CVE-2021-29921Public exploitNetwork attack vector

Access-control bypass via ambiguous IPv4 octet parsing

Published May 6, 2021 · Updated Nov 3, 2025

Improper IPv4 input validation in Python 3.8.0 through 3.8.11 and 3.9.0 through 3.9.4 allows remote attackers to bypass IP-based access controls. The ipaddress _parse_octet method accepts leading-zero octets and converts them as decimal, while consumers such as inet_aton interpret the same text as octal. Exploitation requires an application to accept attacker-controlled address text, trust ipaddress for a policy decision, and pass the original address to a component with different parsing semantics.

CVSS severity9.8
Critical
EPSS probability6.88%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper IPv4 input validation in Python 3.8.0 through 3.8.11 and 3.9.0 through 3.9.4 allows remote attackers to bypass IP-based access controls. The ipaddress _parse_octet method accepts leading-zero octets and converts them as decimal, while consumers such as inet_aton interpret the same text as octal. Exploitation requires an application to accept attacker-controlled address text, trust ipaddress for a policy decision, and pass the original address to a component with different parsing semantics.

The record

CVE
CVE-2021-29921
Published
May 6, 2021
Updated
Nov 3, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
Unavailable
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 6, 2021CVE 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8Sick CodesCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 exploitPublished

Weakness, pattern, technique

No sourced classifications are available.

Public exploit references

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo