Critical 9.8Published May 5, 2021

CVE-2021-29921In Python before 3,9,5, the ipaddress library mishandles leading zero characters in the octets of an IP address string.

Updated under an hour ago (Oct 28, 2026, 9:30 AM ET)

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No exploitation reported and no EPSS estimate yet.

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Exploited in the last 30 days

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: 9.8 CRITICAL (v3.1, NVD)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Weakness (CWE): Not given
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: mitre
NVD analysis: Modified
Record last updated: Oct 8, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

python python python:python	from 3.8.0 before 3.8.12; from 3.9.0 before 3.9.5	NVD
oracle communications_cloud_native_core_automated_test_suite oracle:communications_cloud_native_core_automated_test_suite	1.8.0	NVD
oracle communications_cloud_native_core_binding_support_function oracle:communications_cloud_native_core_binding_support_function	1.11.0	NVD
oracle communications_cloud_native_core_network_slice_selection_function oracle:communications_cloud_native_core_network_slice_selection_function	1.8.0	NVD
oracle graalvm oracle:graalvm	20.3.2; 21.1.0	NVD
oracle zfs_storage_appliance_kit oracle:zfs_storage_appliance_kit	8.8	NVD

References

References

https://github.com/sickcodes
https://docs.python.org/3/library/ipaddress.html
https://github.com/python/cpython/blob/63298930fb531ba2bb4f23bc3b915dbf1e17e9e1/Misc/NEWS.d/3.8.0a4.rst
https://sick.codes/sick-2021-014
https://www.oracle.com//security-alerts/cpujul2021.html
https://github.com/sickcodes/security/blob/master/advisories/SICK-2021-014.md
https://python-security.readthedocs.io/vuln/ipaddress-ipv4-leading-zeros.html
https://bugs.python.org/issue36384
https://github.com/python/cpython/pull/12577
https://github.com/python/cpython/pull/25099
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The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.