CVE-2021-3733Network attack vector

Client hang via quadratic Basic-auth regex

Published Mar 7, 2022 · Updated Nov 3, 2025

Regular expression denial of service in CPython urllib allows remote attackers controlling an HTTP server to hang vulnerable clients. AbstractBasicAuthHandler applies a quadratic-time regular expression to a crafted, invalid HTTP Basic authentication challenge because its scheme token can consume commas. The client must connect to the attacker-controlled server and process its authentication response; the resulting CPU exhaustion interrupts that client application.

CVSS severity6.5
Medium
EPSS probability4.67%
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 CPython urllib allows remote attackers controlling an HTTP server to hang vulnerable clients. AbstractBasicAuthHandler applies a quadratic-time regular expression to a crafted, invalid HTTP Basic authentication challenge because its scheme token can consume commas. The client must connect to the attacker-controlled server and process its authentication response; the resulting CPU exhaustion interrupts that client application.

The record

CVE
CVE-2021-3733
Published
Mar 7, 2022
Updated
Nov 3, 2025
Vendor
Python Software Foundation
Product
CPython
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 7, 2022CVE publishedPublication date reported by the CVE source.
  2. Nov 3, 2025Record 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=Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Python Software Foundation · CPythonversion=Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5.

Published CVSS scores

6.5Amazon LinuxCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
6.5UbuntuCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
6.5NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
4.0NVDAV:N/AC:L/Au:S/C:N/I:N/A:P

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

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

Public exploit references

No public exploit references are available in this record.

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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