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Vulnerability record · CVE-2021-4189 · published 24 August 2022

CVE-2021-4189: Python unchecked return value vulnerability

Python · Python

A flaw was found in Python, specifically in the FTP (File Transfer Protocol) client library in PASV (passive) mode. The issue is how the FTP client trusts the host from the PASV response by default. This flaw allows an attacker to set up a malicious FTP server that can trick FTP clients into connecting back to a given IP address and port. This vulnerability could lead to FTP client scanning ports, which otherwise would not have been possible.

5.3 CVSS 3.1 Medium EPSS 3.2% · top 12.3% CWE-252 · Unchecked return value
5.3CVSS 3.1 base score
3.2%EPSS exploitation probability, 30 days
NoNot in CISA KEV
5Affected product versions listed by NVD
19References
17 Jun 2026Last modified by NVD

Description

A flaw was found in Python, specifically in the FTP (File Transfer Protocol) client library in PASV (passive) mode. The issue is how the FTP client trusts the host from the PASV response by default. This flaw allows an attacker to set up a malicious FTP server that can trick FTP clients into connecting back to a given IP address and port. This vulnerability could lead to FTP client scanning ports, which otherwise would not have been possible.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Affected products

5 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://access.redhat.com/security/cve/CVE-2021-4189 Third Party Advisory
https://bugs.python.org/issue43285 PatchVendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2036020 Issue TrackingPatchThird Party Advisory
https://github.com/python/cpython/commit/0ab152c6b5d95caa2dc1a30fa96e10258b5f188e PatchThird Party Advisory
https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
https://python-security.readthedocs.io/vuln/ftplib-pasv.html PatchThird Party Advisory
https://security-tracker.debian.org/tracker/CVE-2021-4189 Third Party Advisory
https://security.netapp.com/advisory/ntap-20221104-0004/ Third Party Advisory
https://access.redhat.com/security/cve/CVE-2021-4189 Third Party Advisory
https://bugs.python.org/issue43285 PatchVendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2036020 Issue TrackingPatchThird Party Advisory
https://github.com/python/cpython/commit/0ab152c6b5d95caa2dc1a30fa96e10258b5f188e PatchThird Party Advisory
https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
https://lists.debian.org/debian-lts-announce/2024/12/msg00000.html
https://python-security.readthedocs.io/vuln/ftplib-pasv.html PatchThird Party Advisory
https://security-tracker.debian.org/tracker/CVE-2021-4189 Third Party Advisory
https://security.netapp.com/advisory/ntap-20221104-0004/ Third Party Advisory

Track CVE-2021-4189 inside VULONE

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Source: NIST National Vulnerability Database (record CVE-2021-4189), CISA KEV, FIRST EPSS (scores of 2026-10-02). This page is refreshed as NVD updates the record.