CVE-2021-29921

CRITICALNVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

Score 9.8 from GitHub Security Advisory (severity: CRITICAL) published 2022-05-24. NVD baseline CVSS 9.8; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
Trending — 3 sources updated this week
9.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 7%CVSS: 9.8Exploit: NoneExposed: 0

A fix is available — apply it.

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)
EG Risk
—
EPSS
93.1%
KEV
Not listed

Published

May 6, 2021

Last Modified

November 3, 2025

Advisory Details (10)

Auto-updated Jun 22, 2026
🔬 Proof of concept available. Upstream fix merged — awaiting tagged release. Sources: github_pr, github.
generic

NetApp Product Security

https://security.netapp.com/advisory/ntap-20210622-0003/
generic

Python, PyPy3: Multiple Vulnerabilities (GLSA 202305-02) — Gentoo security

https://security.gentoo.org/glsa/202305-02
generic

ipaddress leading zeros in IPv4 address — Python Security 0.0 documentation

https://python-security.readthedocs.io/vuln/ipaddress-ipv4-leading-zeros.html
github🟡 PoC Available

security/advisories/SICK-2021-014.md at master · sickcodes/security · GitHub

https://github.com/sickcodes/security/blob/master/advisories/SICK-2021-014.md
generic

sickcodes (sickcodes) · GitHub

https://github.com/sickcodes
github_pr

bpo-36384: Leading zeros in IPv4 addresses are no longer tolerated

Fix merged in python/cpython PR #25099 on 2021-05-02 — awaiting tagged release

https://github.com/python/cpython/pull/25099
github_pr

bpo-36384: Remove check for leading zeroes in IPv4 addresses

Fix merged in python/cpython PR #12577 on 2019-03-30 — awaiting tagged release

https://github.com/python/cpython/pull/12577
generic

cpython/Misc/NEWS.d/3.8.0a4.rst at 63298930fb531ba2bb4f23bc3b915dbf1e17e9e1 · python/cpython · GitHub

https://github.com/python/cpython/blob/63298930fb531ba2bb4f23bc3b915dbf1e17e9e1/Misc/NEWS.d/3.8.0a4.rst
generic

ipaddress — IPv4/IPv6 manipulation library — Python 3.14.6 documentation

https://docs.python.org/3/library/ipaddress.html
generic

Issue 36384: [security] CVE-2021-29921: ipaddress Should not reject IPv4 addresses with leading zeroes as ambiguously octal - Python tracker

https://bugs.python.org/issue36384

Vendor Advisories for CVE-2021-29921(3)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(6)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntupython3.8-venv (3.8.0-3ubuntu1~18.04.2+esm2) @ bionic2026-05-26ubuntu
ubuntupython3.8-doc (3.8.10-0ubuntu1~20.04.1) @ focal2026-05-26ubuntu
ubuntupython3.8-doc (3.8.6-1ubuntu0.3) @ groovy2026-05-26ubuntu
redhatpython39-devel:3.9-8050020210811100211.d428a79b2021-11-09redhat
redhatpython38-devel:3.8-8050020210811101222.e3d35cca2021-11-09redhat
redhatrh-python38-python-urllib3-0:1.25.7-7.el72021-08-24redhat

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.

Additional Vendor Advisories

(3)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 13× in last 7d / 50× 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.

  1. 2026-07-22 22:47 UTCEG score recompute
  2. 2026-07-22 14:06 UTCEPSS rescore
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  11. 2026-07-01 04:18 UTCOSV refresh
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  38. 2026-06-13 07:13 UTCOSV refresh
  39. 2026-06-12 23:10 UTCEPSS rescore
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  68. 2026-05-26 17:00 UTCEG score recompute
  69. 2026-05-26 17:00 UTCVendor advisory
  70. 2026-05-26 17:00 UTCGHSA enrichment
  71. 2026-05-26 13:42 UTCEPSS rescore
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Related CVEs(same vendor + same CWE)

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 93.1% 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.

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