CVE-2021-3737Network attack vector

Client hang via unbounded interim header reads

Published Mar 4, 2022 · Updated Dec 17, 2025

Denial of service in CPython allows remote attackers controlling an HTTP server to hang an HTTP client with a crafted response. HTTPResponse.begin() repeatedly reads header lines after a 100 Continue response without enforcing the normal total-header limit. The client must request an attacker-controlled HTTP, HTTPS, or FTP service; a stream of short header lines consumes CPU and network bandwidth and keeps the client hung even when a timeout is set.

CVSS severity7.5
High
EPSS probability11.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in CPython allows remote attackers controlling an HTTP server to hang an HTTP client with a crafted response. HTTPResponse.begin() repeatedly reads header lines after a 100 Continue response without enforcing the normal total-header limit. The client must request an attacker-controlled HTTP, HTTPS, or FTP service; a stream of short header lines consumes CPU and network bandwidth and keeps the client hung even when a timeout is set.

The record

CVE
CVE-2021-3737
Published
Mar 4, 2022
Updated
Dec 17, 2025
Vendor
Python Software Foundation
Product
CPython
Classifications
CWE-400, CWE-835, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 4, 2022CVE publishedPublication date reported by the CVE source.
  2. Dec 17, 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 v3.8.11, python v3.9.6, python v3.10.0b2

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 v3.8.11, python v3.9.6, python v3.10.0b2

Published CVSS scores

7.5NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
6.5Red Hat Product SecurityCVSS:3.1/AV:N/AC:L/PR:N/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.

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

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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

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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