Reference page — cumulative record through Monday, October 19, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2021-29921

CVE-2021-29921CRITICAL

n/a n/a — In Python before 3,9,5, the ipaddress library mishandles leading zero characters in the octets of an IP address string.

  AV  AC  PR  UI  S  C  I  A   CVSS    EPSS   %ile   KEV
   N   L   N   N  U  H  H  H    9.8   .0688   94.0     —

AFFECTED
  Product  Versions  Fixed
  n/a      n/a –     —

TIMELINE
  Apr 1   Reserved by mitre
  May 6   Published (CNA: mitre)
  Oct 8   ENRICHED — CVE-2021-29921. Received CVSS 9.8 and CPE data from NVD.
  Oct 8   EXPLOIT PUBLISHED — CVE-2021-29921. Public exploit reference added.

CNA: mitre · CVSS v3.1 · 17 references · NVD status: Modified

Description

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.

Lifecycle

Complete event history — 4 events, chronological
Date	Event	Detail
April 1, 2021	Reserved	Reserved by mitre
May 6, 2021	Published	Published (CNA: mitre)
October 8, 2026	ENRICHED	ENRICHED — CVE-2021-29921. Received CVSS 9.8 and CPE data from NVD.
October 8, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2021-29921. Public exploit reference added.

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
n/a	n/a	—	n/a	—

References (17)

https://bugs.python.org/issue36384 [Issue Tracking, Patch, Vendor Advisory]
https://docs.python.org/3/library/ipaddress.html [Vendor Advisory]
https://github.com/python/cpython/blob/63298930fb531ba2bb4f23bc3b915dbf1e17e9e1/Misc/NEWS.d/3.8.0a4.rst [Third Party Advisory]
https://github.com/python/cpython/pull/12577 [Patch, Third Party Advisory]
https://github.com/python/cpython/pull/25099 [Patch, Third Party Advisory]
https://github.com/sickcodes [Third Party Advisory]
https://github.com/sickcodes/security/blob/master/advisories/SICK-2021-014.md [Exploit, Third Party Advisory]
https://lists.debian.org/debian-lts-announce/2024/12/msg00000.html
https://python-security.readthedocs.io/vuln/ipaddress-ipv4-leading-zeros.html [Vendor Advisory]
https://security.gentoo.org/glsa/202305-02
https://security.netapp.com/advisory/ntap-20210622-0003/ [Third Party Advisory]
https://sick.codes/sick-2021-014 [Exploit, Third Party Advisory]
https://www.oracle.com//security-alerts/cpujul2021.html [Patch, Third Party Advisory]
https://www.oracle.com/security-alerts/cpuapr2022.html [Patch, Third Party Advisory]
https://www.oracle.com/security-alerts/cpujan2022.html [Patch, Third Party Advisory]
https://www.oracle.com/security-alerts/cpujul2022.html
https://www.oracle.com/security-alerts/cpuoct2021.html [Patch, Third Party Advisory]

Authoritative record: CVE-2021-29921 at cve.org

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