Status

Technology

Technical Details

CVSS Vector

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

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
waitress	pip	= 1.4.2	1.4.3

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability stemmed from the regex pattern used for header validation in waitress/rfc7230.py. The commit diff shows critical changes to FIELD_CONTENT and FIELD_VALUE regex components that were causing catastrophic backtracking. Though no specific function name is mentioned, the HEADER_FIELD regex compilation is the direct vulnerable component as it processes incoming headers. The patch replaced ambiguous quantifiers with more specific patterns (+ and non-capturing groups) to prevent backtracking. Test cases added in test_parser.py validate this fix by sending malicious headers that previously caused DoS.

Vulnerable functions

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WAF Protection Rules

WAF Rule

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Reasoning

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5.7

-

Basic Information

Basic Information

Concerned about an active attack path?

CVE-2020-5236: Catastrophic backtracking in regex allows Denial of Service in Waitress

Impact

Patches

Workarounds

Thanks

References

For more information

CVE-2020-5236:

5.7

-

Basic Information

Basic Information

Technical Details

Technical Details

Vulnerability IntelligenceMiggo AI

Root Cause Analysis

Vulnerable functions

WAF Protection Rules

WAF Rule

Reasoning

Vulnerability IntelligenceMiggo AI

CVE-2020-5236: Catastrophic backtracking in regex allows Denial of Service in Waitress

Impact

Patches

Workarounds

Thanks

References

For more information

Basic Information

Basic Information

5.7

5.7

Vulnerability IntelligenceMiggo AI

Root Cause Analysis

Vulnerable functions

WAF Protection Rules

WAF Rule

Reasoning

Technical Details

Vulnerability Intelligence
Miggo AI

Vulnerability Intelligence
Miggo AI

Vulnerability Intelligence
Miggo AI