CVE-2021-29921

CRITICALNVD 9.89.8—

9.8

In Python before 3,9,5, the ipaddress library mishandles leading zero characters in the octets of an IP address string. This (in some situations) allows attackers to bypass access control that is based on IP addresses.

CVSS v3: 9.8
EG Score: 9.8(medium)
EPSS: 84.2%
KEV: Not listed

Published

May 6, 2021

Last Modified

November 3, 2025

References (33)

cve@mitrehttps://bugs.python.org/issue36384
cve@mitrehttps://docs.python.org/3/library/ipaddress.html
cve@mitrehttps://github.com/python/cpython/blob/63298930fb531ba2bb4f23bc3b915dbf1e17e9e1/Misc/NEWS.d/3.8.0a4.rst
cve@mitrehttps://github.com/python/cpython/pull/12577
cve@mitrehttps://github.com/python/cpython/pull/25099
cve@mitrehttps://github.com/sickcodes
cve@mitrehttps://github.com/sickcodes/security/blob/master/advisories/SICK-2021-014.md
cve@mitrehttps://python-security.readthedocs.io/vuln/ipaddress-ipv4-leading-zeros.html
cve@mitrehttps://security.gentoo.org/glsa/202305-02
cve@mitrehttps://security.netapp.com/advisory/ntap-20210622-0003/
cve@mitrehttps://sick.codes/sick-2021-014
cve@mitrehttps://www.oracle.com//security-alerts/cpujul2021.html
cve@mitrehttps://www.oracle.com/security-alerts/cpuapr2022.html
cve@mitrehttps://www.oracle.com/security-alerts/cpujan2022.html
cve@mitrehttps://www.oracle.com/security-alerts/cpujul2022.html

Patch Availability(6)

Vendor / Ecosystem	Fixed in / Patch	Released	Source
ubuntu	python3.8-venv (3.8.0-3ubuntu1~18.04.2+esm2) @ bionic	2026-05-26	ubuntu
ubuntu	python3.8-doc (3.8.10-0ubuntu1~20.04.1) @ focal	2026-05-26	ubuntu
ubuntu	python3.8-doc (3.8.6-1ubuntu0.3) @ groovy	2026-05-26	ubuntu
redhat	python39-devel:3.9-8050020210811100211.d428a79b	2021-11-09	redhat
redhat	python38-devel:3.8-8050020210811101222.e3d35cca	2021-11-09	redhat
redhat	rh-python38-python-urllib3-0:1.25.7-7.el7	2021-08-24	redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

CWE-20Improper Input Validation

All Vendor Advisories

(6)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 12× in last 7d / 20× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

2026-06-08 14:15 UTCEPSS rescore
2026-06-07 15:23 UTCEPSS rescore
2026-06-07 15:23 UTCEPSS rescore
2026-06-06 13:46 UTCEPSS rescore
2026-06-06 13:46 UTCEPSS rescore
2026-06-06 13:46 UTCEPSS rescore
2026-06-05 22:45 UTCEPSS rescore
2026-06-05 22:45 UTCEPSS rescore
2026-06-05 06:09 UTCEPSS rescore
2026-06-05 06:09 UTCEPSS rescore
2026-06-04 13:10 UTCEPSS rescore
2026-06-04 13:10 UTCEPSS rescore
2026-06-02 20:11 UTCEPSS rescore
2026-06-02 20:11 UTCEPSS rescore
2026-06-02 20:11 UTCEPSS rescore
2026-06-01 13:50 UTCEPSS rescore
2026-06-01 13:50 UTCEPSS rescore
2026-05-31 22:29 UTCEPSS rescore
2026-05-31 22:29 UTCEPSS rescore
2026-05-31 00:15 UTCEPSS rescore

Frequently asked(5)

What is CVE-2021-29921?

CVE-2021-29921 is a critical vulnerability published on May 6, 2021. In Python before 3,9,5, the ipaddress library mishandles leading zero characters in the octets of an IP address string. This (in some situations) allows attackers to bypass access control that is based on IP addresses.

When was CVE-2021-29921 disclosed?

CVE-2021-29921 was first published in the National Vulnerability Database on May 6, 2021, with the most recent update on November 3, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.

Is CVE-2021-29921 actively exploited?

CVE-2021-29921 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 84.2% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.

What is the CVSS score of CVE-2021-29921?

CVE-2021-29921 has a CVSS v3 base score of 9.8 (NVD).

How do I remediate CVE-2021-29921?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-29921, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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CVE-2021-29921

Published

Last Modified

References (33)

Patch Availability(6)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-29921?

Same vendor

Same CWE

CVE-2021-29921

📅 Published

🔄 Last Modified

🔗 References (33)

Patch Availability(6)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

❓ Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-29921?

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Frequently asked(5)

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